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-   -   Double Bass Array (DBA) - The modern bass concept! (https://www.avsforum.com/forum/155-diy-speakers-subs/837744-double-bass-array-dba-modern-bass-concept.html)

FoLLgoTT 04-22-2007 05:38 AM

I'm really surprised to see that the "Double Bass Array" (DBA) is not very popular outside Germany. I searched for key words, but found only one thread in the AudioCircle forum.

Maybe someone is interested in this concept, since it is by far the smartest way to get a linear frequency response and completely get rid of room modes.
The DBA was described first in a paper from Klein+Hummel which unfortunately exists only in german. I hope my english is good enough to give a small overview here. https://www.avsforum.com/avs-vb/images/smilies/smile.gif


Configuration

A DBA consists of two identical arrays of subwoofers. One on the front and one one on the rear side.
The subwoofers have to be mounted on special positions on the wall. For example if you have 4 drivers in one array (that means overall 8) ranged in a square, their correct place is at 1/4 and 3/4 of the wall's width and height. Like this (my room in an early stage):
 
https://www.avsforum.com/content/type.../1000/flags/LL
 


The side walls work like mirrors and have the same effect like more equidistant bass sources. This completely eliminates the room modes between the side walls and between floor and ceiling.
With this order the front array produces a plane wave which propagetes through the room. When it arrives on the rear wall the second subwoofer array creates the same signal but with inverted polarity. So both waves compensate each other and no reflection on the rear wall occur. The bass is completely free of modes!

Of course it only works, if the rear array is delayed by the time the sound needs to travel from the front array to the rear (delay = speed of sound / room length). Such a delay can be achieved by using cheap DSP equalizers like the Behringer Ultracurve 2496.
The level of the rear array usually has to be a bit lower than the level of the front array, since there is always a bit loss in real rooms when the wave propagates. But with measuring equipment the best setting is easy to find.

The subwoofers itself should have a low depth so reflection from the mounting wall get minimized. Ideally the drivers are build directly into the wall.

Of course a DBA also works with more or less drivers per wall. It is only important that the distance between 2 drivers is twice as long as the distance between the driver closest to a side wall and the side wall itself.
Both dimensions can be considered completely independent of each other.
For example if you want to use only 2 drivers per array, they have to be mounted on 1/4 and 3/4 of the room width and on the middle between floor and ceiling.
Denser driver grids conclude in a higher frequency where a plane wave will still be formed. With common room dimensions 4 drivers per array are enough to ensure a plane wave up to the LFE cut-off frequency.


Conclusion

The big advantage of this concept is to be completely free of room modes and to get exact the frequency response of the simulation. The maximum sound pressure of the DBA is defined as the number of subwoofers in one (!) array times the maximum sound pressure of one single subwoofer.
Another advantage is that the bass is fine on a large area and not limited to one seat. This makes it suitable for large home cinemas. Also the subwoofers can be integrated perfectly because of their small depth. A simple curtain is enough to hide them completely (my front looks like this now). I have seen other solutions which integrates them into self-built shelves in the living room.
And with a DBA playing "Sokoban" at home is over! https://www.avsforum.com/avs-vb/images/smilies/wink.gif

The only disadvantage I know of is the adaption to the front speakers, since their sound pressure usually lowers with 6 dB when doubling the distance and DBA's sound pressure stays constant in the whole room. So if you adjust a linear frequency response for the first row of seats the second will get slightly more bass (or in fact lower sound pressure from the fronts).

The costs of a DBA strongly depend on the components you want to use. My solution is a very cheap one with about 1100€ for the whole setup. But with more and bigger drivers the costs can easily explode to a multiple of that.



Frequency response of the Simulation (with same lowpass and equalizing like the real one):

http://img541.imageshack.us/img541/2...simulation.gif


1/3 octave smoothed measurement in the middle of my room (looks like a fake, but it is not! https://www.avsforum.com/avs-vb/images/smilies/smile.gif):

http://img822.imageshack.us/img822/2751/dbamessung.gif


The waterfall shows no sign of first order modes:
https://www.avsforum.com/content/type.../1000/flags/LL
http://img259.imageshack.us/img259/2...serfalldba.jpg

clubfoot 04-23-2007 09:06 AM

Where did you take the measurements from, i.e. seat location or infront subs?

FoLLgoTT 04-23-2007 09:17 AM

Quote:
Originally Posted by clubfoot View Post

Where did you take the measurements from, i.e. seat location or infront subs?

The microphone was placed in the middle of the room (about 3m away from the front) in a height of about 1m. The room's dimensions are 5,6 x 3,8 x 3 m.

coyotemoon 04-23-2007 09:39 AM

Interesting....

But I am not sure I understand the real world advantage. You are using perhaps eight sub woofers in what seems to be an average room size, all just to eliminate room mode issues, that can be handled for most users by competent placement, room treatment or even active equalization and room analysis.

However, I do bet it sounds very nice!

Tom N.

armystud0911 04-23-2007 09:50 AM

yeah, but I have never seen a natural response curve that flat, very impressive if all this information is accurate.

FoLLgoTT 04-23-2007 10:19 AM

Quote:
Originally Posted by coyotemoon View Post

You are using perhaps eight sub woofers in what seems to be an average room size, all just to eliminate room mode issues, that can be handled for most users by competent placement, room treatment or even active equalization and room analysis.

Playing around with one or maybe more subwoofers just changes the impact of the modes and strongly depends on the listening position. In fact it is not more than trial and error and the result can never compete with a DBA or good room treatment.

Equalization only is a bad compromise since it only affects amplitude and not reverberation. For true audiophiles not an option.

The only working way is room treatment. The downside is that it is pretty complex and bass absorbers take extra space in the room. You have to measure a lot and higher knowledge about acoustics is needed. Only few users can do this.
But in non cuboid rooms room treatment seems to be the only way to go, because a DBA doesn't work here.


The real world advantages of a DBA are
  • flat frequency response in a large area (perfect for home cinema)
  • no modes and reflections at all
  • very simple to build
  • takes only space on the walls (no big boxes on the floor)
  • can be integrated to invisibility (if done right you get a high WAF )
  • sophisticated concept (you can tell your friends "I have an active absorber at home!" )

FoLLgoTT 04-23-2007 10:26 AM

Quote:
Originally Posted by armystud0911 View Post

yeah, but I have never seen a natural response curve that flat, very impressive if all this information is accurate.

I can ensure you it is. But I have to admit that if a sofa would stand under microphone's position the frequency response is not that flat anymore. Every big furniture will decrease the DBA effect a little.
Right now on my listenings position (about 4m from the front) it doesn't look that flat, but even there it looks much better than with one or more conventional subwoofers placed in the room. You can go around and the bass never really changes audibly, pretty astonishing!

catapult 04-23-2007 11:58 AM

Interesting concept! The only problem I can see is it only fixes the sub but the mains also have modal problems that aren't addressed. Maybe a DBA sub would be a good match for dipole mains pulled well into the room.

Edit: Nils, do you have a room curve with less smoothing? 1/3 octave can cover many sins.

FoLLgoTT 04-23-2007 12:27 PM

Quote:
Originally Posted by catapult View Post

Interesting concept! The only problem I can see is it only fixes the sub but the mains also have modal problems that aren't addressed. Maybe a DBA sub would be a good match for dipole mains pulled well into the room.

In my setup the DBA plays the bass of all speakers up to 100 Hz (with 24 dB/oct like most receivers do). So the fronts don't have problems with room modes. Of course the reflections of frequencies over 100 Hz have to be treated specially.

Quote:


Edit: Nils, do you have a room curve with less smoothing? 1/3 octave can cover many sins.

Here is a measurement with 1/24 octave smoothing and a longer FFT. As you can see the plane wave can only be formed up to about 90 Hz (distance between two drivers = half wavelength), so interferences begin to occur. A denser grid would perform better at higher frequencies.


catapult 04-23-2007 12:31 PM

That 1/24 octave measurement looks really nice.

WallyWest 04-23-2007 01:41 PM

Sounds like a three dimensional interpretation of the Harmon white paper's recommendations.

catapult 04-23-2007 03:05 PM

Quote:
Originally Posted by WallyWest View Post

Sounds like a three dimensional interpretation of the Harmon white paper's recommendations.

It's a bit different in that it uses active cancellation to make it sound like the room is open to the outdoors at the back. The idea is to launch a planar wave off the front wall with a delayed, 180 out of phase wave off the back wall, timed to fire when the front wave reaches it. So the wave just dies when it hits the back wall.

You could do about the same thing with a few feet of fiberglass on the back wall and a front wall IB with the drivers arranged to launch a planar wave.

