Why do subs sound bloated or slow?

Number one reason why subs, or for that matter any speaker producing frequencies below 80 Hz, sound bloated is the room. Adding a second sub will substantially help in reducing room mode peaks and nulls. That's why some people swear by using a full range pair instead of satellites and sub. Corner placement of a single sub is a guaranteed formula for exciting all your room modes to the max. It's impossible to hear whether or not the sub is doing its job correctly when the room is booming.

01-25-11: Martykl
Small movements of the source of deep bass can audibly affect the perceived sound, but once again I'd be inclined to attribute this to room interactions (rather than increased group delay)...

Marty - You, I, and Cbw are all in agreement that small changes in sub placement can result in significant audible changes at the listening position. Where we differ is what we attribute those audible changes to. You attribute them to differences in room interactions, by which you may or may not mean frequency response. Cbw and I attribute them to differences in time alignment, and hence transient response.

Cbw suggested a reason to doubt whether the audible differences at the listening position resulting from small changes in sub placement are attributable to room interactions:

Marty, I don't think the effect I described can be reduced to room interaction because, as I said in my earlier post, I can achieve the same result by adjusting the delay of the mains relative to the subs without physically moving anything.

I have experienced the exact same thing. That is to say, I can produce significant audible changes at the listening position by changing the digital delay on my sub, without altering sub placement.

Having said that, I have to acknowledge that changing the delay on the sub DOES change the "room interactions" insofar as it affects the patterns of constructive and destructive interference between the sub and the mains, which are mediated by the size and shape of THE ROOM. So changing the delay on the sub does change its room interactions. Hence it might be reasonable to conclude that differences in room interactions explain the audible changes at the listening position.

But even if this is true, it still leaves open the question of whether those audible changes at the listening position are attributable to frequency response or transient response. The reason is because constructive and destructive interference, while being forms of "room interaction," alter BOTH the frequency response AND the transient response of the system. Because of this, concluding that the audible changes at the listening position are attributable to room interactions does not definitively answer the question of whether those audible changes are attributable to differences in frequency response or differences in transient response or both.

In my view, the way to determine whether the audible changes at the listening position resulting from small changes in sub placement are attributable to frequency response or transient response is hold one constant, alter the other, and listen for changes. In my reply to Audiokinesis above, I reported just that kind of finding. That is to say, I have had many experiences in which I have moved the sub on the z axis by a small amount and produced significant audible changes at the listening position, EVEN WHEN the measurable frequency response remained the same for both sub placements. It was precisely those experiences that led me to the conclusion that the audible changes at the listening position were attributable to changes in transient response.

Bryon

Not that this benefits this discussion, but I feel the need to thorow out a peeve of mine when discussing "subs" - this supposed magical frequency of 80 Hz. IMHO if you are using 80 Hz as a crossover point you're already off to a bad start with regard to integration. Again, IMHO it is better to cross over the sub at 70% of the -3 dB point of your main speakers and tweak from there. Then worry about level and phase. The wavelengths of sub bass frequencies vs. room size are too mismatched to worry about time alignment. Check out the design of the Vandy 5 line. Richard give a good explanation (and he's fanatical about time and phase alignment).

Shadorne, the flaw in your logic is that a movement of my head is not the same thing as an equivalent movement of the sub. I'm arguing that it is important to get the correct time alignment between the subs and the mains, thus the important parameter is the position of the subs *relative* to the mains. Small movements of the sub relative to the mains will affect how the sound combines in their region of overlapping frequencies. If the speaker and the sub are close together (or on the same axis, anyway), even large movements of my head will not affect their relative distances to my ears (only the absolute distance), and thus will not change their time alignment.

This is, in fact, an argument for keeping your subs close to your mains: if the sub is placed off-axis, then small movements of the head will affect the relative distance of the mains and the subs to the ears, altering the coherence of the signal.

Bryon, while I agree with much of what you say, I think room interactions are secondary. The perceived coherence of the signal is going to be primarily determined by the direct waves from the speaker and sub to the ear. Room interactions may be stimulated to a greater or lesser degree depending on the coherence of the signal but, as you point out later in your post, the effects of small changes in position or delay don't seem to show up prominently in plots of frequency response.

What room interaction does, I think, is produce boomy, one-note bass that is distorted in both the time and frequency domains. Equalization can help with the frequency domain problem, and at the same time, reduce the amplitude of the smear. But room treatment or dynamic equalization is required to reduce its duration.

To make matters worse, bass modes can shift the pitch of the note.

Still, that doesn't really explain why vented subs typically sound mushier than acoustic suspension ones, and acoustic suspension woofers mushier than planars. Cone breakup and nonlinear distortion may have something to do with this, the harmonic distortion on a subwoofer can approach 10%. Cabinet resonances could also play a role. The role of group delay in a vented sub is perhaps more controversial, since as someone pointed out small amounts of group delay don't seem to be audible on musical material, and also since the delay caused by resonances is much greater than the delay introduced by the vent.

Finally, many subs, particularly ported ones, aren't designed for flat response. They're designed to make the loudest thump possible to please people who want to reproduce explosions and dinosaur stampedes. Or to produce lots of bass on the cheap for people who don't understand the difference between loud and deep.

Still, at the end of the day, I haven't seen a truly rigorous explanation of the phenomenon of perceived woofer speed.