Four Subwoofers - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 31 Old 07-01-06, 09:56 PM
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Re: Four Subwoofers

Quote:
Ayreonaut wrote:
Yes it will, if the room is square. If the length and width differ significantly, than the two quarter distances will be sufficiently different to avoid this.
Hm. Maybe I misunderstood the article. Or maybe I just translated it into here incorrectly. I thought it meant if you place a sub in the corner positioned 3' away from both walls, that is bad.
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post #22 of 31 Old 07-02-06, 01:11 AM
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Re: Four Subwoofers

I think one thing to keep in mind is that there are different and often competing metrics used for determining the "optimal" location of the subwoofer(s) and speakers in general. Maximum SPL and flat frequency response are two we are all familiar with, and in the link http://world.std.com/~griesngr/asa05.pdf Dr. Griesinger introduces a third measure; sound pressure gradient.

In large venues (theaters, concert halls, cathedrals) reverberant spatial cues arrive at the listener in a chaotic fashion creating a sense of spaciousness. The attack of the sound comes directly to the front of the listener, but the delayed (50-100ms) reverberant sound seems to come from all directions. This seems to have a high esthetic appeal to most listeners. In our relatively small listening rooms we are able to recreate this effect at higher frequencies with multi-channel sound systems.

Often the low frequency sound in small rooms is lacking in sound pressure gradient, creating the perception that the sound is not external, but coming from inside our head. However, the conventional wisdom is that we can't effectively determine the azimuth of low frequency sound in small rooms, but Griesinger contends that this is not true. With a recording with at least 2 channels of uncorrelated reverberant sound, and by properly placing multiple subwoofers on multiple channels, the listener can experience simultaneously both high SPL and high sound pressure gradient; effectively recreating a spacious sound for low frequencies. However, what is sacrificed in his setup is uniform frequency response.

What Griesinger is trying to do may not be desirable to all people and for all types of music, but it does demonstrate that there are many dimensions to this subject. In general it can be said, if we try too hard to optimize only one aspect of sound reproduction, we will usually screw up two or three others pretty bad.

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post #23 of 31 Old 07-05-06, 07:55 AM
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Re: Four Subwoofers

Quote:
Ayreonaut wrote:
The distances from the walls and corners will reduce the efficiency, but I looked into mutual coupling some more.
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.
Found another thread on mutual coupling:

http://www.audioheritage.org/vbullet...ead.php?t=5896

EV's formula has coupling occurring at 1/4 wavelength, which halves the frequency at which gain occurs.

Either way, I'm thinking (very generally) that subs so spaced might show relatively more gain/be flatter in a FR sweep compared to closer pairs as it's not occurring over the whole bandwidth, just low, as if adding an acoustic shelving filter the corner of which is determined by the spacing of the driver's centers..

I ran some sweeps yesterday comparing a pair of 12" sealed Adire DPL12's and was surprised to find that not only did Mr. Welti's wall-midpoint placement (side walls in my case) show the most uniform FR across my couch (still lumpy, but flat wasn't his objective) but seemed to show more room lift and a lower F3 than the pair together in a corner. He also mentions better "LF factor" for a pair at the midpoints, not sure if it refers to this phenomenon.

Maybe it's the spacing/coupling? Dunno. Anyway, just something I observed from casual measurements that I found interesting. This position also puts the subs closer to the couch, almost near-field, which would be a benefit with the necessarily smaller subs required for such permanent placement.

Last edited by Jack Gilvey; 07-05-06 at 08:08 AM.
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post #24 of 31 Old 07-05-06, 08:13 AM Thread Starter
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Re: Four Subwoofers

Quote:
Josuah wrote:
Hm. Maybe I misunderstood the article. Or maybe I just translated it into here incorrectly. I thought it meant if you place a sub in the corner positioned 3' away from both walls, that is bad.
3' away from both walls would be "bad" resulting in cancellations.

Here's an example of a rectangular room (mine).
Length=18.6'
Width=12.8'
Height=8.0'

Multiply each of them by 1/4 to get the suggested sub location. Then the sub ends up

4.5' off the end wall,
3.2' off the side wall and
2.0' off the floor.

I get three different dimensions which should not create significant cancellations from the corner reflections.