TheEAR 04-23-2007 04:38 PM

Great post,and this setup is a true departure from what most are used to when dealing with room acoustics.

crackyflipside 04-23-2007 04:42 PM

Could the drivers in the rear be mounted in reverse and in same polarity (with some milliseconds of delay)?

WallyWest 04-23-2007 04:52 PM

Quote:
Originally Posted by catapult View Post

It's a bit different in that it uses active cancellation to make it sound like the room is open to the outdoors at the back. The idea is to launch a planar wave off the front wall with a delayed, 180 out of phase wave off the back wall, timed to fire when the front wave reaches it. So the wave just dies when it hits the back wall.

You could do about the same thing with a few feet of fiberglass on the back wall and a front wall IB with the drivers arranged to launch a planar wave.

Ok, I kind of skimmed through it first and didn't get the whole concept.

One question though, the OP says that the array's SPL stays constant through the room. And that's because the exact location of the drivers sets up reflections from the side walls (and floor/ceiling I guess) that reinforce the sound? So basically this design eliminates all room mode problems except for the rear wall. Which you can either treat with this nifty cancellation system, or a ****load of fiberglass.

Nifty.

J_Palmer_Cass 04-23-2007 04:58 PM

Quote:
Originally Posted by FoLLgoTT View Post

I can ensure you it is. But I have to admit that if a sofa would stand under microphone's position the frequency response is not that flat anymore. Every big furniture will decrease the DBA effect a little.
Right now on my listenings position (about 4m from the front) it doesn't look that flat, but even there it looks much better than with one or more conventional subwoofers placed in the room. You can go around and the bass never really changes audibly, pretty astonishing!


Just wondering, are those subwoofers (drivers) in a sealed enclosure, or is that an IB setup?

catapult 04-23-2007 06:26 PM

Quote:
Originally Posted by crackyflipside View Post

Could the drivers in the rear be mounted in reverse and in same polarity (with some milliseconds of delay)?

I don't think it matters much which way you physically mount the drivers. The thing is, whichever way you mount them, when the voltage goes (+), the air pressure goes (-) on the rear drivers. And the rear signal is delayed by a DEQ or whatever by however many milliseconds it takes for sound to travel from the front wall to the rear wall.
Quote:
Originally Posted by WallyWest View Post

One question though, the OP says that the array's SPL stays constant through the room.

I don't think that's strictly true. It's like a line array (except this is a 2D line array or a plane array) where you're always in the nearfield. In the farfield, SPL falls off at 6dB when distance doubles. In the nearfield, SPL falls off at 3dB when distance doubles. So the falloff with distance is less but it's still there. That's why the volume control needs to be a bit less on the rear speakers so they exactly cancel the front speakers' sound that has travelled all that distance.

WallyWest 04-23-2007 06:30 PM

Quote:
Originally Posted by catapult View Post

I don't think it matters much which way you physically mount the drivers. The thing is, whichever way you mount them, when the voltage goes (+), the air pressure goes (-) on the rear drivers. And the rear signal is delayed by a DEQ or whatever by however many milliseconds it takes for sound to travel from the front wall to the rear wall.

Slightly off topic, but can the BFD 1124 do that? Digital delay that is. Or do you have to go to the 2496 to get that feature?

FoLLgoTT 04-23-2007 11:34 PM

Quote:
Originally Posted by WallyWest View Post

One question though, the OP says that the array's SPL stays constant through the room. And that's because the exact location of the drivers sets up reflections from the side walls (and floor/ceiling I guess) that reinforce the sound? So basically this design eliminates all room mode problems except for the rear wall. Which you can either treat with this nifty cancellation system, or a ****load of fiberglass.

Yes, that's exactly the way it works.


Quote:
Originally Posted by catapult View Post

I don't think that's strictly true. It's like a line array (except this is a 2D line array or a plane array) where you're always in the nearfield. In the farfield, SPL falls off at 6dB when distance doubles. In the nearfield, SPL falls off at 3dB when distance doubles. So the falloff with distance is less but it's still there. That's why the volume control needs to be a bit less on the rear speakers so they exactly cancel the front speakers' sound that has travelled all that distance.

I think it is a bit different. Point and line sources lose SPL whith larger distance, because the surface of the wave (sphere/cylinder wave) grows and the sound pressure is always allocated on the whole surface. So the energy is always the same it is just spread on a larger area.

But the surface of the DBA's plane wave remains always the same when traveling through the room. In an ideal infenitely long room (with solid side walls) it could travel until the end of time with the same SPL.
As WallyWest said, the trick is the special placement which let the side walls work like extra bass sources (mirroring from the real sources). So in fact a DBA works like an open transmission line with constant intersection.

The loss in real rooms can be explained by not ideal conditions. For example in my room there is a rather thin wooden door in the side wall which vibrates a bit.


Quote:
Originally Posted by J_Palmer_Cass View Post

Just wondering, are those subwoofers (drivers) in a sealed enclosure, or is that an IB setup?

A time ago I used closed boxes. But since there is no room gain with a DBA (you practically get free air conditions) I had to use a +12 dB shelving filter in the lower end. This dropped max SPL a lot, because of high cone excursion. So right now all subwoofers are vented designs.

J_Palmer_Cass 04-24-2007 01:01 AM

Quote:
Originally Posted by FoLLgoTT View Post


Quote:
Originally Posted by J_Palmer_Class
Just wondering, are those subwoofers (drivers) in a sealed enclosure, or is that an IB setup?



A time ago I used closed boxes. But since there is no room gain with a DBA (you practically get free air conditions) I had to use a +12 dB shelving filter in the lower end. This dropped max SPL a lot, because of high cone excursion. So right now all subwoofers are vented designs.



OK. It's hard to get a perspective on size via your pictures.

What size drivers did you use, and what are the rough dimensions of those wall mounted subwoofers (H X W X D)? Also, with that many speakers, do you need high amplifier power, or do finess that issue wiring speakers in a special series / parallel manner?

Are the ports facing certain directions (like all up), or do they face different different directions for different speakers?

I like the concept myself, and I may try it in the room I am working on.

FoLLgoTT 04-24-2007 01:25 AM

Quote:
Originally Posted by J_Palmer_Cass View Post

What size drivers did you use, and what are the rough dimensions of those wall mounted subwoofers (H X W X D)? Also, with that many speakers, do you need high amplifier power, or do finess that issue wiring speakers in a special series / parallel manner?

I use 25 cm (10") drivers in 100 liters each. The boxes' dimensions are 100 x 80 x 17 cm. I could reduce the depth to 14 cm, but in the case of not enough max SPL I wanted the ability to change to 30 cm (12") woofers. But for my room it is enough (according to simulation about 115 dB down to 20 Hz).

Two of the drivers (8 Ohm) are wired in parallel and two pairs are wired in a serial manner. That results in an impedance of 8 Ohm which is no problem for most power amplifiers.
I use one stereo power amp with 500 W RMS. The front array is driven by the left channel and the rear array by the right. Amplification ist no problem at all with vented enclosures.

Quote:


Are the ports facing certain directions (like all up), or do they face different different directions for different speakers?

The ports are all faced to the side walls not far away from the drivers. This seems to be no problem, because the wavelength is very long at 24 Hz and always result in constructive interference.

Lindahl 04-24-2007 07:23 AM

Very cool concept. I don't think it's feasible for my room, but I like it a lot. I can't imagine trying to do this with the 45 lbs 15" monsters I recently purchased, or even any of the smaller drivers people use here. But, for 10" drivers, no problem, and it sounds like it worked great. Maybe for my next theater...

Jonomega 04-24-2007 08:00 AM

nice concept, and thats a good "natural" 1/24 octave graph!

Mark Seaton 04-24-2007 08:55 AM

Good stuff FoLLgoTT,

This is a good solution to what I've observed many times over...

Placement in a room can easily deal with interactions in two dimensions of a closed room. The part that throws the wrench in the mix and causes all of the headaches is the back wall.

In looking at a room with this approach, you will be better served to consider the walls as reflective boundaries instead of looking at things from a room mode perspective. An ideally solid boundary effectively creates a mirrored image of the real sources. If you have a symmetrical spacing in one dimension and have mirrors on opposing sides, like two parallel walls, the virtual behavior is akin to an infinitely repeating array. How close the sources are placed to eachother and to the boundaries determines the upper frequency at which they couple to form effectively a homogeneous source. If you have side walls and floor-ceiling, you can then have effectively a planar source like described above. To the degree that the walls are more reflective than transmissive, this behavior will be observed in the real world.

I have not personally tried the cancelling idea at the rear wall, although it makes sense so long as you can clearly predict the interaction of both sources with the room in the cancelling dimension. In other words, if the subwoofer spacing isn't close enough for the upper frequency limit, the behavior will be more location dependent.

