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Double Bass Array (DBA) - The modern bass concept! - Page 2

post #31 of 321
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
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post #32 of 321
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.
post #33 of 321
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).
post #34 of 321
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
post #35 of 321
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'?
post #36 of 321
Thread Starter 
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).
post #37 of 321
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?
post #38 of 321
Thread Starter 
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.
post #39 of 321
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.
post #40 of 321
Thread Starter 
Of course. Thanks for the correction.
post #41 of 321
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
post #42 of 321
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.
post #43 of 321
Thadman,

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

post #44 of 321
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.
post #45 of 321
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.
post #46 of 321
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.
post #47 of 321
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!
post #48 of 321
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 .
post #49 of 321
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?
post #50 of 321
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.
post #51 of 321
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.
post #52 of 321
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.
post #53 of 321
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 .
post #54 of 321
Thread Starter 
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.
post #55 of 321
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?
post #56 of 321
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?
post #57 of 321
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.
post #58 of 321
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!
post #59 of 321
Thread Starter 
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.
post #60 of 321
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.
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