Why do subs sound bloated or slow?

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.

01-25-11: Cbw723
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.

I'm not sure I agree with this point. I agree with Josh358 that the "perceived coherence" of the signal is significantly affected both by the direct sound and the indirect sound, and my suspicion is that the indirect sound is even more relevant, since the indirect sound is mediated by room modes, which are highly audible. But this is not a major source of disagreement. What IS a major source of disagreement is the importance (or lack thereof) of transient response at low frequencies, and hence the importance (or lack thereof) of time alignment at low frequencies.

It just occurred to me that there is a plausible argument about the importance of time alignment at low frequencies that has nothing to do with with improving transient response, but only with improving frequency response, namely:

Time alignment between the sub and the mains improves low frequency response by minimizing the patterns of constructive and destructive interference around the crossover frequency.

An explanation of why this is so can be read here (see the second section entitled "Timing is Everything"). For reasons of improving frequency response, some manufacturers of subs explicitly advocate the time alignment of the sub with the mains. For example, Rythmik Audio says:

In order to get the most out of your subwoofer, it is critical that it is correctly integrated with the rest of the system...

...The simple method is to compensate by changing the speaker distance setting on your receiver. Bass management in HT receivers has a speaker distance adjustment which process the signal on digital domain. If one puts distance of the sub x feet further away than its physical distance relative to other speakers, the HT receiver will put out the signal to the sub x/1000 sec before it puts out signals to other channels. That essentially puts a negative delay on the sub which can be used to reduces the "phase lag" on the sub and therefore reduces the phase difference between the sub and the front speakers. This trick enables us to use the speaker distance as a tool for phase adjustment between subwoofer and front speakers.

The point of all this is that there may be compelling reasons to time align a sub with the mains that have nothing to do with transient response, but only with frequency response.

Bryon

This is getting way too serious.

Is there a moratorium on serious conversation?

Bryon

My bottom line is that when I optimize my subs for smoothest FR in a co-planar arrangement it sounds no better (or worse) than when the subs are FR optimized for placement 2 1/2 feet behind the mains. It does, however, require significantly more EQ to get to the same place, so I stuck with the flush to the wall sub placement.

I'm pretty confident (can't be 100% sure, though) that no one would characterize the bass that I'm currently producing as either bloated or slow - and I do have room (via the adjustable Q on the Rythmiks) to skinny it down further if it did sound that way to someone.

So, my experience just runs counter to your theory. OTOH, while I'm extremely happy with the character, extention, and integration of the bass I'm getting from my subs, any of you folks might adjudge it differently.

The ol' YMMV.

Marty