I'll have to do some more looking into this and experimenting. Of course the trade off here is potentially needing more subwoofers for similar SPL, especially at very low frequencies, but I could certainly see this being a useful option.

The idea that comes to my mind is in using some creative filtering to use this approach down to the lowest length mode and then either cut off the rear sub, or use an all-pass filter to shift the two subs back into polarity at the lowest frequencies so we don't give up VLF gain in the room. It certainly makes a strong case for a two way subwoofer for above and below the modal regions.

andy_puiu 04-24-2007 09:15 AM

Fascinating idea, and it makes perfect sense.

Two implementation thoughts that occured to me...

- A partially finished basement would be perfect for the fiberglass insulation option suggested above. Build a false back wall loaded with fiberglass, let the sound pass through this (attenuated), where after bouncing around back there and dissipating some more, it would have to again pass through your false fiberglass wall and get attenuated more. Wouldn't be ideal, compared to the active wave cancellation, but would save half the cost (half the drivers, half the amp eliminated) and also eliminate the need to tune the volume and time delay for wave cancellation.

- Could this work with a sonosub? Two tall sonosubs, with a driver at both ends? Might be hard to build/unstable, but assuming 8 ft ceilings, put a driver 2 feet off the floor (bottom) and another at 6 foot elevation (top). Maybe do cutouts and covering on the tube to make it look like it goes floor to ceiling but actually opens up at both ends. I don't know, just thinking out loud.

andy_puiu 04-27-2007 02:06 PM

We obviously need a better, catchier name for this... there hasn't been much interest/activity in this thread.

cameronthorne 04-27-2007 07:00 PM

I'm definitely interested, but I've only been lurking. It sounds like a fascinating concept, but nothing I would be able to use or even investigate for quite some time. I have only open, lossy rooms.

If I ever build my dream house with a dedicated room, this would be an interesting thing to combine with an infinite baffle alignment.

crackyflipside 04-27-2007 08:33 PM

Quote:
Originally Posted by andy_puiu View Post

We obviously need a better, catchier name for this... there hasn't been much interest/activity in this thread.


An Acronym!

Well it already has that; a DBA sub.

FoLLgoTT 04-28-2007 03:16 AM

Quote:
Originally Posted by andy_puiu View Post

We obviously need a better, catchier name for this... there hasn't been much interest/activity in this thread.

I think my titles often sound a bit too dry or too technical to get many clicks. Maybe something like "Ultimate subwoofer setup! The search is over!!!" would get the attention of every user, but I don't like such titles...

FoLLgoTT 04-28-2007 07:58 AM

Another interesting idea appeared at a german Hifi forum. The idea is to use only a Single Bass Array (SBA) on the front wall and to add a time shifted signal with inverted polarity to the original signal.
So the wave will be reflected at the rear wall exactly one time and then it will be canceled by the additional signal.

This works so far, but only down to the first room mode as you can see in the frequency diagram. At lower frequencies the room works as a compression chamber and the sound will not be reflected by the rear wall anymore.

A possible solution could be applying a high pass with a cut-off frequency of the first room mode to the additional signal. Sadly with IIR filters the whole phase response changes and doesn't fit to the original signal anymore. I played around a bit with delay and level, but I could not find working settings. Maybe a bit more time is needed...


SBA only:




SBA with additional inverted signal with inverse polarity:




DBA:


catapult 04-28-2007 03:50 PM

My favorite notion is still an array up front and absorption at the back. That will work in 'open' rooms as well as sealed cubes. As the frequency goes down below the first room mode, the absorption becomes less effective, and the room gets pressurized as it should, with no fancy electronics. Two feet of fiberglass would probably do a pretty good job in most rooms. More would be better yet of course. Quoting Terry Montlick:

Quote:


It's hard to measure accurately at low frequencies, but we've done a lot of modelling down there using transfer matrix methods. A two foot thickness of fiberglass batt should give you a diffuse absorption coefficient of about 0.76 at 50 Hz. With a one foot thickness, this drops to about 0.47 at 50 Hz. Denser fiberglass performs substantially worse at these thicknesses.

- Terry
_________________
ALPHA CERTIFICATION
The Home Theater Acoustical Standard
Demand Nothing Less
Terry Montlick Laboratories LLC
www.tmlaboratories.com


Mark Seaton 04-28-2007 04:09 PM

Russ Berger of studio design fame was in town Thursday night for a local AES event at Shure (I live a few miles away). I had the pleasure of sitting through a TEF workshop next to Russ Berger a year or two ago and I had a chance after the meeting to pick his brain on this concept.

It was curious to find that he is in fact working with others in developing specific algorithms that would better work with some of the irregularities in real spaces.

He had a good phrase for the concept which he described as a "Pitch and Catch Approach." That makes sense, as it's exactly what you are trying to do. Apparently the limitations arise in the transient case, where it can work extremely well in a steady state observation for limited locations.

I'm starting to see it as something that could be used well over some limited bandwidth and with some creative processing might still allow the gain of the room or even both front & rear subs depending on the approach.

bwaslo 04-28-2007 05:31 PM

That has to be just about the most interesting article I've read here. Very clever concept! A welcome change toward better behaved bass (rather than just "more bass"). I'd really like to hear a setup like that.

Seems like for this, someone should be developing some ultra-shallow large front surface sub cabinets, with lots of driver area (so they don't need too much excursion and can still be shallow) -- maybe an array of 8x 6" drivers in each flat box, under motional feedback control to get them to go down low? A box like that might be made to look like a 'false wall' maybe (for those of us who don't have dedicated listening rooms).

Gir_1337 04-28-2007 08:57 PM

Quote:
Originally Posted by bwaslo View Post

That has to be just about the most interesting article I've read here. Very clever concept! A welcome change toward better behaved bass (rather than just "more bass"). I'd really like to hear a setup like that.

Seems like for this, someone should be developing some ultra-shallow large front surface sub cabinets, with lots of driver area (so they don't need too much excursion and can still be shallow) -- maybe an array of 8x 6" drivers in each flat box, under motional feedback control to get them to go down low? A box like that might be made to look like a 'false wall' maybe (for those of us who don't have dedicated listening rooms).

I'm up for a 8x18" =D

andy_puiu 04-30-2007 07:51 AM

Quote:


Another interesting idea appeared at a german Hifi forum. The idea is to use only a Single Bass Array (SBA) on the front wall and to add a time shifted signal with inverted polarity to the original signal. So the wave will be reflected at the rear wall exactly one time and then it will be canceled by the additional signal.

I'm not sure I understand this SBA idea. Though that would improve/correct decay times and such, wouldn't you still have constructive and destructive interference happening throughout the room, from the primary and reflected waves, such that the frequency response would vary throughout the room? i.e. Wouldn't you still get peaks and valleys for your in room response due to room size, proportions, etc...?

The fascinating thing about the DBA is that it largely eliminates (or tries to) the tremendous negative effect the room can have.

The DBA obviously looks the most impressive out of those three images you posted, not just in decay time but even initial response. What exactly is that graph simulating and 'where'?

FoLLgoTT 04-30-2007 08:05 AM

Quote:
Originally Posted by andy_puiu View Post

I'm not sure I understand this SBA idea. Though that would improve/correct decay times and such, wouldn't you still have constructive and destructive interference happening throughout the room, from the primary and reflected waves, such that the frequency response would vary throughout the room? i.e. Wouldn't you still get peaks and valleys for your in room response due to room size, proportions, etc...?

Yes, that's true. I measured in different places with this configuration and the frequency response depended much more on the microphone's position. It is just a compromise to a DBA.

Quote:


The DBA obviously looks the most impressive out of those three images you posted, not just in decay time but even initial response. What exactly is that graph simulating and 'where'?

This measurement was taken with ETF in the middle of the room. It shows a waterfall plot which visualizes the decay of sound pressure over the time (usually reflections).
Sadly I have always a bit noise from the street in my apartment, so real measuring of RT60 is impossible (only RT30).

thadman 04-30-2007 07:42 PM

Is there a formula available (such as those made available by Jim Griffin in his white paper for Line Arrays) that will calculate how high in frequency the DBA effect will dominate the listening domain?

Please email me if you have any knowledge concerning this and wish to view my listening room to form a conjecture concerning it. The forum isnt allowing me to post my drawing (until >5 posts)

I was wondering if the rear wall was necessary for the DBA effect, or if I could place the rear subwoofers at a predetermined distance in the room and cancel them at that point? I'd love to incorporate a double bass array into my HT/listening area, but since my listening room opens up it doesnt seem very desirable. Could I place the rear subwoofers at the mouth of the HT area and achieve the same effects as if a wall were there?

