Two Subwoofers... Comb Effect

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:

http://www.harman.com/wp/pdf/multsubs.pdf

Duke
dealer/manufacturer

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

Duke

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.

Duke

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:

http://www.audiocircle.com/circles/index.php?topic=60103.20 (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.

Duke

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.

Duke

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:

http://www.audioasylum.com/audio/speakers/messages/208728.html

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.

Duke