To add a (hopefully interesting) note, the comment of Andrew's that I was referring to is from last year in his excellent Sound and Vision interview on subwoofers
here.
S&V: Would you recommend S&V readers buy four smaller subs, two midsized subs or one large sub, if the price for the different packages were about the same?
AW: I think the sweet spot is to buy two. That’s going to give you a pretty decent balance between output level and linearity. If all you’re concerned about is output and you think your room correction system can fix the problems—although it can’t—go for the single $1,500 sub. If it’s a smaller room or you don’t listen that loud, absolutely put in four subs. We did a study on this a few years ago and four subs will always give you the smoothest response. With two you get fairly decent output but it’ll be smoother than just one.
People think when you put more subs in you get +3 or +6 dB more output. That might happen if you stacked them in a corner. But if they’re in different positions, where there might be a peak with one sub’s output it might be a dip with another sub, and they’ll cancel each other out. You may get a little more output and you’ll certainly get more headroom. But two subs doesn’t mean +6 dB more output compared to one.
I wouldn't want to put words in his mouth, but it seems safe enough to extrapolate a bit. With multiple subs, placed at different locations, the way in which they excite the room modes, interact with the boundaries, and transmit sound to your location may be distinct. These two sources will interfere and tend to cancel each other's imperfections to some extent. The difficulty of the problem is that it does not exist just in the global frequency domain, but additionally varies locally from point to point. With two perfectly co-located subs, you will double the pressure output. The power is proportional to this quantity squared, so it goes up by a factor of 4. 10*Log_10 ( 4 ) = 6 dB. But, since the output of the two is spatially identical, the 6 dB boost is contributing to exactly the same peaks in exactly the same locations, and not improving the smoothness at all. If all that one wants is power, then doubling the number of subs will actually quadruple the power delivered at a location where peaks coalesce, at least for the selected frequencies. Of course, there is a price for this. Doubling the number of subs must, on average, precisely double the delivered power. This is not 6 dB, but 10*Log_10 ( 2 ) = 3 dB. You can't get 6 dB everywhere, it violates energy conservation, and the dynamic imbalance between peaks/nulls will be made worse at the spots where you do get it (for certain frequencies). If the subs are not co-located, then the interference may have the effect of spatial averaging, which would be a good thing for smoothness of response in various locations, limiting the artificial inflation or negation of frequencies that interact in particular ways with the room geometry.
Also relevant is the next Q/A:
S&V: How do you feel about the automatic EQ and room correction technologies that are built into receivers and pre/pros?
AW: As long as you can force them to work only at low frequencies, they can be very beneficial. Broadband room correction we are in total disagreement with because there’s so much variability, not only in the way the different systems work but in the target curves they’re attempting to correct to.
A properly designed loudspeaker will work in any decent room. If you have a bad room, room EQ may help a little bit, but it’s not going to fix it. Also, the EQ curve shouldn’t have a boost anywhere. You cannot fill in a valley caused by a room mode. You’re just going to waste amplifier power. Either use multiple subs or acoustic treatment to fix the problem. You might get a couple of dB measured improvement by trying to fill in the dip with EQ, but you’ll probably have to boost +6 dB or more, which means you’ll be hitting the limiter earlier, which will affect the sound.
Cheers - DSQ