I'd also love to substitute this for a dipole and run them all the way up to 300hz (RSS390HF can handle extremely high crossover points, considering the first high amplitude breakup is in the 1700-1800hz region). What would be necessary and how many would I need to reach 300hz?

FoLLgoTT 05-01-2007 01:50 AM

Quote:
Originally Posted by thadman View Post

Is there a formula available (such as those made available by Jim Griffin in his white paper for Line Arrays) that will calculate how high in frequency the DBA effect will dominate the listening domain?

A plane wave can be formed up to frequencies whose half wavelength is smaller than the distance between two woofers (or twice the distance between wall and outer woofer). Again vertical and horizontal dimensions are independant of each other.

The formular for the cut-off frequency is

fc = speed of sound / (2 * distance between drivers)

I my case the horizontal distance is limiting and let the DBA work up to 340 m/s / 1.9 m = 89,5 Hz. At higher frequencies the frequency response is not that flat anymore, because of destructive interferences.


Quote:
I was wondering if the rear wall was necessary for the DBA effect, or if I could place the rear subwoofers at a predetermined distance in the room and cancel them at that point?

The problem is that subwoofers are point sources with spherical radiation. So you will get reflections from the back wall which will interfere. I think it will not work well. Even dipoles with a better directivity emit sound to the back.

Jonomega 05-01-2007 04:31 AM

Quote:
Originally Posted by FoLLgoTT View Post


The formular for the cut-off frequency is

fc = speed of sound / (2 * distance between drivers)

I my case the horizontal distance is limiting and let the DBA work up to 340 m/s / 1.9 m/s = 89,5 Hz. At higher frequencies the frequency response is not that flat anymore, because of destructive interferences.

You mean 340 m/s / 1.9 m = 89.5 Hz right? Two velocities divided into each other gives dimensionless velocity rather than inverse seconds.

FoLLgoTT 05-01-2007 07:25 AM

Of course. Thanks for the correction.

Jonomega 05-01-2007 08:03 AM

Quote:
Originally Posted by FoLLgoTT View Post

Of course. Thanks for the correction.

No prob. this is all new stuff to me, and I just wanted to make sure I wasnt confused

thadman 05-01-2007 02:44 PM

To what extent will the DBA effect contribute if pieces of furniture/lossy walls are in the area? If its not demonstrated in a perfect room is the concept totally destroyed...or does it still do a fairly good job of destroying room nodes?

Also at what point does mono become necessary? Could I take these up to 150hz without penalty? I'd like to cross to a pair of dipoles (baffle width/drivers have not yet been determined) and I'd like to know a desirable crossover point for them to mate seamlessly. Is the DBA effect or dipole effect more effective at >50-300hz frequencies?

If I have 4 woofers, how do I arrange them on the wall in proportion to the proper CTC spacing? Is the distance to the walls supposed to be the same as the distance to the drivers? Do the drivers have to be front firing or can I place them in raised sonotubes (I driver at the top, 1 driver at the bottom)? I would have 4 sonotubes, 2 against each wall.

Thomas-W 05-01-2007 05:03 PM

Thadman,

As Dennis H. explained in your other thread about this, your room isn't suitable for this concept....


thadman 05-01-2007 05:38 PM

Quote:
Originally Posted by thadman View Post

To what extent will the DBA effect contribute if pieces of furniture/lossy walls are in the area? If its not demonstrated in a perfect room is the concept totally destroyed...or does it still do a fairly good job of destroying room nodes?

Also at what point does mono become necessary? Could I take these up to 150hz without penalty? I'd like to cross to a pair of dipoles (baffle width/drivers have not yet been determined) and I'd like to know a desirable crossover point for them to mate seamlessly. Is the DBA effect or dipole effect more effective at >50-300hz frequencies?

If I have 4 woofers, how do I arrange them on the wall in proportion to the proper CTC spacing? Is the distance to the walls supposed to be the same as the distance to the drivers? Do the drivers have to be front firing or can I place them in raised sonotubes (I driver at the top, 1 driver at the bottom)? I would have 4 sonotubes, 2 against each wall.

Quote:
Originally Posted by Thomas-W View Post

Thadman,

As Dennis H. explained in your other thread about this, your room isn't suitable for this concept....

Even if that room isnt suitable, I'd still like my specific questions answered.

andy_puiu 05-02-2007 01:28 PM

Quote:
thadman
If I have 4 woofers, how do I arrange them on the wall in proportion to the proper CTC spacing? Is the distance to the walls supposed to be the same as the distance to the drivers? Do the drivers have to be front firing or can I place them in raised sonotubes (I driver at the top, 1 driver at the bottom)? I would have 4 sonotubes, 2 against each wall.

This was answered at the start of this thread. The drivers should be located at 1/4 and 3/4 of the width and height of the room. In other words, the distance between subs should be twice the distance from the sub to the nearest wall or floor/ceiling.

I wouldn't *think* it matters if the drivers are front firing or not... as I suggested the exact same thing earlier (2 x dual driver sonotubes) At subwoofer/LFE frequencies, the sound/energy isn't very directional. I'd be interested to hear if anyone else thinks non-front-firing is or is not a good idea, though.

Your HT room looks to me like it would work if you were willing to partition it off... using the 2 feet of insulation/false wall method.

Gir_1337 05-02-2007 04:40 PM

I would think that you need a front firing driver, since you're making a wave front traveling towards the back of the room. Without the front firing, you're dealing with all the reflections and I'd imagine the drivers in the back of the room wouldn't work properly then. Perhaps I'm wrong, I'm no expert.

JinMTVT 09-16-2007 09:13 PM

Very interesting concept...
currently designing my soon to be new house with a room only for HT
( 25 by 20 with irregular shape )

interested in learning more about planar wave for subwoofers

does it matter if the room isn't square walled?
i tried to design my room with angle walls so to limit modes as much as possible ...

I was planning on 16 18" ( 8 per side ) for subwoofer in IB configuration
directly on the front wall, on each side of the projection screen

how should i place those woofers to get a planar wave effect?
is it possible ?
i am quite restraint by the screen position as it will be of 100" large
thus leaving only ~50" on each side of the screen for speakers
( will be using line arrays from floor to ceiling for all mid/high loudspeakers)

wonder if there would be a way to take advantage of this to further eliminate modes, back reflection will be easy to deal with as i have already planned a very absorbative rear!

nice nice ideas here i believe!

krholmberg 09-18-2007 01:34 AM

JinMTVT...

My take is you should have 8 on the front wall and 8 on the back wall... maybe you could have 4 left of the screen in a vertical line and 4 right of the screen in a vertical line. Then, in the back, you'd have two more line arrays of four each that mirror the front and are exactly out of phase with the proper time delay. This way you maintain symmetry and shouldn't lose the effect (I think). If you had the volume of the subs louder than the rest of the speakers (say 6dB hot), I don't see why you couldn't use a parametric EQ to slowly trim the response down to the same volume of the rest of the speakers at the frequency in which they cross over. You would effectively be adding a house curve without boosting the low end (and of course you really only want to cut output). Since this DBA alignment theoretically causes the same frequency response throughout the room, wouldn't this house curve be the same throughout the entire room? If so, that would be pretty cool and a hell of a lot better than the small sweet spot we are accustomed to. Obviously you'd need lots of drivers to get the output you'd need... but for you that clearly won't be a problem. And, since you're using line arrays for your mains, the drop in output with distance will be less of a concern. For you I'd say the only real concern is the fact that your room is oddly shaped.

BTW, since you're going IB, you'll definitely want a house curve. I have a dual Ascendant Audio 18" Avalanche IB (one manifold front and center). I initially had it flat to 10 Hz at the seated position, but the bass was so clean, that it left me wanting more (it felt quieter than it was). I dialed in a house curve that crept up from even at 70Hz to 10dB hot at 30Hz, stayed 10dB hot from 30Hz down to 20Hz, and then slowly dropped off to even at 10Hz. It sounds incredible. Most would probably consider a 10dB house curve excessive, but the sound is so clean, I absolutely love it. If you can pull this off, everyone in your theater will be able to experience the same sound ! In my theater, it only occurs in once specific location .

krholmberg 09-18-2007 01:48 AM

FoLLgoTT...

Two questions...

Would extensive bass traps have a negative effect on the working of the DBA alignment? I ask because you stated having large furniture in the room has a negative effect.

Also, if the subs were ported, where would you put the ports?

krholmberg 09-18-2007 09:53 AM

JinMTVT...

