Theory on best subwoofer placement for rectangular rooms. Can someone prove this wrong? - AVS Forum
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post #1 of 23 Old 08-12-2012, 11:16 AM - Thread Starter
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This here should be the best sub config, at least that I've been able to find, specifically for a quad config via ripple tank simulations.
Essentially you want the least number of streaks or fuzzy turquoise colored regions. It should be a uniform mode field of equal pressure (in theory). Especially after the first 40ms when most waves collapse into room modes (i.e. the first reflection ripples).

I can't prove this because my room is J-shaped and unloads into two L-shaped walkways (YAY).

Looks like the lowest this simulator goes is about 60hz, relative to the size of this room vs the speed of sound, if this were reality.
Can someone, prove this ripple tank wrong? Or at least find a better config for differing numbers of point sources, like 2 and 3 subs (the max in this software is 4)

Also, someone needs to code a proper 3d/spherical computational fluid dynamics simulator, with adjustable parameters like a house-builder app has, with stacked subs etc etc... that would be super-cool. tongue.gif
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post #2 of 23 Old 08-13-2012, 10:18 AM
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Dual zoned bass array will be better...

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post #3 of 23 Old 08-15-2012, 11:56 AM
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This is just plain neat!

Very nice illustration of mode density. You see the effect of frequency - initial sims are in the ray-trace range, moving through the transition region to fully modal in the last sim. This last, however, is still just 4th harmonic (count the simultaneous light/dark areas), and it's showing terrible room response to my eye.

A link to the simulator would be useful, as it would be far more enlightening to start with one source and see what it does. The value is in mapping simulator response into actual room response. Maybe my eye is off...

And what happens to the lower order harmonic modes that cause most of the real problem?

Have fun,
Frank
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post #4 of 23 Old 08-15-2012, 03:19 PM
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Originally Posted by fbov View Post

This is just plain neat!


Yes it is.

http://www.falstad.com/ripple/

 

Originally Posted by Bill Fitzmaurice:
It's also the very sort of Voodoo Engineering that should never be done.

 

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post #5 of 23 Old 08-15-2012, 08:42 PM
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If you want a generic 4 sub concept for a rectangular room then there are two prime candidates. One would be Welti/Toole with four midwall subs. The other would be a single bass array with four subs placed on the front wall, each inset one quarter of the respective boundary to boundary distance. That takes out all modes but longitudinal due to the creation of a plane wave. The fix the longitudinal, you either add a second time aligned phase inverted array, or seriously treat the rear wall with tuned absorbers. I introduced this idea here:

http://redspade-audio.blogspot.com.au/2012/04/double-bass-array-dba-bass-integration.html
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post #6 of 23 Old 08-20-2012, 05:51 AM
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My method was necessary because my mains subs were up front and too heavy to move. So i ended up with one sub on either side of my LP, its in my signature...
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post #7 of 23 Old 08-20-2012, 08:20 PM
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The Welti approach mentioned earlier is different from most other methods since it makes no attempt (zero, zip, nada) to improve the frequency response of the bass, just minimize spatial variance. The recommendations of the Welti paper (four subs: one in each corner or one at the midpoint of each wall) are intended to give you the same bass across all your seats. Doesn't matter if the frequency response is crappy, as long as it is equally crappy across all seats. (In practice, Welti's subwoofer placement recommendations will make the frequency response smoother, even though that is not his intention.)

One of the biggest issues with equalizing bass response is that fixing a problem in one seat can result in worse response elsewhere. Welti figured out a way to minimize that by finding subwoofer locations that yield the most consistent response across multiple seatings. Once this is done, you can EQ knowing that fixing problems at one location will likely fix them everywhere.

Which brings up some caveats: Welti only tested 2D (all of the subwoofer locations were on the floor), only tested rectangular rooms (which the OP doesn't have), and equalization is mandatory to shape/smoothen the frequency response. IF you're setting up a room with a single listening position, then you're better off using the Geddes approach rather than Welti's method. IF you're concerned with all the seats on your couch, then it's a good idea to look into Welti's approach or the double bass array mentioned by Paul (especially useful if you're not sure of the final seating locations).

