Four Subwoofers

**Ayreonaut** · **06-26-06, 03:06 PM**

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, there’s 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 didn’t consider.
Floyd Toole: It’s 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 that’s 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?

**Sonnie** · **06-26-06, 11:08 PM**

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...

**JCD** · **06-27-06, 02:14 AM**

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?

JCD

**Chrisbee** · **06-27-06, 04:30 AM**

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.

**Ayreonaut** · **06-27-06, 10:18 AM**

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.

**Ayreonaut** · **06-27-06, 10:22 AM**

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 Öhman 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.

**Ayreonaut** · **06-27-06, 01:39 PM**

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 ½ a wavelength of one another (85Hz upper crossover frequency ½ 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.

**Wayne A. Pflughaupt** · **06-27-06, 02:36 PM**

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

**Ayreonaut** · **06-30-06, 09:47 AM**

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, you’ll get different results at every listening position.

5. Experiments in sub placement are relevant only in the room they’re 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

**Sonnie** · **06-30-06, 12:15 PM**

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.

**Ayreonaut** · **06-30-06, 01:14 PM**

Sonnie,

I've been doing a lot of research on sub positioning lately, as you know. I discovered this article: link

Its a bit complicated, but after carefully going over it I was able to do the math. The gist of it is that for one sub in a rectangular room the best sub placement to avoid room resonances is in a quarter point. 1/4 of the room width off the side wall and 1/4 of the room length off of the end wall.

That may not be practical is some living rooms, but sonically its the best spot. Fourtuantely I can put mine very near one of the quarter points. I moved my sub there last night, and measured. I lost a few dB comapred to corner placement, but I got the most even FR (pre EQed) of any position I've tried. It really works.

Why does this matter? Can't I just EQ for any position? Well, I recently tried corner loading and then EQing out the ensuing resonances. Even though I gained a few dB abnd the EQed output measured flat, the sub still sounded boomy. I don't really know why. My guess is that the decay times come in to play since the room continues to ring after the sub stops. If you position in a quarter point, those modes get much less excited.

Theoretically, the only thing better for dealing with room modes is to use multiple subs in every quarter point. Instead of mitigating the LF axial room modes, it elimintes them. (I should add that two subs in the two front quarter points will eliminate the width modes and mitigate the length modes.)

I suggest poisitioning your two subs on the two front quarter points and measuring at all of your listening positions. Obviously this isn't a practical set up to keep, but it should give you the same response as subs mounted in the same poisitions in the ceiling above.

Please let us know!

**brucek** · **06-30-06, 02:26 PM**

Quote:

The gist of it is that for one sub in a rectangular room the best sub placement to avoid room resonances is in a quarter point. 1/4 of the room width off the side wall and 1/4 of the room length off of the end wall.

Interesting stuff. Wish I knew more about it. I have certainly read opposing views to the theory and article you site though.

Floyd Toole seems to support the theory that if we have proper room dimensions where we can spread the axial modes (and lesser so the tangential and oblique modes) evenly, then the optimum position would be corner loaded so as to most efficiently acoustically couple to the room modes.

This provides us with added gain and headroom. Hopefully the resulting first order harmonics are evenly spread and not much equalization is required. That's the rub for sure - many are dealing with fixed room dimensions designed for ascetics and not subwoofer placement.

Your method essentially is in direct opposition to that theory in that you're trying to locate the dead zones so as to basically place the sub in a null and not generate any resonances. Fine, but unfortunately you're trying to power your way out of the null - you'll need a large amp. I suspect for optimum placement for a true null of the ceiling fundamental with your method, you'd have to mount the sub in the air about a quarter way between the floor and ceiling. Not possible with an IB or really very practical for any sub. Although the ceiling harmonic would be considerably higher than the other dimensions.

Who knows.....

brucek

**Sonnie** · **06-30-06, 03:02 PM**

I can see it now... four hanging subs... who will be the first to try it? heeheehe

I'm gonna be doing some experimenting this evening with moving one of my SVS subs around the room some. The 1/4 point definitely couldn't be the permanent spot, but it would be interesting to see the different response compared with each other.

Last night when I was measuring the different spots in the ceiling I noticed the response was not a lot different from where my subs are now. There was a tad better extension down to 15hz and the dip I have moved from 40hz to 35hz.

What I'm thinkin' is moving the left sub down the left wall in various locations while leaving the other sub in check. I'll test the subs jointly as I have previously and also adjust phase on the one I'm moving and retest. I can stand to move that one sub to a couple of different spots along that wall... so I might pick up that weak spot at 40hz... or move the weak spot with that sub to another frequency area.

Then eq'ing them might be more of a challenge too... being they will no longer be symmetrically located.

**Ayreonaut** · **06-30-06, 04:59 PM**

Quote:

brucek wrote:

Interesting stuff. Wish I knew more about it. I have certainly read opposing views to the theory and article you site though.

So have I. I've tried it both ways now.

