An idea of reducing rear wall reflections. Looking for feedback. - AVS Forum
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post #1 of 24 Old 07-11-2014, 02:45 PM - Thread Starter
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An idea of reducing rear wall reflections. Looking for feedback.

I was just thinking about some common acoustic problems. One of these is that when we move our speakers well away from walls, we get a null off the rear wall around 80hz. So I thought of using DSP in a three way using front to rear opposing woofers. In this example I'll let's consider a 15" woofer with a bandwidth of say 30 to 150hz. If you're using subs, this can still work. Here is my illustration:



Immediately you can see some benefits. Dual opposed woofers means reduces cabinet vibrations. Well away from the rear wall, about 1.5m to be precise. This pushes nulls from the rear wall reflection way down in frequency. Two woofers means lots of output.

Ok, but the woofers are way out of time, they'll cancel each other at 150hz because the cab is 1.1m deep. So I propose to add delay of 3.2ms to the top cab and front woofer. Now the woofers are in time.

The real advantage I see hear though, is that the woofers will be out of time 1/2 wavelength at 75hz when BEHIND the speaker, as shown here:



This is because the front woofer is delayed 3.2m and offset in space 1.1m. So they totally cancel at 75hz if you are behind the speaker. I expect this to be fairly deep low q notch in the response. Additionally, as you move to the sides of the speaker, the speakers are totally out of phase at 90 degrees and 150hz, so there is less radiation to the sides as well.

I'm sure someone has done this before. Not much to discover these days. So can someone link me to an example if this actually works. Or explain to me the flaw I'm not seeing? Thanks.

Downsides, expensive and uses a lot of DSP power. This is becoming a non-issue these days.
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post #2 of 24 Old 07-11-2014, 03:26 PM
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Originally Posted by tuxedocivic View Post
One of these is that when we move our speakers well away from walls, we get a null off the rear wall around 80hz.
An 80Hz null would occur if the radiating plane is 3.5 feet from the wall. With most subs that wouldn't be a problem. If a sub was so large that the baffle was that far away from the wall the simple cure is to aim it at the wall.
Allison Effect is more problematic with mains than subs, as with an 80 Hz crossover you need to have the main baffle over 3.5 feet from the wall. The best way to get around that is with walled in speakers, if you have the option.

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post #3 of 24 Old 07-11-2014, 03:40 PM - Thread Starter
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Hi Bill, Thanks.

Yes, I agree that with subs you should just locate them better. But what about a 3 way mains like I'm describing, using pro 15" woofers from 30hz and up. A lot of people these days want full range mains. Part of why they're a compromise over using a subwoofer system is because of this rear wall reflection problem.

Even if using subs, this could still be a reasonable solution for say a 0.85m deep woofer section that plays from 80 to 200hz.

I agree, going in wall is best, and that's exactly what I personally do.

Any more thoughts on that? Thanks again.
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post #4 of 24 Old 07-11-2014, 04:32 PM
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post #5 of 24 Old 07-11-2014, 04:45 PM
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But what about a 3 way mains like I'm describing, using pro 15" woofers from 30hz and up. .
Rear fire the woofer. If you cross it at 200Hz or lower dispersion and integration isn't a problem. You can then have the midbass baffle far enough and the woofer baffle close enough to the wall at the same time. I do something similar with my David 3 way tower, which has the sub mouth openings to either side of the cab, rather than the front, placing the sub radiating plane closer to the rear wall than the midbass radiating plane.

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post #6 of 24 Old 07-11-2014, 05:28 PM - Thread Starter
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Interesting Bill. No integration problems at 200hz? How much distance do you have between your mid woofer and your woofer (horn loaded I assume based on your description).
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post #7 of 24 Old 07-11-2014, 05:28 PM - Thread Starter
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Scott, I said it first
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post #8 of 24 Old 07-11-2014, 07:28 PM
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Interesting Bill. No integration problems at 200hz? How much distance do you have between your mid woofer and your woofer (horn loaded I assume based on your description).
The woofer horn mouth is at the floor, the MTM section has the tweeter axis 39 inches from the floor.

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post #9 of 24 Old 07-11-2014, 09:44 PM - Thread Starter
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But is there a large distance horizontally, to keep the mid woofer away from the rear boundary?
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post #10 of 24 Old 07-12-2014, 05:32 AM
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Originally Posted by tuxedocivic View Post
But is there a large distance horizontally, to keep the mid woofer away from the rear boundary?

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post #11 of 24 Old 07-12-2014, 09:14 AM
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I like this thread. I love reading this stuff you guys post.

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post #12 of 24 Old 07-12-2014, 10:28 AM
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with two drivers (off set in the z axis) run out of phase with one delayed, the result (as you probably know) is going to be some flavor of cardiod response.


there are several places where this is done with varying degrees of success.


a search on 'cardiod bass' will bring up a large amount of information ranging from theory to measured applications.

