Why do subs sound bloated or slow?

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.

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.