Just thought of something else. IBs have the natural slow roll off of a sealed box. Room gain and massive displacement is what allows it to have such VLF potential. With room gain taken out of the equation, I'm not so sure you'll have the VLFs that others with IBs have (like myself). Now, having said that, with 16 18" woofers, you'll have a crazy amount of displacement, but this has to be something to consider before you build. The problem I see is even if you can turn up the volume high enough to get the VLFs, the 20-40Hz range will probably be 20 or so dBs louder purely because of the roll off. That would require the use of a parametric EQ to make the response flat let alone have a house curve. I suppose the best way to build it is so that you have the option to build a moderately sized enclosure behind the subs (on the other side of the wall) and port them into the room. In other words, I think it would be smart if you built it so it would be easy to convert your subs to an LLT.

Lindahl 09-18-2007 10:31 AM

It's easier just to use the $300 Behringer DEQ2496 to adjust the frequency response. You're going to need a good processor, anyway, just to perform the delay and inversion.

krholmberg 09-18-2007 10:53 AM

In thinking about this more, the DBA arrangement lends itself extremely well to a combination of LLT sub enclsures and sealed box line arrays. All you'd need for acoustic treatments are to tame the first point reflections on the side and rear walls. This would be extremely dynamic and easy to set-up. Adding a some Eighth Nerve Adapt treatements to the bi and tri corners would probably be helpful, too, but I doubt you'd need anything more than that. This type of setup would create an extremely wide sweet spot and an extremely dynamic system.

krholmberg 09-18-2007 11:07 AM

Quote:
Originally Posted by bwaslo View Post

Seems like for this, someone should be developing some ultra-shallow large front surface sub cabinets, with lots of driver area (so they don't need too much excursion and can still be shallow) -- maybe an array of 8x 6" drivers in each flat box, under motional feedback control to get them to go down low? A box like that might be made to look like a 'false wall' maybe (for those of us who don't have dedicated listening rooms).

Get a bunch of Adire Extrem(i?) drivers at close out discount (is that possible?). Hmm, that would be .

FoLLgoTT 09-18-2007 12:17 PM

Quote:
Originally Posted by krholmberg View Post

Would extensive bass traps have a negative effect on the working of the DBA alignment? I ask because you stated having large furniture in the room has a negative effect.

I think it would have a negative effect since the back array would not fire against a plain wave anymore. The bass traps and the back array have the same function. With a DBA bass traps are destructive for the function.

Quote:


Also, if the subs were ported, where would you put the ports?

The wavelength is so large under 100 Hz that it is nearly irrelevant where it is placed. There will be always constructive interferences. But enclosures with large front sizes (like mine) I would put them in the near of the driver.

noah katz 09-18-2007 12:22 PM

Hi Mark,

"Apparently the limitations arise in the transient case, where it can work extremely well in a steady state observation for limited locations."

At least for short transients that should be OK, as there won't be enough cycles to build modes to full amplitude.

"...Wouldn't you still get peaks and valleys for your in room response due to room size, proportions, etc...?"..."Yes, that's true. I measured in different places with this configuration and the frequency response depended much more on the microphone's position."

I would hope that DBA gives a smoother response over a wider area than conventional EQ, which can also give a smooth response at one location; otherwise, why bother?

Thomas-W 09-18-2007 02:50 PM

Quote:
Originally Posted by krholmberg View Post

In thinking about this more, the DBA arrangement lends itself extremely well to a combination of LLT sub enclsures and sealed box line arrays.

Question being why would he want to live with the colorations created by any type enclosure, when he can have an IB and no box colorations?

krholmberg 09-18-2007 04:38 PM

That's a whole 'nother question . I suppose it depend on how bad his room dimensions are and what his priorities are. If it is deep, effortless, clean sound, then nothing but an IB will do. If it's a good even response across the room for multiple seating locations, then a DBA of LLT subs would probably be better. Only he can will have the answer.

JinMTVT 09-18-2007 10:01 PM

hi all!

well i have set my mind for IB ..and you pointed the reason why
i don't see why i would use ANY OTHER configurations than IB since the design of my house (by meyself) allows it!

that set,

there are a few reasons why i have chose line arrays for my HT
the importants are :

- less distortion for the same SPL wich is important in HT
- more dynamics
- possibility to place from 4 to 8 people in the sweet spot for all the drivers in the room ... ( nearfield on all loudpseakers ..and i don't need /want reflections )


now that leaves 1 area lagging, and it is sub-bass room response
for 4-8 people ..wihc is hard to do normally

that is why i am interested in the "planar wave" thing of this thread


so please , teach me more about setting up my IB subwoofers array
to get this effect ...i will probably settle for a 1-2' depth of foam at the back of the room so i do not believe i will need the cancelling woofers at the back
i prefer leaving 8 more woofers at the front and get lower cleaner spl bass

how to achieve a good planar wavefront with subwoofers?
i understand that the placement vs the sidewalls is critical?
will that remove all side and floor/ceiling modes for bass?
( well ALL is never achieved..so "most" would be probably more correct)

let me know the theory under this pleasE!

FoLLgoTT 09-19-2007 03:28 AM

Quote:
Originally Posted by JinMTVT View Post

how to achieve a good planar wavefront with subwoofers?
i understand that the placement vs the sidewalls is critical?
will that remove all side and floor/ceiling modes for bass?
( well ALL is never achieved..so "most" would be probably more correct)

I described the theory in the first post.

Quote:


Of course a DBA also works with more or less drivers per wall. It is only important that the distance between 2 drivers is twice as long as the distance between the driver closest to a side wall and the side wall itself.
Both dimensions can be considered completely independent of each other.

For example if you want to use only 2 drivers per array, they have to be mounted on 1/4 and 3/4 of the room width and on the middle between floor and ceiling.
Denser driver grids conclude in a higher frequency where a plane wave will still be formed. With common room dimensions 4 drivers per array are enough to ensure a plane wave up to the LFE cut-off frequency.

And yes, all side and floor/ceiling modes are removed. With a single bass aray you only get the length mode. But with maximum amplitude.

catapult 09-19-2007 04:12 PM

As Nils said, the theory is in the first post. Short version, array the drivers across the wall so driver-to-driver distance is twice edge-driver-to-wall/ceiling/floor distance. Some of the drivers will need to be behind the screen. It's no problem for bass frequencies to go through a screen but, depending on how close the screen is to the wall, the screen may vibrate. If you have an acoustically transparent screen or can provide a couple feet of space, it should be no problem.

JinMTVT 09-19-2007 06:21 PM

what happens when we have more than 4 drivers?
the spacing between them is descreased thus the spacing VS walls and floor/ceiling also decreases?

With 16 time 18", i'd probably be looking at the complete front wall though
ahhaha nice image in my head right now


is there any problem with those drivers used in Stereo ?

is there any theory on that. i just want to understand why this specific spacing gives a wavefront ( maybea drawing with wavefront ?? )

what are the CONS of this technique ?

the amplitude of the front to rear modes will be more than usual since it will take all of the power from the other dimensions wich are not present??

FoLLgoTT 09-19-2007 11:53 PM

Quote:
Originally Posted by JinMTVT View Post

what happens when we have more than 4 drivers?
the spacing between them is descreased thus the spacing VS walls and floor/ceiling also decreases?

Yes, as you can see in the picture below, the distance between the wall has to be 1/2 of the distance between the drivers itself to form a planar wave. The source (S0) is mirrored on the side wall (S1) which leads in addition to the driver grid to a waveform of an infinite plane source.

So the formular for the positioning on the front and back wall is

pX = (2 * n + 1) * widthWall / (2 * numDriversX)
pY = (2 * n + 1) * heightWall / (2 * numDriversY)

n = 0, 1, 2, 3,...

LL

andy_puiu 09-20-2007 10:08 AM

Just saw this thread revived. To help anyone having trouble visualizing the layout, and what is going on here, I'll try expressing it differently. It's easier if you think only in the horizontal plane, as if you are looking down from above. Everything is the same in the vertical direction.

I've attached a drawing to help show the mirror effect.

As to why the spacing is critical... moving a speaker to the right, moves its reflection to the left, and would lead to inconsistent spacing between the drivers and their reflections:
Instead of this: _x__x_|_x__x_|_x__x_
You'd get this: x____x|x____x|x____x
LL

krholmberg 09-20-2007 12:47 PM

I get the front wall. As for the back wall, the woofers are time delayed so their cones are effectively moving back the same distance as the front woofers did, thus they are essentially catching the same air thats thown at them. A poor analogy is what the winning team of an egg thowing contest would do. One person gently throws the egg with two hands, and ever so gently, with the same motion and in the same direction, the other person catches the egg. Those who have the same catching and releasing motion can essentially go on forever.

JinMTVT 09-20-2007 07:50 PM

not sure i get it there
if i use your formula for 4 drivers on X with wall width of 10 units
i get positions at : 2.5-5-7.5-10
???
am i dumb again! ?