Sanjay
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post #8 of 23 Old 08-21-2012, 11:58 AM
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Quote:
Originally Posted by sdurani View Post

The Welti approach mentioned earlier is different from most other methods since it makes no attempt (zero, zip, nada) to improve the frequency response of the bass, just minimize spatial variance. The recommendations of the Welti paper (four subs: one in each corner or one at the midpoint of each wall) are intended to give you the same bass across all your seats. Doesn't matter if the frequency response is crappy, as long as it is equally crappy across all seats. (In practice, Welti's subwoofer placement recommendations will make the frequency response smoother, even though that is not his intention.)
One of the biggest issues with equalizing bass response is that fixing a problem in one seat can result in worse response elsewhere. Welti figured out a way to minimize that by finding subwoofer locations that yield the most consistent response across multiple seatings. Once this is done, you can EQ knowing that fixing problems at one location will likely fix them everywhere.
Which brings up some caveats: Welti only tested 2D (all of the subwoofer locations were on the floor), only tested rectangular rooms (which the OP doesn't have), and equalization is mandatory to shape/smoothen the frequency response. IF you're setting up a room with a single listening position, then you're better off using the Geddes approach rather than Welti's method. IF you're concerned with all the seats on your couch, then it's a good idea to look into Welti's approach or the double bass array mentioned by Paul (especially useful if you're not sure of the final seating locations).

Good post.

JSS
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post #9 of 23 Old 08-22-2012, 04:49 AM
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Quote:
Originally Posted by sdurani View Post

The Welti approach mentioned earlier is different from most other methods since it makes no attempt (zero, zip, nada) to improve the frequency response of the bass, just minimize spatial variance. The recommendations of the Welti paper (four subs: one in each corner or one at the midpoint of each wall) are intended to give you the same bass across all your seats. Doesn't matter if the frequency response is crappy, as long as it is equally crappy across all seats. (In practice, Welti's subwoofer placement recommendations will make the frequency response smoother, even though that is not his intention.)
One of the biggest issues with equalizing bass response is that fixing a problem in one seat can result in worse response elsewhere. Welti figured out a way to minimize that by finding subwoofer locations that yield the most consistent response across multiple seatings. Once this is done, you can EQ knowing that fixing problems at one location will likely fix them everywhere.
Which brings up some caveats: Welti only tested 2D (all of the subwoofer locations were on the floor), only tested rectangular rooms (which the OP doesn't have), and equalization is mandatory to shape/smoothen the frequency response. IF you're setting up a room with a single listening position, then you're better off using the Geddes approach rather than Welti's method. IF you're concerned with all the seats on your couch, then it's a good idea to look into Welti's approach or the double bass array mentioned by Paul (especially useful if you're not sure of the final seating locations).

Not sure i follow your comment. Wouldnt flattening the response, also improve it. Adding more subs actually flattened it at the LP in my room.
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post #10 of 23 Old 08-22-2012, 11:26 AM
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Originally Posted by kgveteran View Post

Wouldnt flattening the response, also improve it.
Sure, but that's not the "improvement" that Welti was going for. His approach is to first maximize consistency across all seats (using multi-sub placement), because he plans to flatten the response later (using EQ). His goal isn't to improve things only at the LP.
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Originally Posted by kgveteran View Post

Adding more subs actually flattened it at the LP in my room.
Depending on how you place them, adding more subs can do different things: stacking them in the same corner can maximize output (if you have low powered subs) whereas placing them at the midpoints of opposite walls can maximize consistency across the seating area (if you have multiple listeners). Or you can arrange them to flatten the response at the main listening position, which seems to be the most popular use of multiple subs. By comparison, Welti's approach is unique.

Sanjay
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post #11 of 23 Old 08-22-2012, 11:54 AM
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I think it was the zip,zero,Nada, that really isnt so..... A flatter response, is what it is, an improvement , even if the main theory is room modes....just sayin
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post #12 of 23 Old 08-22-2012, 11:55 AM
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"His goal isn't to improve things only at the LP."

to clarify, His goal isn't to improve things only at one LP.