Quote:

brucek wrote:

Floyd Toole seems to support the theory that if we have proper room dimensions where we can spread the axial modes (and lesser so the tangential and oblique modes) evenly, then the optimum position would be corner loaded so as to most efficiently acoustically couple to the room modes.

This provides us with added gain and headroom. Hopefully the resulting first order harmonics are evenly spread and not much equalization is required. That's the rub for sure - many are dealing with fixed room dimensions designed for ascetics and not subwoofer placement.

Floyd Toole has a list of rules for good sound in rooms, and what we are coming up against is a conflict between two of them. Maximize the output from the subwoofer(s), but achieve a uniform performance over the listening area. For maximizing the output, corner loading is a good idea.

Quote:

brucek wrote:

Your method essentially is in direct opposition to that theory in that you're trying to locate the dead zones so as to basically place the sub in a null and not generate any resonances.

Exactly. When we are trying to "achieve a uniform performance over the listening area" we can use what Toole calls Selective Mode Cancellation.

Quote:

Floyd Toole wrote:

So, why would we want to do this? Aha! Would it not be a good idea for everybody in each row of a home theater to hear the same bass sounds? Would it not be a good idea for a recording engineer to be able to move from one end of the console to the other without experiencing huge changes in bass? Well, this is how it can be accomplished. We are not saying, yet, that it is good sound, merely that it is the same sound. Once things are equalized in the sense of getting everybody hearing more or less the same sound, we then may need to equalize in the sense of changing the frequency response of the system.

I’ve tried it both ways. I equalized both responses and set the volumes the same. What I found was that in my room the position employing Selective Mode Cancellation does yield a more uniform response over several listening positions and a lower total level. But I also found that it sounds much cleaner. Tighter, or damped. That’s what I prefer. Many might prefer the higher output of corner loading and excited modes. It’s probably a misnomer, but some might call the difference HT sound vs. Musical sound.

Quote:

brucek wrote:

Fine, but unfortunately you're trying to power your way out of the null - you'll need a large amp.

You’re right. To achieve the same output level you might need more sub and amp. Do you think Sonnie’s eight woofer IB and two amps will do the trick?

Quote:

brucek wrote:

I suspect for optimum placement for a true null of the ceiling fundamental with your method, you'd have to mount the sub in the air about a quarter way between the floor and ceiling. Not possible with an IB or really very practical for any sub. Although the ceiling harmonic would be considerably higher than the other dimensions.

Yes, a sub on the floor or in the ceiling will excite all the vertical axial modes. Fortunately only the first resonance will be in the subwoofer range for a typical room height. In my room with an eight foot ceiling it falls at 70 Hz. But since all of the listeners in a room will be seated at about the same height, one band of EQ will correct it for all positions, if it proves to be a problem.

**Ayreonaut** · **06-30-06, 05:07 PM**

Quote:

Sonnie wrote:

I can see it now... four hanging subs... who will be the first to try it? heeheehe

I think that to cancel the mechanical vibrations, he'll have to build four boxes and use eight woofers. Wait a minute ... weren't you going to buy eight woofers?

Quote:

Sonnie wrote:

I'm gonna be doing some experimenting this evening with moving one of my SVS subs around the room some. The 1/4 point definitely couldn't be the permanent spot, but it would be interesting to see the different response compared with each other.

That spot worked for me with one sub, but be sure to try two in the quarter points some time.

Quote:

Sonnie wrote:

Last night when I was measuring the different spots in the ceiling I noticed the response was not a lot different from where my subs are now. There was a tad better extension down to 15hz and the dip I have moved from 40hz to 35hz.

A spot on the floor and the one above it on the ceiling should measure very much the same.

**Josuah** · **06-30-06, 05:14 PM**

Hm. The article posted on Audioholics just now about room acoustics states that placing a speaker equidistant from two boundaries creates a problem. So I assume placing a subwoofer at 1/4 in from a corner would create that same problem. ???

**Jack Gilvey** · **06-30-06, 06:09 PM**

Quote:

That may not be practical is some living rooms, but sonically its the best spot. Fourtuantely I can put mine very near one of the quarter points. I moved my sub there last night, and measured. I lost a few dB comapred to corner placement, but I got the most even FR (pre EQed) of any position I've tried. It really works.

Great stuff. Definitely going to give it a try. I can certainly use a front corner for this placement on a permanent basis for a single sub. Even elevating the sub to 1/4 height might be do-able, at least with one of my little sealed 12" workhorses.

Quote:

Why does this matter? Can't I just EQ for any position? Well, I recently tried corner loading and then EQing out the ensuing resonances. Even though I gained a few dB abnd the EQed output measured flat, the sub still sounded boomy. I don't really know why. My guess is that the decay times come in to play since the room continues to ring after the sub stops.

Yeah, I think you hit it. Eq-ing out some peaks isn't the same as not exciting the modal ringing in the first place.

What do you think this you linked to above...make sense?

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