Two Subwoofers... Comb Effect

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 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. -
jim

Duke,

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.

Marty

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