Two Subwoofers... Comb Effect

is there such a thing like 'comb effect' as result of having two subwoofer (stereo) in the same room? And how do I know it?
Comb filtering is not an issue at low frequencies because the room response swamps out any lobing due to multiple sources. Because the ear is literally slow to register bass frequencies, it cannot distinguish between the first-arrival sound and the room response.

On the other hand, multiple subwoofers widely spaced is an effective way to smoothe out the in-room response in the modal region. Todd Welti of Harmon International wrote a paper on the subject:

Above link is a very recommended read.
Key appears to be asymmetrical placement.
I believe wavelengths to be too long for such effects.
50hz has over a 20ft long as many rooms.
thank you very much. That is very informative and reliable, so it seems.
Make sure you use symmetry along the room length I find that if you vary
placment too much along the room length then you can indeed run into
problems => bass becomes muddy instead of punchy. - it loses dynamics or
correct phase on transients. I find best placment is close to wall where you
place speakers and about 4 to 6 feet out (not in a corner) and right up against
side wall. This seems to reduce tangential room modes which have nulls
down the middle. Your only big problem then becomes room length modes
which require acoustic treatments at each end and notch filtering in the bass
on the modal usual YMMV.
HSU Research recommends 'near field' sub placement. When I was up there (OC, SoCal) I sat in there listening/audition area and sat virtually next to the sub(s) under test. Still difficult to localize and the 'blend' was good.
If I could work it out, I'd TRY my sub in back of the sofa, concealed (sort of) under the sofa back table. However, I got lucky and near corner placement (3 feet out) and against a long wall made it happen.
I have the advantage, by sheer accident, of having a room with 8 or so walls, 3 45degree corners and an off-center asymmetric ceiling of 12' max and 8' at one side. This room seems to break up or not be conducive to the formation of 'bad' standing waves....(so called peaks or suckouts).

Treatments ARE in the offing, but the idea would be to kill first reflection points from my Maggies and deaden the back of the room....the back wall is a reflection monster, too.

MAAB, please be careful, and Shad, this is meant with all due respect, of any and MOST prescriptive advice. Your listening area is a unique problem and Opportunity.
Experiment around. Even if you can't leave something there due to WAF, try it. Send everyone OUT for the day. Invest in some movie tickets for 'em all. Get RID of them so you can move stuff around and experiment. That's how I discovered, many years ago, that the one hard/fast rule of Magnepans.....IE, tweeters OUT doesn't apply in ALL instances. You will learn stuff nobody could tell you about your room / system / ears / interactions.
It'll all be good.
Duke, your room sounded great at RMAF; I especially loved the clean off-axis presentation! Kudos to you for thinking outside the (subwoofer) box!
I should clarify that the punchy feeling of coherent bass is a kind of compression - it is not something so much as you hear in the sense of a note but more like something you feel. I find if I place the subwoofer too far away from the mains (for example the middle of the room or behind the listener then the notes are still audible but the "attack" is less...I suspect the transient response just becomes wrong...the hit from the beater on the drum head arrives later than the bass by however many feet you are closer to the sub than the speaker (6 feet = 6 millisecs and I think we can begin to perceive this even with bass frequencies.)

Hope this clarifies - I certainly agree that for the sound of the note itself it does not seem to matter where it is placed - so getting an even response makes sense.

If you have a DSP then you can set the distance (sub to listener) and have the processor delay the output to the sub to maintain phase. However this may not work for several listeners seated at different distances to sub and speakers...
I think it's an advantage to place the subs as close to the mains as practical running in mono to minimize phase issues. There is going to be some overlap of frequency and you don't want the same frequency coming from sources several feet away from each other. Speaker makers go to alot of trouble to align drivers. Depending on the crossover slope it might also be beneficial to angle them within 15 degrees or so of the mains. Yes it's all room dependent and you could end up with something quite different but I would start with the sub drivers and the main drivers on the same plane as close together as possible with a similar angle. In a normal sized room treatments are likely to be needed - Jim
One key is to keep the x-over as low as practical. This will make sure you can't localize the bass. My Maggies, 1.6s are x'd-over at about 40-45hz. Seamless and invisible.
Phase, however, is another matter and does make a difference.
I wish I had a fully variable 0->180 control rather than the 0 OR 180 I now have. Small point, but don't forget that sound travels at about 14"/millisecond, so a small movement of the sub can produce phase changes, too.
A reason to place stereo subs as close to mains as possible is the inevitable "leakage" of sound above the crossover level. On paper, a driver contributes 50% to overall spl at crossover frequency and a sub continues to "roll off" above from there.
Listen to your system with only the subs powered up, don't even need an spl meter.
All that "supra-sub" content from the subs would seem to affect imaging, etc. the least by such positioning, though I've not done subjective testing. My stereo subs will stay where they are - immediately adjacent to each main speaker.
A Velodyne SMS-1, Rives PARC and other of their ilk will do a reasonable job smoothing peaks to make the nulls less toublesome at one's listening position.
Thanks Todd,
That Harmon International link is not only is the first quasi scientific documentation that appears to prove that multi subs are the only real way to go.
Sorry....I meant thanks Duke!
Halcro, Todd Welti et al published a couple of thoroughly scientific (nothing "quasi" about them) papers on the same subject in the Journal of the Audio Engineering Society a few years ago. What you see at the link above is the version he wrote for non-engineers.

