Some pretty cool ideas floating around here. "If" we could keep the stereo bass AND kill room nodes with a multi-woofer setup, that would be wonderful. Delaying the subs that are far from the mains and maybe bringing down their level a little could be a good line of attack. As long as it doesn't sound distracting. Don't want to kill the stereo effect of the bass, just smooth the response.
Can you ellaborate why you wouldn't crossover that high? Is it problem when you signal align?
I don't see why its not possible with a layout that is symmetrical down the median line with a even number (minimum of 4) subs.
In the harmon paper recently linked they go to some length to justify summing the bass to mono before applying it to all the subs.
They present two alternatives:
1) Sum the bass - the pro being that when you position the subs for optimal modal smoothing and EQ the result for the flattest response, you'll get the same frequency response and modal excitation regardless of whether the bass in the music is mono or panned to the side.
The con being that the bass is mono so all out of phase information is eliminated.
2) Don't sum the bass - the con being that its not possible to optimise modal smoothing for both mono summed bass and for bass unique to one channel or the other by itself, since mono bass won't excite lateral modes in a symmetrical layout while single channel bass will. Also you have half as many woofers to provide smoothing making the smoothing less effective.
Although they hedge their position somewhat they seem to be in the sceptical camp when it comes to the effects of stereo perception of bass, so prioritise flat frequency response regardless of stereo offset of the bass as a higher priority thus choose summed mono as easier to optimise.
However I suggest that this may be based on a faulty premise - that you somehow MUST have perfectly flat bass response for out of phase or single channel components - why must this be the case when such signals are supposedly in the minority ?
The Griesinger paper makes it fairly clear I think that for low frequency envelopment the frequency response of the out of phase components doesn't necessarily have to be flat, as it is only used to "modulate" the apparent source location of the in phase components in a semi random fashion - in fact the "effect" only works across certain discrete frequency ranges where the right conditions are met which varies on a room by room basis anyway.
In fact it relies on excitation of lateral room modes to generate the required particle velocity, so the lateral room mode excitation from out of phase sound is required, not something to be avoided, as least if you agree with the approach presented.
So if you optimise the modal smoothing for mono bass whilst maintaining medial line symmetry and left/right drive of subwoofers you get the best of both words - for the "majority" of recordings with mono bass the system performs exactly as a summed mono system would, and for recordings with out of phase content that content is reproduced and excites lateral modes as desired. (rather than being completely suppressed before it even reaches the speakers)
The only real drawback is that single channel bass would not be optimally flat compared to mono bass or a summed mono system but I can almost guarantee that it will still be far flatter than a system with no distributed subs...(don't let perfect be the enemy of good!)
The Griesinger paper suggests that a second pair of subwoofers beside the listener would actually be beneficial for envelopment not detrimental.
As I said, I'm not sure that delaying the rear subs is the right thing to do, but you would certainly want to low pass filter them <= 80Hz while the fronts could go a little higher. I would probably face the cones towards the wall away from the listener too - that tends to eliminate most high frequencies that might cause unwanted localisation of the sub location quite effectively.
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Well, 1ms at speed of sound in free air at +20C in dry conditions equals 0.343cm. 1 meter = 3ms was chosen as easier base point for calculations. My suggestion is to delay rear subs only by the time/distance margin by which they are closer to listening spot compared to time/distance from the listening spot to front subs.
For 6m long room you'll get strong 28.6Hz axial mode with maximum at the middle of the room. If that's what you want to achieve, then symmetry without any delay is fine. Rear sub delay will sure spoil the mode cancellation symmetry.. which actually may be a good thing. Remeber, sub placement 1/4 from walls gave the smoothest bass all over the room (see Harman tests again). Corner placement was slightly worse, still much more practical. Delay would slightly offset subs "virtually".
You've got this wrong - a 6 metre long room will have a 1st order (half wavelength) axial mode at 28.6Hz indeed, but the pressure maximums are at both ends of the room with no gain (pressure minimum) from the mode at the middle of the room.
Thus there will be excess bass at 28.6Hz at both ends of the room but a lack of bass at the same frequency in the middle of the room. The 2nd order mode at 57.3Hz will have maximums at both ends AND the middle.
All modes have pressure maximums at the room boundaries but the distribution within those boundaries depends on the order and odd order modes do not have a pressure maximum at the half way point. For any order start at the boundary and measure half a wavelength to find the next pressure maximum. (In the 1st order mode half a wavelength is the far end of the room not somewhere in between)
A sub in phase at the opposite end of the room cancels this first order mode thus the pressure peak at the room boundaries is eliminated without affecting the dip in pressure near the middle. The result is that the pressure gradient at this frequency no longer exists along the room so there is very little variation as you walk along the room. The rear sub does nothing to eliminate the 2nd order mode however.
Placement 1/4 out from every corner won't fly in most living rooms, if wives have anything to do with it. 😀 If corner placement is only "slightly worse" but still infinitely better than no multi-sub set up, its a good practical approach.