....


"I think it was the zip,zero,Nada, that really isnt so..."

flat (or at least consistent) response across all seats is important. what was said was that if you can get the seat-to-seat variation to an absolute minimum, then you can eq out any peaks and dips.

but if you have large variation from seat-to-seat, then no amount of eq can fix the sound for everybody.

it just so happens that the same solutions that minimize seat-to-seat variation also provide for a flatter response. that shouldn't really be a surprise, but it is good to see in the data.

Listen. It's All Good.
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post #13 of 23 Old 08-22-2012, 12:54 PM
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Originally Posted by kgveteran View Post

I think it was the zip,zero,Nada, that really isnt so.....
No, that part is factually correct. Read his paper (linked earlier) and see for yourself whether he ever mentions improving the frequency response. As I said earlier: "In practice, Welti's subwoofer placement recommendations will make the frequency response smoother, even though that is not his intention."
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Originally Posted by kgveteran View Post

A flatter response, is what it is, an improvement , even if the main theory is room modes....
By that logic you can claim that Welti was trying for louder bass, since using 4 subs instead of one can yield greater output. But that's not his intent, as made clear by his paper. Is there a reason you're not willing to separate the intent of Welti's approach with possible byproducts (smoother, louder response) of his approach?

Sanjay
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post #14 of 23 Old 08-22-2012, 12:58 PM
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Originally Posted by LTD02 View Post

"His goal isn't to improve things only at the LP."

to clarify, His goal isn't to improve things only at one LP.
LP = listener position. "Position" is singular, as in one. I don't understand the semantic correction you're trying to make since I didn't imply multiple listening positions.

Sanjay
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post #15 of 23 Old 08-22-2012, 03:07 PM
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"LP = listener position. "Position" is singular, as in one. I don't understand the semantic correction you're trying to make since I didn't imply multiple listening positions."

some folks might be thinking that if there is more than one seat, there is more than one listening position. i was just trying to clarify that you were talking about a single listening position. that's all.

Listen. It's All Good.
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post #16 of 23 Old 08-24-2012, 06:34 AM
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Then i guess its a free lunch, eliminate peaks and nulls and improve the response. Any way you look at it, the theory is rock solid with many benefits.

I was quite suprised at the amount of cabin gain, i never had to LT the system....
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post #17 of 23 Old 08-24-2012, 11:08 AM
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Originally Posted by kgveteran View Post

Then i guess its a free lunch...
It can be, depending on room and subwoofer placement. If your goal is to improve frequency response using subwoofer placement, then Welti's approach is the not a good starting point since his paper never mentions improving the frequency response, only improving seat-to-seat consistency. You're better off using the Geddes approach to smoothen/flatten the response, since that is his primary concern.

Sanjay
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post #18 of 23 Old 08-24-2012, 03:59 PM
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Quote:
Originally Posted by sdurani View Post

It can be, depending on room and subwoofer placement. If your goal is to improve frequency response using subwoofer placement, then Welti's approach is the not a good starting point since his paper never mentions improving the frequency response, only improving seat-to-seat consistency. You're better off using the Geddes approach to smoothen/flatten the response, since that is his primary concern.

Sanjay,

Your contributions are great, and I'm somewhat of an acoustics nube, but I respectfully take exception to mere semantic delineation.

Please, readers of this thread need to know that a multi-sub approach, that may resemble Welti's, is a solid and highly recommended technique to achieve a lessening of frequency response aberrations. Typically any approach that acoustically feeds the room in opposite polar segments, can address issues in that room axis. spread the drives across the front wall horizontally, then most likely the horizontal modes are minimized. Spread your sub front and rear, then minimizing of front to back irregularities is often achieved. Whichever axis one spans the midpoint; vertical, side to side, or front to back, that's likely where the benefit arises.