(Mine is on the floor with the center of the woofer at just 0.7', but there's not a problem with the first vertical resonance.)

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post #25 of 31 Old 07-05-06, 08:45 AM Thread Starter
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Re: Four Subwoofers

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reed.hannebaum wrote:
...Dr. Griesinger introduces a third measure; sound pressure gradient... What Griesinger is trying to do may not be desirable to all people and for all types of music, but it does demonstrate that there are many dimensions to this subject.
Quote:
Dr. Griesinger wrote:
Successful results depend on: 1. having an input recording that includes at least
two channels where the reverberation is independently recorded, and thus uncorrelated
with the other channels...
This does illustrate yet another goal in what we could aim for. But there are few recordings that meet the requirements set forth by Dr. Griesinger. A lot of stereo music has mono bass and moviel LFE is mono. Dr. Griesinger uses the difference signal between two channels to drive one of the subwoofers in his proposed setup.
Quote:
reed.hannebaum wrote:
In general it can be said, if we try too hard to optimize only one aspect of sound reproduction, we will usually screw up two or three others pretty bad.
You're right, there are many parameters that define performance, and we each have to figure out our own priorities. I do think that anyone with one sub should try it in a corner, and in the middle of the front wall, and at a quarter point of the room and decide for themself. Likewise, anyone with two or four subs should try some of the arrangements suggested in the whitepapers referenced above and see what they think. A short list of logicaly calculated alternative locations is probably better than the old method of giving the sub your chair and crawling around on your hands and knees!

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post #26 of 31 Old 07-05-06, 10:03 AM Thread Starter
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Re: Four Subwoofers

Quote:
Jack Gilvey wrote:
...not only did Mr. Welti's wall-midpoint placement (side walls in my case) show the most uniform FR across my couch (still lumpy, but flat wasn't his objective) but seemed to show more room lift and a lower F3 than the pair together in a corner. He also mentions better "LF factor" for a pair at the midpoints, not sure if it refers to this phenomenon.
Nice work! I wish I had a four identical subs on hand to play with. (My wife is getting sick of me messing around with it, though she loves the results.)
Quote:
F Toole wrote:
The LF factor metric is simply the sum of the energy over the bandwidth of interest (20-80 Hz here) produced by a given configuration, assuming a correction factor to normalize for the number of subwoofers: 20log10n.
I think that LF factor is basically total output per sub and is related to the room lift that you mentioned.

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post #27 of 31 Old 04-28-10, 05:39 PM
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Re: Four Subwoofers

So what ever became of this? Did anyone ever try 4 IB's located a 1/4 from each wall. I'm gearing up to purchase four 18" Fi IB3s and I'm trying to decide if I should place them:

1) all four in one baffle centered between front speakers (about five feet off front wall)
2) two per baffle along the front wall, 1/4 off the front and side wall
3) two per baffle midway along each side wall 1/4 off the side walls
3) four baffles at 1/4 locations at each corner

My room is 27'L X 15'W X 10'H.

The room doesn't currently have a ceiling, it's open to the joists, and it seems any measurements would be skewed by the lack of a ceiling.
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post #28 of 31 Old 04-28-10, 09:51 PM
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Re: Four Subwoofers

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superfly19 wrote: View Post
So what ever became of this? Did anyone ever try 4 IB's located a 1/4 from each wall. I'm gearing up to purchase four 18" Fi IB3s and I'm trying to decide if I should place them:

1) all four in one baffle centered between front speakers (about five feet off front wall)
2) two per baffle along the front wall, 1/4 off the front and side wall
3) two per baffle midway along each side wall 1/4 off the side walls
3) four baffles at 1/4 locations at each corner

My room is 27'L X 15'W X 10'H.

The room doesn't currently have a ceiling, it's open to the joists, and it seems any measurements would be skewed by the lack of a ceiling.
I havent read anything about any one doing that method but have read on other forums about the benefits of four subs in different locations around the room on other forums.
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post #29 of 31 Old 04-29-10, 04:23 AM
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Re: Four Subwoofers

I find this article quite good:
Which hopefully will be seen in my next 5:th post
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post #30 of 31 Old 04-29-10, 04:24 AM
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Re: Four Subwoofers

i have my subs equidistantly placed along the front wall...
maybe a link in the next post....
www.sonicdesign.se/subplace.html
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