Certainly boundary placement whether corners or along walls is the only practical option in most rooms from an ergonomic standpoint, except perhaps for a subwoofer disguised as a coffee table, but then you have wires to worry about...or ceiling mounting, but then we're getting into real dedicated territory... 🙂
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For most rooms that high would no longer be modal and there certainly could be an effect on perception from two widely separated sources at that frequency. Also, I have never found it necessary or beneficial. About 120 Hz. is about as high as I have ever taken a sub.
I see that as a no-brainer - just duplicate the multi-subs for one channel in the other channel (the EQ for each channel is done separately of course. I just don't see it as necessary and a real waste of good subs. But I certainly don't think that it would hurt anything either.
This is why I use only one main in my room mode simulation. Just do this twice, one for each channel and you have stereo. (Just remember to reverse the room for one channel.)
Dangerous cold here. Can't go outdoors, and heat barely keeps up. The wind chill will be -40 degrees early tomorrow. That's like Antarctica.
That's because it enhances it. That's the problem with the idea of "mode cancellation" because alternate modes do opposite things. There is no free lunch here. That's why I encourage random as opposed to symmetric. Less likely to cause one problem fixing another.
It is simply not true that if you have enough subs the modes go away. What happens is that you reach the power response of the room where there is still gain at the modes, but no holes, and it is spatially uniform and as such EQ works globally.
I keep getting e-mails from a friend in Edmonton who tells me that's like . . . normal . . .
Not true if the subs are on opposite walls and one is delayed and inverted . . . essentially the DBA approach. In that case the "mode cancellation" is frequency independent, since it is the reflection that is being nulled regardless what modes are, or are not, excited.
I know you realize this because we've been on many of the same threads here, and come to many of the same conclusions or at least strong working hypothesis: This is the approach I've advocated for a decade or so, literally since the beginning of the multisub movement. I call this approach the "flanking sub" or "helper woofer" configuration. Earl Geddes and I have discussed this at some length as well. This configuration can be done in conjunction with other, more distant, distributed multisubs.
Yes, that was in one of the early multisub papers written by Todd Welti. He proposed four wall midpoints, four corners or two wall midpoints as optimal locations. Geddes prefers a more random approach. But both agree once you get to four subs, placement of individual sources begins to matter less and less, as long as none are clustered acoustically close to one another (defeating the purpose of distributed placement).
I don't think this is correct ? For the 2nd order mode both subwoofers at opposite ends of the room on their own would generate the same pressure standing wave pattern and polarity along the room, thus would simply sum to a 6dB increase with the same pattern along the room and same standing wave amplitude when both are running.
So as I said - 1st order mode and pressure gradient along the room cancelled out, 2nd order mode unchanged apart from a 6dB increase in pressure everywhere.
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Yes we've talked about it at length in other threads and I share your position on the flanking sub approach. I experimented with it a little bit on my own years ago but never followed it up seriously but talking about it here over the last couple of years has encouraged me to take serious steps to address it on the next system I build.
It disheartens me a little when people are willing to spend a lot of effort on multi-sub modal smoothing for bass below 80-100Hz but are willing to dismiss with the wave of a hand the massive hole that almost always occurs around 120-140Hz as if it just doesn't matter. It does matter! 🙂
That frequency range is right smack in the middle of typical male vocal fundamentals for starters, it also seems to play a significant role in the impression of depth and "space" of the sound stage with a significantly "flatter" sound stage with holes present in that frequency range.
Helper woofer is probably a better description of the two "subs" that need to flank the main speakers to achieve this - they need to driven in stereo and run a bit higher in frequency than more distant subs (160Hz may be a bit unnecessary, perhaps 140Hz is sufficient depending on where the notch falls) and they can't be too far from the main speakers - approximately a metre plus or minus a bit, with the precise location based on optimising the notch around 120-140Hz.
The reason I bring it up again is that I can see this approach with 4 subs making it possible to achieve three goals at once - the two front flanking subs can solve the notch at 120-140Hz issue, the four subs working together can provide good (if not perfect) modal smoothing <100Hz, and the symmetrical layout of 4 subs driven in stereo can provide stereo bass reproduction that can potentially reproduce envelopment cues encoded in out of phase information in the recording which would simply be discarded in a summed mono sub system.
Thanks, I'll have a re-read of that one too.
We can chase blue birds in the stratosphere forever, but if microphones in the church were centimeters apart, how would you reproduce that realistic air flow from organ using woofers meters apart from each other? 🙂
That's exactly what I said. Where did the communication go wrong? If the mode amplitude is up 6 dB, I don't consider that "unchanged".
I did mean to say that "mode cancellation" doesn't work with just subs. If you add DSP EQ to each sub then you can get anything that you want within the confines of the number of sources that you have - i.e. the multiple source concept. Source's can become sinks, non-sources or whatever you want them to be.
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