Another very scientific proponent of multiple subwoofers is Earl Geddes, though he hasn't published any peer-reviewed papers on the subject. His approach differs from that described in the Welti papers somewhat; Earl advocates asymmetrical placement. Here is a brief study he did comparing four-corners placement with asymmetrical placement of four subs:

I use a simplified quasi-Geddes placement strategy; I do not elevate one of the subs as he recommends.

I was logged in on the account of a physically challenged friend (with his permission) when I posted above.

The 'study' room was 2x as long as high. and 'medium' damped, whatever that is. Was it meant to be 'average'?
All you can say is that IN THIS CASE...the results were as noted. I'd love to see this repeated in my room with 8 walls, 2 of which are at 45's, and an asymmetrical vaulted ceiling.
In the test room, the 1st plots 100hz (aprox) peak can be accounted for since the height of 3.5m is about 95hz at 1100ft/sec.
What I take away from this is that a listening area is not just floor space, but a 3d problem, something I knew before.
I think it's an advantage to place the subs as close to the mains as practical

That has been my experience too. I fear one can get caught up trying to even out the room modal response and lose "transient" response if you move the subs too far from the mains. In a sense, time coherent bass with the rest of the response may be as important as getting rid of the room modes - in practice I use a bit of give and take....I move the sub out into the room a bit in front of the mains but not so far as I feel that transient response suffers. This leaves me with perhaps less than optimal "room modes" but I am streets ahead of the situation with sub in the corner and still get reasonable transients.

FWIW: If you have freestanding speakers then you may not notice this effect - already free standing speakers are sending bass in an omnidirectional pattern - so you lose bass impact from quarter wave rear wall cancellation from the get go ( a set of nulls in the bass across the entire length of the room).
I ditto the experience with sub(s) placed up near the front of the room, with the mains. Either multiple subs in one location, perhaps just off center on the front wall - near the center - , or similar, or subs near the mains, perhaps in stereo, maybe mono, depending.
It's almost always easier to get a better blend, even from multiple seating positions, and still maintain excellent cohesion from the subs to the mains, from the listing positions. It also eliminates any potential for hearing "voices" from your subs, overlap problems, and just plain integrates easier in my opinion.
While I think, carefully done and considered, you can build an excellent system around subs placed around the room, I think the benefits of doing subs up near the mains offers better integration and benefits - especially when you consider today's DSP and EQ technology applied!
Also, I don't personally like to see subwoofer sitting around the room. The gear is an "eye-soar" as it is, mostly. I'd rather be able to have the option to hide the mains and subs behind some facade if I so chose, in a custom set up. And this is all easily done with all the speakers up front, and side/rears hidden, somewhat. But, that's me.
Still, I can't argue with the excellent results I've had over the years with either one, or all of the main/front subs (usually rears crossed over to these subs also) up front. Clients have been ecstatic to!
Still, in each application, you likely will need to "tinker", and adjust for maximum results, and keep a balance.
Bottom line though, is the results is all that matters
If you have a steep-slope lowpass filter (24 dB per octave), then you can get away with spreading subs around the room. Most subs have a 12 dB per octave lowpass filter, and so will give away their presence and not blend well because they allow too much lower midrange energy to pass through. Since the ear localizes by arrival time as well as intensity, it makes sense to me that if the subs are passing audible lower midrange energy you'd either want them close to the mains, or farther away from your listening position than the mains, or both.

