Four Subwoofers - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 31 Old 06-26-06, 03:06 PM Thread Starter
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Four Subwoofers

Have you read this interview with Floyd Toole and Todd Welti from Harman?

(I've seen this repeated elsewhere that four subwoofers provide the best solution.)

Quote:
Todd Welti: ...four subwoofers seems to do the trick -- and we came up with three configurations out of about 100 that we looked at that are the cream of the crop. Actually, thereís a fourth solution -- one that Floyd came up with -- that worked even better than the others, but it involves bringing the subwoofers out from the walls, which I generally didnít consider.
Floyd Toole: Itís not very practical, unless you could put them in the ceiling. It uses four subwoofers located at the 25% points from all four walls.
Todd Welti: In other words, you shrink the whole room by 25% and put the subwoofers at the corners of that virtual room. You get incredible performance, but thatís just not practical in most rooms...
link to the white paper

link to interview

Well, I'm planning to implement a four driver IB sub in the fall. And I'm thinking that this is a perfectly practical solution. Instead of co-locating all of the drivers in one enclosure, I can mount each one in the ceiling at the quarter points as described. What do you think?

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post #2 of 31 Old 06-26-06, 11:08 PM
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Re: Four Subwoofers

Sounds interesting indeed. You are teasing us though... you shouldn't wait til fall... do it now so we can get a users opinion on how well it works. I agree about the only way it could work is via the ceiling. I can see four SVS PB12's sitting at 25% of my four corners...
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post #3 of 31 Old 06-27-06, 02:14 AM
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Re: Four Subwoofers

I've had this thought before.. but wasn't sure if the coupling between the various drivers would be as effective. I think I've determined that it's ok in the end.

I posted a similar q on the Cult and was told to "save myself the headache and mount them all centered on the front wall" -- for reference, I was thinking I might mount them in the corners in the front wall and create a "wall of sound".

Anyway, that's what I got -- not much though huh?

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post #4 of 31 Old 06-27-06, 04:30 AM
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Re: Four Subwoofers

If you put them all together in an IB you get a coupling effect. Just like you do if you co-locate box subs.

Four box subs pulled out from the walls is not the same as an IB with widely seperated drivers.

I would imagine they would all be competing out of phase at certain frequencies depending on their spacing.
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post #5 of 31 Old 06-27-06, 10:18 AM Thread Starter
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Re: Four Subwoofers

Quote:
Sonnie wrote:
...you shouldn't wait til fall... do it now so we can get a users opinion on how well it works...
Sure Sonny, you guys can chip in the cash so I can do it now.

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post #6 of 31 Old 06-27-06, 10:22 AM Thread Starter
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Re: Four Subwoofers

Quote:
JCD wrote:
...I posted a similar q on the Cult and was told to "save myself the headache and mount them all centered on the front wall" -- for reference, I was thinking I might mount them in the corners in the front wall and create a "wall of sound"...
This guy has a similar idea here.

Quote:
Ingvar ÷hman wrote:
One example of an excellent placement of four
subwoofers is an even distribution horizontally
behind the main speakers.

When the four real subwoofers come to an end,
the reflections from the side walls will continue,
creating a line source of infinite length.

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post #7 of 31 Old 06-27-06, 01:39 PM Thread Starter
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Re: Four Subwoofers

The distances from the walls and corners will reduce the efficiency, but I looked into mutual coupling some more.
Quote:
Genelec Website wrote:
When two subwoofers are positioned close to one another mutual coupling is the fortunate by-product. This is due to the long wavelengths, associated with low frequencies, causing constructive superimposition. For mutual coupling the subwoofers must be place within Ĺ a wavelength of one another (85Hz upper crossover frequency Ĺ wavelength is approx. 2m). For example, two subwoofers give a 6dB increase in acoustical output at the listening position - see table below.
My room is 12'-10" x 18'-7". So located at the quarter points the four drivers would be 6'-5" and 9'-4" apart. By calcualting the half wavelengths, I found that all four drivers will be mutually coupled together below 60 Hz. This would seem to indicate no loss of power at the really low, power hungry frequencies.

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post #8 of 31 Old 06-27-06, 02:36 PM
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Re: Four Subwoofers

But you’ll still have the phase issues Chris spoke of, unless all of them are equidistant to the seating position. This will show up as ragged, saw-tooth response, and since it is a time-alignment issue, you’ll get different results at every listening position. You might note that Mr. Welti stated in the quote field from your initial post that four subs “aren’t practical in most rooms.”