FoLLgoTT 09-20-2007 11:54 PM

Quote:
Originally Posted by JinMTVT View Post

not sure i get it there
if i use your formula for 4 drivers on X with wall width of 10 units
i get positions at : 2.5-5-7.5-10
???
am i dumb again! ?

I don't know if you are dumb, but you definitely didn't used the formular!

OK, here we go (widthWall = 10, numDriversX = 4):

n = 0:
pX1 = (2 * 0 + 1) * 10 / (2 * 4) = 1.25

n = 1:
pX2 = (2 * 1 + 1) * 10 / (2 * 4) = 3.75

n = 2:
pX3 = (2 * 2 + 1) * 10 / (2 * 4) = 6.25

n = 3:
pX4 = (2 * 3 + 1) * 10 / (2 * 4) = 8.75

JinMTVT 09-21-2007 09:58 AM

lol ....
i was doing 2 * (n+1) instead of 2*N+1
my bad ...thanks very much for the nice formulas



Now onto the real beast,

what will happen if the side walls aren't paralell ??
would it work ??

JinMTVT 09-24-2007 07:12 PM

ok
LAST QUESTION PLEAS PEOPLE!

i need answeres to that so i can be done with my new house design
( well at least the HT room )

Is there any problem with plaanr waves if the side walls aren't paralell ?
is it a requirement ?

krholmberg 09-25-2007 09:43 AM

For optimum performance it is necessary. Then again, for optimum performance, there should be no bass traps and no furniture in the room. Deviations from perfection result in less than ideal performance. If you room is relatively rectangular, I would say you're probably fine. If it is far from rectangular, the results may be far from perfect. Ultimately nobody knows how well your room will do until you try and take measurements.

andy_puiu 09-27-2007 01:49 PM

Only guessing here: assuming they are remotely close to parallel, I wouldn't think it would have a big effect if you're doing the passive absorbtion at the rear. I would expect a much bigger difference if you were doing the active time-delayed phase-reversed subs at the rear of the room (since the rear wall would then be a different size, requiring different placement distances, etc...)

JinMTVT 09-27-2007 06:48 PM

thinking of it...it is probably a similar wave front as experienced within most horns,
wich doesn't have paralell walls ??
so i guess that it would work to some extent!

i'll have to see about the rear of the room, i'll probably try to absorb most of it
to get rid of the longitudinal mode as much as possible
it will also help achieve my nearfield listening space

i guess that with such a planar wave, nearfield like loss of sound per distance applies?

andy_puiu 10-01-2007 02:23 PM

No. At least in theory, there would not be any loss of sound per distance. (at least in a perfectly executed DBA with straight sides)

In point source examples, the energy being produced, i.e. the wavefront, occupies a larger and larger sphere as it radiates out. Fixed amount of input energy, stretched over a larger and larger surface - thus the decrease in energy in any one small part of that spherical wave.

Line arrays radiate out in a cylindrical wave, therefore less surface area per distance and better retention, if you will, of that original energy.

The DBA produces a plane wave - which continues to occupy the exact same amount of surface area as it travels through the room - therefore no decrease in local intensity/energy level.

noah katz 10-01-2007 03:44 PM

"The DBA produces a plane wave - which continues to occupy the exact same amount of surface area as it travels through the room - therefore no decrease in local intensity/energy level."

Since the entire front wall is not occupied by drivers, this is certainly not so near it.

It would be interesting to see at what distance from the front wall the drivers' output is more or less fully integrated.

krholmberg 10-01-2007 05:00 PM

Noah... that is an interesting thought. It is to 3-D as the line array is to 2-D. There is a certain area one must be in where you are effectively in the near field response but comb filtering is minimized. I suppose it's a function of the spacing between the woofers (the greater the spacing, the lower the low pass filter and the further away you must be).

JinMTVT 10-01-2007 07:47 PM

Do you guys have any sources for all that info ?
i'd like to read more papers ( not in german please ) about planar waves
and DBA setup ...



What would it do if the SUB array would be used for STEREO bass ???
would it work the same ?
i guess not ..

FoLLgoTT 10-02-2007 11:18 AM

Quote:
Originally Posted by noah katz View Post

Since the entire front wall is not occupied by drivers, this is certainly not so near it.

It would be interesting to see at what distance from the front wall the drivers' output is more or less fully integrated.

I measured my DBA and placed the microphone in the middle of the room's width and changed the distance to the front. The graph shows the distances 1m, 2m, 3m and 4m at 1/3 octave filtering. The room's length is 5.6m.
As you can see, the frequency response and the overall level stay nearly constant. Only in the upper bass where the driver grid is not dense enough differences occur. But these are also pretty small in comparison to single woofers.




@JimMTVT
Quote:


What would it do if the SUB array would be used for STEREO bass ???

A DBA is monaural, so you have to sum the bass of both stereo channels. But since most mixes contain only mono bass information and our ear is not good at locating bass this is no big problem.

FoLLgoTT 10-02-2007 11:44 AM

Btw, my front looks like this now. Only the 3m wide screen is visible, nothing else.


noah katz 10-02-2007 12:15 PM

"As you can see, the frequency response and the overall level stay nearly constant."

Yes; thanks for the measurements.

I realize that my statement is in error; I was overfocusing on the statement mentioning the front wall, completely forgetting that there are also drivers on the back wall.

Ydope 10-28-2007 08:55 AM

Hi Follgott,

Quote:
Originally Posted by FoLLgoTT View Post

I measured my DBA and placed the microphone in the middle of the room's width and changed the distance to the front. The graph shows the distances 1m, 2m, 3m and 4m at 1/3 octave filtering. The room's length is 5.6m.
As you can see, the frequency response and the overall level stay nearly constant. Only in the upper bass where the driver grid is not dense enough differences occur. But these are also pretty small in comparison to single woofers.

Thanks for your measurements. I think it's indeed very interesting to see measurements at diffferent locations in the room.
Could you post the same graph, but without smoothing and maybe some waterfall plots at different locations?
This would allow for an evaluation of how far this concept can really eliminate the modes.

Thanks and besten Gruß
-Y

FoLLgoTT 11-09-2007 06:50 AM

@Ydope
Sorry for the late answer. I'm very busy with my diploma thesis at the moment. When I get some time I will post more measurement plots.

dwk123 11-09-2007 04:02 PM

Fascinating thread - thanks to Follgott for initiating it.

So, my room is tiny. As in small. I can stick a single driver in the center of the front and back walls and still be less than about 5' from any boundary. Given the c/2x equation, this seems to indicate that such a setup would satisfy the conditions up to ~100hz or a bit better.

It can't really be that easy, can it?

I have a couple of the original NHT1259's that I guess I'm just going to have to throw into an experiment. Like my project list wasn't already long enough.

krholmberg 11-09-2007 05:38 PM

As long as they are half way up the wall, I don't see why not. That would be a pretty easy experiment to try. Please reply with your impressions.

Ydope 11-10-2007 11:32 AM

Quote:
Originally Posted by FoLLgoTT View Post

@Ydope
Sorry for the late answer. I'm very busy with my diploma thesis at the moment. When I get some time I will post more measurement plots.

Ah, thanks for the info. Good luck with the thesis!

Maybe you can tweak the topic a bit and include the DBA measurements.
;-)


Greets Y

Avus_M3 11-10-2007 05:01 PM

I don't think my wife would like me hanging 4 18's on the wall...not her type of art

MrHelpful 11-05-2008 07:33 PM

Hate to bump up a dead topic (not that it should be), but is it possible to do this with subs inward of the walls? Say a few subs 1 meter from the front wall and a few 1 meter from the rear wall?

Terry Montlick 11-06-2008 05:25 AM

Quote:
Originally Posted by MrHelpful View Post

Hate to bump up a dead topic (not that it should be), but is it possible to do this with subs inward of the walls? Say a few subs 1 meter from the front wall and a few 1 meter from the rear wall?

Maybe. I've run a good number of simulations. Performance (spacial uniformity of sound field) was relatively constant up to about 1.5 feet distance from the front and back walls. It then started to fall off, reaching a rate of about 1 dB standard deviation per foot at 2.5 feet.

This distance was from the acoustic centers of the subs. The acoustic center is a relatively new concept. It is the point at which spherical sound waves eminate from the sub. For an unported sub, this is a few inches in front of the driver. For a rear ported sub, it is somewhere within the sub box.

However, you can always face the subs toward the walls! Then the acoustic center of an unported sub can be put arbitrarily close to the wall. "But with the sub facing away from the room, the sound would get blocked by the sub," you might think. No. Subwoofer frequencies are truly omnidirectional. It makes no difference which direction the sub is facing. It has no directionality.