Since the low-pass driver is phase-lagging relative to the high-pass driver, I don't see any theoretical disadvantage in the bass region to having some of the subs a bit closer to the listening position than the main speakers - provided the above recommendation regarding crossover slope is observed. That really is a crucial aspect of a scattered multisub setup, in my opinion.

4 th order crossovers just never sound quite right to me despite the obvious theoretic advantages of 24db LR. Less overlap and summing to zero at the crossover as well as being able to crossover higher and not localize them. As a result I'm crossing over lower than I might otherwise. As far as the subs being a BIT closer or further away than the mains Duke is right in most real world situations but audible/non localizable frequencies ( 20hz-100hz) if they are in the crossover range will be arriving from two different sources at two different times. This of course may not make a pennies worth of difference in any given situation with any given listener - Jim
Most subs have a 12 dB per octave lowpass filter, and so will give away their presence and not blend well because they allow too much lower midrange energy to pass through.

That is what I have - 12 db/octave. That may explain why I prefer it up front.
Ok, points are definitely noted from those here responding about phase on the subs.
So here's some questions that even a novice might ask then. Which is, If phase doesn't matter on the subwoofers, in relations to each listner and the speakers, then why do subwoofers and even some processor offer phase adjustment in the first place? I mean, if you have one subwoower 3 feet from your listening position, and another 10 feet away, and maybe another 14 feet away from you, doesn't the phase get all confused and out of sync with the rest of your system?
I mean why does a full range speaker's woofer even have to be in phase with the rest of the drivers then? You mean it doesn't matter as long as you have relatively flat response from your sub-woofers, and proper volume level?
Maybe someone can address these questions concisely. Cause otherwise I think all the speaker engineers maybe put too much thought into making their large full range speakers so coherent in the first place. And not only that, but all the pre/pro's out there that have you specifying sub to listener position distance were just adding unnecessary steps to the process...
That's why I x-over as low as possible...40->45hz. And while my Sub IS fairly close to the LH speaker, it is still pretty seamless. Don't forget that recordings. ..with the possible exception of the Telarc recording of Wellingtons Victory are MONO below about 80hz.
The Telarc disk has very localized CanonShots which are a real system test.
SMALL point...using 1ms/ft is probably OK, but sound is actually a little quicker....maybe 14"/ a said, a very small point......
Phase matters alot
There is a 180 degree phase shift in the bass phase anyway - which happens across resonance. So the whole thing is debatable however but purists will argue that even the 180 degree shift is bad - so it is best to get resonance way down and ultimately a system Q of 0.3 to 0.5 will give a better transient response (unfortuntely very inefficient in terms of SPL output).

See this article which explains the trade offs of small light weight woofers with small magnets in ported boxes (cheap - great extension but poor transient response) versus big heavy woofers with large magnets in sealed boxes.

The author says:

"There thus evolved two camps of woofer design: those with strong magnets, having better transient accuracy but worse LF response [he means poor LF extension], and those with weaker magnets having good LF response [better LF extension] but poor transient response. However, the poor transient response of a sealed box with a woofer having a weak magnet pales into insignificance alongside the wholesale demolition of the waveform that takes place in reflex, bandpass and transmission line speakers."

=> this may explain why small ported speakers with impressive bass extension just do not sound right with percussion (at least to my tin ears).
Phase matters alot

If you feel that way then consider active designs with digital filters - manufacturers can correct phase in the crossover region as well as below woofer resonance in the bass...yes you heard me correctly it means "perfect phase". I am not sure if Meridian do this but mathematically it can be done. As far as I know ATC do not specifically correct phase in the bass roll-off but they do so in their active crossovers and overall the speaker is phase adjusted as a whole using analog electronics.
Shadorne, I disagree with the basic premise of the author of that article. His third sentence describes his foundational belief:

"I soon learned that realistic loudspeaker reproduction requires reproduction of the input waveform, which seems somewhat obvious."

Not all that seems obvious is necessarily correct. The ear does not hear waveforms. Instead, to quote Dr. Earl Geddes:

"You are making a huge assumption here that the ear hears "waveforms". It doesn't. It deconstructs the waveform into a pattern of excitations in the ear (along the Cochlea) which are detected in complex ways."

How then to explain away the consistently-noted subjective superiority of low-Q sealed boxes? Doesn't that prove that good group-delay behavior is the key to natural-sounding bass?

Well, let's take a look into what we know to be audibly significant. Is low frequency group delay audible? The body of published research is inconclusive on the subject. Apparently it is on test tones, but not necessarily on music. But we'll concede that it might make a slight audible difference.

