Two Subwoofers... Comb Effect

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 peaks....as usual YMMV.

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

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).

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.

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.

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?

Duke,

Thanks. I read all your posts with great interest.