Regards,
Terry

Mark Seaton 11-06-2008 07:23 AM

Quote:
Originally Posted by Terry Montlick View Post

Maybe. I've run a good number of simulations. Performance (spacial uniformity of sound field) was relatively constant up to about 1.5 feet distance from the front and back walls. It then started to fall off, reaching a rate of about 1 dB standard deviation per foot at 2.5 feet.

This distance was from the acoustic centers of the subs. The acoustic center is a relatively new concept. It is the point at which spherical sound waves eminate from the sub. For an unported sub, this is a few inches in front of the driver. For a rear ported sub, it is somewhere within the sub box.

Regards,
Terry

Hi Terry,

What does "1dB std deviation/ft" translate to in average and maximum deviations? At first glance that sounds quite good in terms of real rooms.

As a minor correction/addition, a ported or other higher order subwoofer doesn't really have a different origin, but where the origin is constant for a sealed subwoofer, the origin shifts at low frequencies in a ported design. It is generally better to conceptualize it as a multi-way device (driver + port or PR) than as a lumped factor, as this better explains real world observations related to subwoofer orientation and placement.

Terry Montlick 11-06-2008 08:13 AM

Hi Mark,
Quote:
Originally Posted by Mark Seaton View Post

Hi Terry,

What does "1dB std deviation/ft" translate to in average and maximum deviations? At first glance that sounds quite good in terms of real rooms.

The 1 dB std deviation/ft is a guestimate based on the graph of composite spatial standard deviations (multiple experiments) vs. sub distance from wall, over the range of 0 to 3 feet distance. These spatial variations were determined by sampling at evenly spaced microphone positions in a plane at a fixed distanced from front and back walls. At 0 feet subwoofer wall distance, the spacial standard deviation was 1.3 dB. At 3 feet distance, it was 2.3 dB.

Yes, this is quite good in terms of real rooms. The microphone positions sampled included the corners of the plane! The performance from 0 feet to 3 feet changed from phenomenal to merely great.

Quote:
As a minor correction/addition, a ported or other higher order subwoofer doesn't really have a different origin, but where the origin is constant for a sealed subwoofer, the origin shifts at low frequencies in a ported design. It is generally better to conceptualize it as a multi-way device (driver + port or PR) than as a lumped factor, as this better explains real world observations related to subwoofer orientation and placement.

See John Vanderkooy's April 2006 AES UK presentation. That's where I got the info. Vanderkooy has a number of papers studying the acoustic center.

Regards,
Terry

FoLLgoTT 01-07-2009 07:22 AM

Introduction
On the christmas days I had much time to take measurements of different configurations of my eight subwoofers.

The following configurations were measured:


DBA:
  • front array active
  • back array with inverted and time delayed signal

SBA:
  • only front array active

Multi sub:
  • both arrays are feeded with the same signal



Measurements with ARTA

Frequency response (not calibrated, without window, smoothing 1/24 octave)

SBA:


Multi sub:


DBA:




Burst Decay


SBA:


Multi sub:


DBA:




RT60


SBA:


Multi sub:


DBA:





Measurements with RoomEQWizard

RT60 (blue: SBA, red: DBA):


SBA:


DBA:




Btw, maybe someone is interested in translating the DBA article at Wikipedia from german to english.

catapult 01-07-2009 10:40 AM

Thanks for the updates, Nils! Google seems to be getting better at translating German to English. That wikipedia article is actually understandable.

http://translate.google.com/translat...hl=en&ie=UTF-8

penngray 01-07-2009 10:51 AM

Very interesting, I will have to use this design in my next HT build, if the housing market ever turns around

yogaman 02-14-2009 06:57 PM

I would like to create a DBA in my nicely rectangular 7'11-1/2" x 14'9" x 21'5-1/2" (4.50 m x 2.43m x 6.54m) WxHxL room.
Nils's flat subwoofers would work, but sufficient spaces behind one end wall and above the entire ceiling practically beg for Infinite Baffle subwoofers.
So, in striving to combine IB's very low bass extension with DBA's suppression of modal effects:
(1a) Can I use ceiling-mounted IBs at the non-IB-able end to cancel any of the SBA's wavefront?
(1b) What if I put a quad array of IB subwoofers at the end where I can, and use planar ported subs for the opposite wall?
(1c) What about IBs at the ceiling of one end with only floor-level IBs at the opposite end?
(1d) How is the distance from IB drivers to manifold opening related to manifold area?
(For my case, could I make 6-ft tubes to bring the manifold down from the ceiling?)
(1e) Ceiling IBs both ends? (Definite FAF improvement, but don't properly delayed ceiling-only IBs do at least some mode-quenching magic?)

For extra credit, I'd like to increase the upper cutoff frequency above the room's quad-array limit of 76.6 hz.
So, I need to decrease the minimum driver separation.
One driver in the center of each wall would help, but the room won't permit.
The IB wall can handle any configuration of drivers, but the other end won't allow drivers in the middle 45% of the bottom 55%.
So,
(2a) add one ceiling IB center one end, floor IB other?
(2b) add one each ceiling IBs both ends?
(2c) Nils's flat subwoofer a little above center at non-IB end, and another at opposite end above/at/below center
(2d) other ideas?

Lastly, how should I go about beating down higher-frequency reflections?
DBA's infinite array of mirror-image subwoofers relies on having little absorption in the listening room walls, floor and celing.
But the room will still resonate at all the higher harmonics of 1/2 the room dimensions,
so these frequencies still require treatment.
I guess the usual approach is cloth-covered "rigid fiberglass" (gotta research this still),
but (3a) how might one best approach the tradeoffs between absorption and subwoofer imaging?
(3b) Is there a better approach than slapping a rug/pillow over each of the mid+tweeter images?

Thanks in advance,
Yoga Man

Terry Montlick 02-15-2009 04:57 AM

Some general answers, yogaman:

For a DBA to work properly, the rear wall sub positions must exactly match the front sub positions. Front and back subs should be the same type so that their frequency response is the same. I supposed this could be equalized for with different sub types, but then you have to worry about time delay through the equalization filter.

The sub positions on the walls cannot be chosen arbitrarily. The inter-sub spacings must be 2x the spacings from sub to nearest wall, floor or ceiling. The idea of this is that if you created the mirror images of the subs reflected in side walls, floor, and ceiling, the original sub positions plus the mirrored sub positions will all lie on a reguler rectangular grid. If you research the "image source method" of acoustic modelling, you can learn why this has to be so.

Regards,
Terry

vasyachkin 02-15-2009 08:47 AM

i described the same exact concept on Partsexpress forum when i was discussing various possibilities for active room correction ...

if i thought of it - it must be good

yogaman 02-15-2009 10:21 AM

Vasyachkin,
Kudos to you for prior art, but which scenario did you favor? Did you implement? Results, pointers, etc, appreciated.

Terry,
Thanks for a first jab at my problem.

I can see that my presentation has not conveyed my design questions very well.
Please allow some review and more detail.

As we all know, the simplest DBA is one speaker each in the center of the front and back walls.
The walls create acoustic images of these drivers every room-width to each side;
the ceiling and floor image the drivers every room-height above and below; and
this repeats, including all the diagonals, even out to infinity, for perfectly reflective walls, floors and ceilings.

A two-speaker DBA array centered vertically doubles the horizontal density of this simplest DBA,
but leaves the vertical density the same.
The two-speaker DBA array also shifts the driver horizontally by 1/4 of the room width, but the vertical offset is unchanged.
Nils's beautiful quad DBA then doubles the vertical density, too, also shifting the vertical placement by 1/4 room-height.

In all these configurations, the assumption is that the rear wall array is configured identically to the front wall array.
But it's also clear that the number of speakers and their horizontal and vertical "phasing" can be varied.
So, my fundamental inquiry is, "What would happen if the front and rear arrays were not identical?"

For 1a, the proposed arrays are Nils-style quad at one end, and dual ceiling at the other end.
Note that the horizontal alignment is the same at both ends, with two vertical lines of driver images half a room-width apart, 1/4 room-width on either side of each wall image.
The dual ceiling drivers are vertically offset by 1/4 of the room-height relative to the quad array, and every other driver (image) is missing.
Still, can't the sparser array cancel at least some of the back wall reflections? (How much?)

Regarding 1b: I understand that mismatches of the original and reflected wavefronts will result in non-cancellation,
but wouldn't "different but equalized" speakers get you at least most of the way there?
(I'm gonna have furniture, too; is this speaker mismatch more distorting than furniture?)

The images in 1c similarly are aligned horizontally and offset vertically between the end walls, this time in an attempt to minimize the number of drivers.
How well will this work?
(I observe that one way to think about this configuration is to treat the wavefront as moving along the diagonal
rather than parallel to the walls. Is that useful? Btw, I bet this configuration is most susceptible to furniture.)