Is there anything else that we know makes a significant audible difference?

Yes - frequency response! We know that the large peaks and dips in the bass region are audible if they are far enough apart to not be smoothed out by the ear's averaging characteristic (which averages and smoothes the much more closely-spaced in-room peak-and-dip patterns at midrange and treble frequencies). We also know that broad, gentle trends in the frequency response are even more likely to be audible than are narrow-band peaks and dips, even though the latter look much worse on paper.

Given that frequency response is known to be highly audible, could the subjective superiority of low-Q sealed boxes be related to frequency response issues? At first glance, that doesn't seem to be the case: A vented box that measures "flat" to 30 Hz sounds boomy, while a low-Q sealed box that is -6 dB at 30 Hz sounds tight and natural. But wait - that's not the whole picture! We are leaving out a very important piece of the puzzle!

And what is that? The room, of course! Typical room gain is +3 dB per octave below 100 Hz. So, our vented box is probably up 5 dB at 30 Hz - no wonder it sounds boomy. On the other hand our low-Q sealed box is -1 dB at 30 Hz, which is much better.

So once we examine bass reproduction taking into account the room's effect on frequency response, the subjective superiority of a slow-rolloff bass system (low-Q sealed box) makes a lot of sense.

But wait - wouldn't the ideal be -3 dB per octave rolloff? How about an ultra-ultra-low Q sealed box? It turns out that the shallowest rolloff you can get from an unequalized sealed box is about 4.5 dB per octave. With equalization, it would be possible to achieve a -3 dB per octave rolloff starting with a more conventional sealed box alignment (assuming adequate excursion capability). But ironically, a unequalized vented box can be designed which comes very close to the theoretical ideal -3 dB per octave rolloff, at least down to system tuning.

Here's a link to a subjective evaluation of a room-complementary-tuned multisub system: (scroll down to post by ro7939, near the bottom of page 2)

Now I would agree that doing the same thing with large-magnet, large-displacement woofers in room-complementary-equalized sealed boxes would be even better. But, it would cost many times more. When comparing bass systems, there has to be some apples-to-apples basis, or else the biggest and/or most expensive always wins. We must compare approximately equal dollar solutions, or equal size solutions, or equal output-level solutions, or something like that.

So to recap, yes low-Q sealed boxes sound better than most vented boxes, but I believe the reason is that they produce a far more desirable in-room frequency response, rather than their superior group-delay performance making an audibly significant difference. And there are more cost-effective techniques for acheiving a desirable in-room frequency response.

Interesting post Duke. What do you think about the concept expresed in the "subwoofer conundrum" under the subheading 'Timing is everything' at It's not the greatest techno reference but it's quick and expresses what I was trying to get at with variable distances.It mirrors what I have experienced in my home and what the folks at jl audio preach. Thanks - Jim
Aldavis, in my opinion that author makes a mistake in assuming an anechoic environment for the sake of simplicity. I believe the room's influence to be the dominant factor in determining the perceived low-frequency characteristics, assuming competent speaker system design.

Apparently the author is aware that by the time we even hear a low frequency tone, we're well past any sort of "first-arrival" time window and into the time region where the frequency response is totally dominated by the room response. I quote him here:

"A 40Hz bass fundamental cannot be fast or slow - it is simply a 40Hz (transient) tone, and our hearing is depressingly bad at even hearing such frequencies until they have been present for several cycles."

Why he then proceeds to focus on the first less-than-a-cycle, which he pretty much just told us is virtually inaudible (or audible only insofar as it makes up part of the first several cycles), I do not understand.

Also, I've never seen a setup like his "Figure 1 - Typical Listening Room Setup". That looks somewhat contrived to me, probably to better make his point; but I certainly wouldn't call it "Typical".

I'm sure it's possible to come up with a positioning scheme and crossover frequency and slope that gives some sort of worst-case scenario, but consider this: The lowpass driver will be phase-lagging the highpass driver by 180 degrees for a second-order crossover, and 360 degrees for a fourth-order crossover. So the argument might well be made that the subs should be placed either 1/2 wavelength closer to the listener than the mains at the crossover frequency, or 1 wavelength closer to the listener, depending on the crossover type. I wouldn't make that argument personally, but my point is that placing the subs the exact same distance away as the mains doesn't time-align them anyway, because there is still the issue of a frequency-dependent time delay (a phase lag).