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Last edited by LTD02; 07-12-2014 at 02:15 PM.
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post #13 of 24 Old 07-12-2014, 12:01 PM
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I've been beating my head against this problem for a while trying to get the speaker close enough to boundaries to avoid nulls. I've reluctantly concluded its far simpler just to put enough absorption on the walls to kill the nulls.

The problem with corner speakers is that the tighter you tuck the drivers into the corner, the less volume you have to work with behind them for generating bass. If you space them out from the corner so you can have some interior volume, then you need absorption on the wall to avoid nulls. If you put absorption on the walls then you kill some of the corner gain, but not down low where you really need it. I think there is a good solution there but I'm still circling around it.

Speaker Dave just posted on this topic on another forum.
http://www.diyaudio.com/forums/multi...peakers-6.html
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post #14 of 24 Old 07-12-2014, 01:34 PM - Thread Starter
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LTD02, you're right. It is cardioid I suppose. I thought they usually delay the rear woofer rather than the front. I'll do some reading.

Jack, yes I do think its probably simpler to just absorb it. Thanks for your thoughts on the problem.
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post #15 of 24 Old 07-12-2014, 02:26 PM
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"I thought they usually delay the rear woofer rather than the front."


that is so the wavefront from the front driver has time to reach the rear driver. then the rear driver fires out of phase and the result is cancellation in the direction of the rear driver.


if you delay the front, the rear driver will fire toward the rear with no cancellation and so it will not be cardiod.

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post #16 of 24 Old 07-12-2014, 08:06 PM
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Well, since my system was blown apart for upgrades I thought I'd try this..........and I deleted the data. Sorry.

I will say that it improved the 100hz null I had from the front wall, but it made the rest of the midbass response worse. I didn't play with positioning or anything so maybe it could have been improved. I'm having pretty good luck with my main speaker out from the wall about 32" to the center of the baffle, then a flanking 15" woofer on the floor to the inside of the main pushed against the wall. Both forward firing. Gives pretty good midbass smoothing and took care of my 100hz null. I get a pretty good sized null at 60hz with this configuration though so it doesn't really help with your goal in this thread..........but the distributed subs take care of that.

I goofed with cardioid while I was at it. IIRC, an active cardioid would have the rear woofer out of phase and lower (like 10dB) in level. Or maybe it's delayed and lower in level. Need to check out John K's site again to be sure. Decent mid bass response, better than a single 15" monopole anyway but I don't want to give up the efficiency or have the speaker that far out into the room. I do intend to try an active cardioid setup at some time, with 10" drivers on each side of the 15" woofer cab.
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post #17 of 24 Old 07-13-2014, 01:13 PM
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How about placing the woofers against the front wall and delaying the MT? Wouldn't be perfect everywhere, but should be good near-axis.

Or, build shallow quasi-in-walls with wide flared sides ending at the wall(s).

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post #18 of 24 Old 07-13-2014, 01:17 PM
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Would multiple woofers with different depths work ?

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post #19 of 24 Old 07-13-2014, 02:47 PM
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It was years being interested in the science of speakers before I came across the concept of floor bounce (and subsequently decide to not worry about it, head-in-the-sand ostrich style), and now you guys had to bring up this stuff. I hate this place sometimes.
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post #20 of 24 Old 07-13-2014, 03:00 PM - Thread Starter
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Ltd02: Right. That's a little different but the same challenge. They're trying to limit the full bandwidth of LF, but that reduces overall potential output. I think this might be high output potential and focus on a narrower frequency range. I think I can learn a lot about this concept on the coat tails of cardioid users though. Thanks.

Nate, so sounds like it worked but introduced some other issues maybe. I honestly thought it would reduce issues out to the sides of the speaker as well. Hmm... Thanks for the feedback. If you do more testing please come back

pnw, I thought of that after the fact. And that makes perfect sense. Also thought after the fact that the dual opposed benefit actually doesn't exist once you delay one woofer. So what you're saying might be the simplest way to go. But something I want out of a 3 way is to mitigate floor bounce (as antisuck mentioned below and I know you're aware of as well). I thought there might be some additional benefits like reduced sideways (horizontal) output, dual opposed, increased output. But maybe not. Or at least not to the amount I had thought.

Mfusik, not sure what you mean.