1d reflects ignorance about IB manifold design.
I realize this may not be the best thread to pose this question.
Sorry, but if someone can point me to...

1e is similar to 1c, but the vertically sparse emitters (and images) are at the same heights
on front and back walls (and images), so does this mean they'll cancel each other better or worse than floor vs. ceiling?
(My guess is 1e is better than 1c.)

The added drivers and their images in my part 2 above are attempting to decrease the maximum separations
between emitters (real and virtual) so that the cutoff frequency could be raised.
Thus, they would be placed in the horizontal center.
2a and 2b enumerate the IB options.
Again, sure, the match won't be perfect, and/or the array will not be fully filled in, but won't there be *some* bandwidth gain?
Which should be better?
(How much?)

In 2c, regarding vertical alignment between the two ends, one suggestion is to place them symmetrically, ie, where one end is slightly above and the other below the horizontal midline;
another suggestion is maintaining the same height from the ceiling/floor.
Or misalign them, and place one end in the exact center where possible.
My guess is symmetry is best, especially for small offsets. But can I get increased bandwidth?

Part 3 seeks advice on how to simultaneously achieve low-frequency reflection and high-frequency absorption.
I suspect the key is that the LF reflections utilize the whole walls, ceiling, and floor, while the HF reflections
can be suppressed by treating their reflections as narrower beams,
but I hope a DBA enthusiast might have already resolved this dilemma.

Thanks to everyone who has taken the time for considering my "out-of-the-box imaginings".
(Seriously evil pun intended.)

Yogaman

vasyachkin 02-15-2009 11:14 AM

Quote:
Originally Posted by yogaman View Post

Vasyachkin,
Kudos to you for prior art, but which scenario did you favor? Did you implement? Results, pointers, etc, appreciated.

here is the original thread:

http://techtalk.parts-express.com/sh....php?p=1532641

having looked over it just now i don't see myself talking about this particular design. probably because i didn't think it was practical for a residential setting. i was thinking for a dance club to have the entire floor as one array and entire ceiling as another ( delayed and inverted ). that way the entire dance floor, no matter the size, would have perfect bass.

read the whole thread, well, not the whole just up to the point where i stop posting ( because i am banned ) ...

catapult 02-15-2009 11:15 AM

Yogaman,

I don't think most of your questions can be answered without running a simulation of the specific layout. It would just be speculation for us to try.

I will say, I don't think ceiling speakers by themselves would work very well. The driver-driver spacing should be under 1/2 wavelength and the d-d spacing of a ceiling speaker is twice the room height. For an 8' ceiling, that means your max 'recommended' frequency would be about 35Hz. I have a room similar to yours where I could do an IB array at one end but not at the other. I have considered doing shallow floor-ceiling boxes that are open to the attic at the top and lightly stuffed to prevent pipe resonances. Pretty close to IB. Or I could just build sealed boxes and use a little EQ.

Terry,

I don't think using digital EQ is a problem since it's one of the requirements of the DBA that the back speakers be delayed by the time of flight. The Behringer DEQ2496 or DCX2496 would be very cost effective and give you powerful EQ as well as the required digital delay.

I've wondered if you couldn't do some EQ trickery to fight nulls like the one Nils has up around 80Hz. You can't boost a null but what you can do is put a notch filter on the inverted rear signal. Would it work? I dunno. The same concept could be used to take advantage of room gain below the first longitudinal mode. The DBA, in theory, cancels all room gain. However, you could roll off the rear array at very low frequencies so the front array gets its room gain.

Bottom line, I think you could deviate quite a bit from the 'ideal' DBA and still take advantage of the DBA concept with some clever EQ, adjusting the front and rear arrays separately, not as a single unit. My living room has openings to other parts of the house and I'm not inclined to install doors. I suspect, with a 6-channel DCX2496, I could still dial in different parts of the array so the sum is pretty good.

PS -- I think we can safely ignore Vas's prior art as this thread is two years older than his thread at PE.

vasyachkin 02-15-2009 11:35 AM

Quote:
Originally Posted by catapult View Post

PS -- I think we can safely ignore Vas's prior art as this thread is two years older than his thread at PE.

while i still haven't read this thread ( nor do i plan to ) i do believe the subject matter is somewhat different from the one we had at PE ...

this whole bass control is such a big subject - it makes me really reluctant to write an article about it - i am too lazy to even read about it in fact lol

yogaman 02-15-2009 01:42 PM

Thanks, Dennis, for the helpful suggestions, especially your insight that the upper cutoff of ceiling-only drivers is 35 hz.
Happily, this reduces the number of choices for my DBA configuration.

What tool (free, I hope?) would you recommend for simulating non-identical arrays?

Regarding your question about improving the null:
The planar wave from the front array should not reflect at all from the back wall, thanks to the rear array.
(And not from the walls, floor or ceiling because it's planar, of course.)
So, if the null you discuss is due to reflection(s) from the walls floor or ceiling, then notching the cancellation wave from the rear drivers should make the null worse.
So whence does the null arise? Furniture? Near-field effects?
If so, then, yes, maybe you will see more amplitude at that null location,
but you'll also be reintroducing all the room modes at that frequency.
I suppose that could turn out to be what the room needs, but it seems unlikely.

I also thought about making cabinets that could open through the ceiling as you describe.
I asked it as a manifold distance question.
I think the IB design doesn't care where drivers are placed, as long as you avoid constricting the flow.
Your idea to stuff openings for tuning seems like a good one.
Were you thinking to put both drivers in the same enclosure?
If so, does that mean the enclosure volume should double to 20x Vas?
Should the ceiling aperture area then be 2x the area of a single driver?

Besides those IB questions, my new list of DBA questions is:

4a) Should I put IB drivers where I can, and use Nils-style flat drivers such as Nils's, EQing out any response mismatches?
4b) Or should I use only Nils-like drivers everywhere?

5a) Does the distributed (ie, non-lumped) model of Nils-style subwoofers mean that he ignored the locations of four of his sources, ie, the ports?
5b) If so, could these extra radiators be considered as increasing the effective density of the driver arrays, and thereby the cutoff frequency for the DBA?
For example, leave speakers and ports at 1/4 and 3/4 room-height, but move speakers to 3/8 and 5/8 room-width, and recognize the port radiators at 1/8 and 7/8.

Uh, in ascii, that's something like:

0 1 2 3 4 5 6 7 0 <- eighths of a room-width
| | | | | | | | |
v v v v v v v v v

------------------------- Ceiling
| +-------+ +-------+ |
| |p D| |D p| | <-- 3/4 room-height
| +-------+ +-------+ |
| | <-- 1/2 room-height
| +-------+ +-------+ |
| |p D| |D p| | <-- 1/4 room-height
| +-------+ +-------+ |
------------------------- Floor

+-------+
|D p| = a subwoofer with a single driver (D) and a single port (p) located as shown
+-------+

(Display the above in a fixed width font such as Courier to make it look right. -ym)

6) And, renaming (3), how can I integrate DBA's reflection-based, nearly anechoic bass with the need to minimize higher frequency reflections?

7) Hey! How about designing a subwoofer with multiple ports? Flat drivers seem to be the perfect shape for this. Not bunched together like SVS PB12 tuning ports, but strategically placed according to the wall coordinates. Wouldn't this yield an array with even greater radiator density?

Thanks again to all,
Yogaman

PS. After hearing vasyachin describe his invention and his approach to knowledge, I'm disinclined to recognize his claim to the concept of mismatched arrays. Mismatched arrays seems like a fairly obvious concept, though. Equally obvious, I think, is the observation that the port and its images can be treated as additional array elements, although if it's so obvious why didn't I think to ask about it until this morning? I bet RF engineers have already thought a whole lot about this stuff...

catapult 02-15-2009 02:44 PM

Quote:


What tool (free, I hope?) would you recommend for simulating non-identical arrays?

Sorry, I'm not aware of any reasonably priced software that could do it. Terry Montlick (above) uses his own custom room software but I doubt he'd be willing to run custom sims for you unless you wanted to pay his hourly rate.

About nulls, they're surprisingly easy to create and the way to fix them isn't always obvious. JohnK over at the PE board modified his room software to simulate a 2-front, 2-rear DBA. In a perfectly reflective room (concrete cave) it was almost perfect up to 70Hz or so. However adding some wall absorption introduced some deep nulls at (I think) 22 and 66 Hz. I'm 99% sure it's because he didn't decrease the gain on the rear array, to account for the sound of the front array being absorbed by the walls, but he wasn't interested in modifying his software (again) to check it out. There was also some question about the mic position (too near the front or rear wall, I forget).

As for the rest, we're all just guessing until we build something.


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