I think that if you can get the in-room frequency response in the bass region correct (and not just for one microphone location), you've already fixed all the major problems. At that point timing might become an audible issue, if the initial setup was something like Figure 1.

Thanks for the thoughtful answer Duke. Your posts are always well resoned. Reguarding your 1/2 wavelength remark I agree that you need to compensate for second order crossovers by inverting phase( 180 degrees available on most subs). Once you have done that does the distance argument not hold ? You can attempt to compensate for various distance discrepancies by utilizing continuous phase adjustment on the sub but you have to pick a frequency to correlate this with.( adjusting to be in phase at one frequency will take you further away in another. Figure 1 I think is an exagerated 'sub in one corner placement'. Your dead on with the major deal being in room frequency response at all likely listening positions which far outways this one argument. I just think this is a simple practical place to start. -

Thanks - that we hear frequencies or cycles and NOT transients in the bass makes sense - you have convinced me that John Watkinson may be barking up the wrong tree. Your contention that integration is better with a sub close to the mains when using only 12 db/octave cut-off makes sense (because of higher frequencies above sub cut off).

However, there must be a limit to what extreme phase shifts can be allowed and not mess things up. Perhaps a full cycle is too much? This would be 30 feet of distance at 40 HZ - and so many small ported speakers have a 3 db point at 55 Hz and a port at 40 Hz and will therefore achieve this!

While I can't say for such large 360 phase shifts (are they really audible at all) However, my impression is that an EXTRA cycle is certainly audible/perceptible. In fact, I suspect the overdamped response of a low Q system is so pleasant and "tight" in the bass precisely BECAUSE you get absolutely no extra cycles as you would from an underdamped ported resonant design (the better in room roll off response may certainly be part of it too). So John may be right but for the WRONG reasons?
This thread is interesting to me because I've spent the last 10 days with a room analyzer, a PEq, 2 subs and 2 ears. There seems to be a lot of theory being argued here and I wonder how many people have worked through this process in their room. I only say this because my experience 'till now has been one of near total unpredictability: What works best for one combo of sub/main fails miserably for the next.

I understand that my experience is limited, and I do not want to overstate my expertise, but so far, trial and error appears to be the only way to a good answer. To date, being wed to one philosophy of phase coherence, crossover slope, etc would seem to be a recipe for disaster. I know this is not a satisfying conclusion, but it's the one I've reached thus far.

Thanks, Aldavis. Just so you know, I don't mean to discount your personal experiences and observations, such as regarding 4th order crossovers, but it's possible that the problems you heard could be traced to a frequency response issue arising from poor driver integration. For example, in a typical 6" two-way speaker, an abrupt change in power response is more likely with a steep crossover than with a gentle one, and in my experience that's likely to be audible. In fact, that's why I pattern-match in the crossover region in my two-way speakers.

Shadorne, I know it seems insane for me to make excuses for a phase rotation equal to 30 feet of distance or so... but, that's my understanding of how the ear processes deep bass information. I could be wrong.

I concede that a sealed box comes to a full stop (and a full start) faster, which is beneficial for impact but perhaps not for pitch recognition, assuming the vented box doesn't sound boomy. This post by Earl Geddes has some quite interesting thoughts on the subject of low frequency reverberation towards the end:

Several months ago I performed an unscientific experiment, comparing equal-size, equal-efficiency small subwoofer boxes. The contestants were a 6.5" room-complimentary-tuned vented box, and a 10", Qtc = .5 sealed box. The 10" woofer was nearly three times the price of the 6.5" woofer.

On kick-drum, the sealed woofer sounded more tight and solid. On literally every other bass instrument I could come up with, the RGC-tuned vented woofer had a more natural-sounding tonal characteristic. On deep synthesized bass ("Tiger" by Paula Cole), the sealed woofer went into severe audible distortion at levels where the little vented woofer still sounded solid, and the little vented woofer was making the room shudder which the sealed woofer utterly failed to do. I think the ideal would have been an equalized sealed woofer with several times the displacement capability of the one I was using, but that would have been many times the price of the little vented woofer, not counting the equalizer and greater amplification required to get there. On the other hand, a subsequent vented box with a more expensive woofer (still considerably cheaper than the sealed box woofer) also had better solidity on kick-drum. Unfortunately, I've sold the sealed-box woofers so can't make that comparison to see if the better vented woofer evens things up in that area. I suspect the sealed box would still win out on kickdrum by a small margin, but would be even more outclassed elsewhere.


Thanks. I read all your posts with great interest.