A step farther, the highly touted DBA, whereby each drive unit is spaced according to selective mode cancellation in that axis, yielding a planar launch to the same array on the back wall,...but out of polarity. Kind of a pitch and catch scenario. Or more akin to a no rear wall scenario. But, no PVG benefit, unless one utilizes some measure of HP filtration below modal region, yet within the area of concern.

Thanks

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post #19 of 23 Old 08-24-2012, 04:54 PM
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But, no PVG benefit, unless one utilizes some measure of HP filtration below modal region, yet within the area of concern.

No PVG "benefit" until you implement a group delay filter (easy to do).

For accurate reproduction, I would argue that you don't want PVG. For movies and people that just like special effects, I would probably leave the PVG in.

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post #20 of 23 Old 08-24-2012, 05:39 PM
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No PVG "benefit" until you implement a group delay filter (easy to do).
For accurate reproduction, I would argue that you don't want PVG. For movies and people that just like special effects, I would probably leave the PVG in.

MBentz,

How would I implement a group delay filter easily?

JSS
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post #21 of 23 Old 08-24-2012, 09:03 PM
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Originally Posted by FOH View Post

I respectfully take exception to mere semantic delineation.
It's not mere semantics. IF your primary goal is to use subwoofer placement to improve frequency response, do you pick an apporach that never ever mentions improving frequency response (Welti) or do you pick an approach based on improving frequency response (Geddes)? The fact that a multi-sub approach (whether it resembles Welti's recommended layouts or not) improves frequency response has more to do with using multiple subs in the first place than the particular layouts in Welti's paper. Geddes doesn't recommend placing subs in the four corners of the room nor the midpoints of opposite walls, but his method does an excellent job of smoothening/flattening out the frequency response. So much so, that improvements are small after the third sub.

If people want to use Welti's approach to improve the frequency response and use Geddes' method to get better seat-to-seat consistency, then that's up to them. If they want to set up a double-bass array primarily to get louder bass, then they can. Heck, I once drove a nail into a wall with a pipe wrench because that's what was in my hand at the moment. But that tool wouldn't be my first recommendation to drive nails. Likewise, if my goal was to get the smoothest/flattest bass response I could get at the LP using subwoofer positioning, then Welti's approach wouldn't be my first choice. There are better methods to achieve that, because they are targeted specifically to that goal.

Do you really believe that the Welti paper never mentioning any improvement in frequency response is mere semanitics?

Sanjay
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post #22 of 23 Old 08-25-2012, 04:18 PM
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Too bad Toole doesn't mention the Geddes method in his book. He does mention that although spatial variance can be minimized with a Welti approach, that the subsequent response may require more headroom than is available to EQ flat. Some get lucky and acheive both with one or the other methods with little EQ. Due to the lack of symmetry and the natural human tendency to avoid it, I would bet the Geddes method doesn't get used as much in homes. Has anyone written a whitepaper on the Geddes method complete with before/after measurements? I have read what other's have written about the method, but not the man himself...

JSS
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post #23 of 23 Old 08-26-2012, 01:46 AM
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Quote:
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Has anyone written a whitepaper on the Geddes method complete with before/after measurements? I have read what other's have written about the method, but not the man himself...
Part of the problem with having a comprehensive whitepaper is that the Geddes method keeps evolving. For example, I think he might have softened his position on not using any equalization. Another example is the change in measuring technique:

In 2008 he posted at DIY Forums that you "Take your mic and move it through large spatial positions near but arround the prefered listening position. Be carefull as small bumps of the cable can generate large eroneous signals into the mic. The sweeping has to be smooth. When the analyzer has completed its run you will have a plot of the frequency and spatial averaged LF sound field."

Three years later he posted at his own forum "I am now not so adamant that spatial averaging is necessary at these LFs. The differences over a small area are just not that great and it now seems that a single median point is a good representation of the nearby area - at least up to 150 Hz or so."

If you want to read stuff written by the man himself, there is plenty out there. He's posted at various forums (including having had a long running thread here at AVS). To get you started with a couple of his papers:

http://seriousaudioblog.blogspot.com/2012/05/two-great-articles-on-multiple.html#!/2012/05/two-great-articles-on-multiple.html

Sanjay
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