It also should be noted that experience in sub placement like the ones evaluated in that white paper are relevant only in the room they’re performed in. Unless your room is identical, or at least pretty close, your results will be different.

Regards,
Wayne
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post #9 of 31 Old 06-30-06, 09:47 AM Thread Starter
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Re: Four Subwoofers

I wrote an e-mail to Todd Welti to see what he thought of our ideas.

Quote:
Daniel in an email to Todd Welti wrote:
Dear Mr. Welti,

I would like to seek your input on this. I'm an audio enthusiast, and I recently discovered your paper "Subwoofers: Optimum Number and Locations." and Floyd Toole's "Loudspeakers and Rooms for Multichannel Audio Reproduction." I also read the interview that you did with Wes Philips in 2004 on the subject.

I am very interested in implementing the solution of using four subwoofers located at the 25% points from all four walls. My room is rectangular with two open doorways. I plan to install four subs in my ceiling in an "infinite baffle" configuration. When I flew this idea past my friends at the Home Theater Shack forum they posed several reservations. Can you tell us if these objections are valid?

1. The four subwoofers will not be mutually coupled (and output may fall short.)

2. Four box subs pulled out from the walls would perform differently than four Infinite Baffle subwoofers at the same locations.

3. The subwoofers would be competing out of phase at certain frequencies depending on their spacing.

4. Phase issues will exist unless all of the subs are equidistant to the seating position. This will show up as ragged, saw-tooth response, and since it is a time-alignment issue, youíll get different results at every listening position.

5. Experiments in sub placement are relevant only in the room theyíre performed in.

Would you please share your opinion on these? You can respond to my e-mail or feel free to answer in the forum where we are holding this discussion at Home Theater Shack. Thanks for your time and efforts.

Sincerely,

Daniel
Mr. Welti was very kind to take time and respond to all of these. (Even though he's on vacation!)

Quote:
Todd Welti in an e-mail to Daniel wrote:
Hi Daniel,

first off, the most recent and complete reference regarding the subwoofer work we did can be found in the May issue of the AES Journal. There is some new stuff in there that might interest you.

To address your questions:

1. The total acoustical output into the entire room space may be less, but that does not necessarily mean the output at the individual seats is less. Of course, you are using four subs. That will be in your favor.

2. I wouldn't expect the response of the subs to change too much when baffle mounting them. I assume you are crossing over at 80 Hz. The acoutical center of a subwoofer mounted right next to a wall is probably 12 to 18 inches from its reflected image, i.e. only a fraction of a wavelength at 80 Hz. So baffle mounting shouldnt make too much difference. Even if it did, it would be a difference in the overall (global) response, and would certainly not affect the seat to seat consistancy we are after. Afterthought: if baffle mounting be sure to isolate mechanically from wall.

3. This is true, but dont forget that even if they are IN phase, you can still have cancellations at a particular seat due to room modes, which will be spatially and frequency dependent (the worst kind of cancellation). This is what we are trying to avoid. There is no point in having more acoustical energy if it is only at certain frequencies.

4. I am assuming you have more than one seating position - optimized sub layouts such as the one you are interested in are only really relevent in this case. In such a case you cannot have equal distances form all subs to all seats. If your seating layout is symetrical, and your sub layout is symetrical you could at least have consistant sets of path lengths from the subs to the seats. In any case, unless the room is very dead, the room modes cannot be ignored. You cannot just think about the direct paths.

5. True only to a certain extent. It's not the dimensions so much (for example, wall midpoints are first order mode null points regardless of the dimensions). I did include an analysis of different dimensions in one of my AES papers, and found that the results held. (Sorry, I dont have the reference with me right now, I'm on vacation.) The wall midpoint solution is allways best. If the room has a large opening into another space, or wall construction that is very unsymetrical (for example brick on one wall and single layer sheetrock on the opposing wall), there may be divergences from the theoretical best sub solutions. In this case you may want to look into another technique such as is described in our recent AES paper, Sound Field Management.

You can copy this to your forum if you want.

Cheers, Todd

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post #10 of 31 Old 06-30-06, 12:15 PM
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Re: Four Subwoofers

Very interesting... now toss in this whitepaper on multiple sub locations.

ThomasW has actually suggested the possibility of multiple sub locations for my IB project. Possibly 2 subs in 4 locations or 4 subs in two locations.
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