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Discussion Starter · #1 · (Edited)
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.



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):
 

 


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!


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):




1/3 octave smoothed measurement in the middle of my room (looks like a fake, but it is not!
):




The waterfall shows no sign of first order modes:

 

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Discussion Starter · #3 ·

Quote:
Originally Posted by clubfoot /forum/post/0


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.
 

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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.
 

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Discussion Starter · #6 ·

Quote:
Originally Posted by coyotemoon /forum/post/0


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!"
    )
 

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Discussion Starter · #7 ·

Quote:
Originally Posted by armystud0911 /forum/post/0


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!
 

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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.
 

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Discussion Starter · #9 ·

Quote:
Originally Posted by catapult /forum/post/0


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.

 

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That 1/24 octave measurement looks really nice.
 

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Quote:
Originally Posted by WallyWest /forum/post/0


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.
 

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Great post,and this setup is a true departure from what most are used to when dealing with room acoustics.
 
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Quote:
Originally Posted by catapult /forum/post/0


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.
 

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Quote:
Originally Posted by FoLLgoTT /forum/post/0


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?
 

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Quote:
Originally Posted by crackyflipside /forum/post/0


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 /forum/post/0


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.
 
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Quote:
Originally Posted by catapult /forum/post/0


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?
 

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Discussion Starter · #19 ·

Quote:
Originally Posted by WallyWest /forum/post/0


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 /forum/post/0


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 /forum/post/0


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.
 

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Quote:
Originally Posted by FoLLgoTT /forum/post/0



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.
 
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