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post #21 of 24 Old 07-13-2014, 03:22 PM
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Quote:
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But something I want out of a 3 way is to mitigate floor bounce (as antisuck mentioned below and I know you're aware of as well).
That's the nice thing about a 3-way. You can put the woofer near the floor and if you cross it low enough the floor bounce is pushed out of band. Then the MT is up higher where it's easier to use directional speakers. Of course this does nothing for the front wall reflection...........
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post #22 of 24 Old 07-13-2014, 08:53 PM
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The more I think about this stuff, the more I think that if you can't do something that approaches soffit mounting you are almost forced into a bass-managed system, and the closer your mains are to the front wall the higher you need to cross over to a modular woofer system of some kind. And those woofers darn well better be close to the front wall and close to the floor, with the tops delayed to match. Flanking subs, anybody? Dunno if Parham has ever talked about using delay, makes sense to me though.

Over on gearsluts where Ethan Winer and some of the other room gurus post, it's pointed out though that if you push the mains back close enough to the wall then you can absorb the problematic reflections that are now much higher in frequency with wall treatments, something that is impractical at 80 hz or below. So if you are going to do treatments anyway, maybe that's a valid direction to go.

JackNC, the solution you are circling involves tilting your entire house backward approximately 40 degrees until the front wall/side wall/floor junctions are at ear level, then mount the drivers outside to make giant 3-sided Synergy horns. Then it's a simple matter to construct a platform for your couch and some type of catwalk system to move around.
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post #23 of 24 Old 07-14-2014, 04:37 AM
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Quote:
Originally Posted by antisuck View Post
The more I think about this stuff, the more I think that if you can't do something that approaches soffit mounting you are almost forced into a bass-managed system, and the closer your mains are to the front wall the higher you need to cross over to a modular woofer system of some kind. And those woofers darn well better be close to the front wall and close to the floor, with the tops delayed to match. Flanking subs, anybody? Dunno if Parham has ever talked about using delay, makes sense to me though.

Over on gearsluts where Ethan Winer and some of the other room gurus post, it's pointed out though that if you push the mains back close enough to the wall then you can absorb the problematic reflections that are now much higher in frequency with wall treatments, something that is impractical at 80 hz or below. So if you are going to do treatments anyway, maybe that's a valid direction to go.

JackNC, the solution you are circling involves tilting your entire house backward approximately 40 degrees until the front wall/side wall/floor junctions are at ear level, then mount the drivers outside to make giant 3-sided Synergy horns. Then it's a simple matter to construct a platform for your couch and some type of catwalk system to move around.
When you say sofit mounting, are you referring to having the LCR's and subs mounted flush into a baffle wall? I know and understand the benefits of flush mounting the mains, but, are there benefits of flush mounting subs?

Would it be possible for vented subwoofers that have their ports in the front to be flush mounted in a baffle wall?
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post #24 of 24 Old 07-14-2014, 05:00 AM
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Quote:
Originally Posted by antisuck View Post
The more I think about this stuff, the more I think that if you can't do something that approaches soffit mounting you are almost forced into a bass-managed system, and the closer your mains are to the front wall the higher you need to cross over to a modular woofer system of some kind. And those woofers darn well better be close to the front wall and close to the floor, with the tops delayed to match. Flanking subs, anybody? Dunno if Parham has ever talked about using delay, makes sense to me though.
corner mounting at ear level is the same as soffit mounting as far as boundary nulls go. For flanking corner woofers, top and bottom firing dual opposed woofers will play highest before showing a boundary null.

It didn't take much playing with REW's room sim to convince me I needed an absorber wall somewhere and the front wall works best for my room. (There went 1/3 of the boundary support for corner loading.) Speaking of Parham, note that his corner horns don't integrate with a front absorber wall but IMO use of a midhorn is compelling.

Once you have that absorber wall, you can sink your mains into it and remove some of your boundary nulls. You may find you can't get the mains far enough away from the corners so that they don't show side wall reflection nulls. If that is the case, then bring that absorber around the corner and up the wall or push the speakers into the corner.

I like the free thinking evident in your room/house design idea and I love the Synergy concept. If I had as much design freedom for the room as I do for the speakers in it, I'd have started construction already but, while my wife tolerates (reasonable sized) speakers and loves music, she hates construction projects. So I'm waiting for the right moment to bring up the subject...

Speaking of Synergy, a full range point source wouldn't be optimal for dealing with floor and ceiling bounce or room issues in general. I'm working towards a 3-way with a woofer and midhorn overlapped in the 100-200 hz region. (Not far above 200, the woofer starts showing a boundary null, which moves lower the larger I make the cabinet.) That could be done in a Synergy but a Synergy would be too much of a science project for me and perhaps not as good as SEOS above 1 Khz. My midhorn will be (at least that is the way it looks today) conical with (2) offset drivers, like a Synergy and with most of its benefits, but the CD will go on a SEOS18. And this, I think, is the perfect usage model for the SEOS18 - crossed at 1Khz before it loses pattern control to a 24" wide midhorn to provide horizontal pattern control down below 400 Hz.

So yes I'm paralyzed by analysis but so far I've avoided a number of false tries
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