Why do subs sound bloated or slow?

01-23-11: Plato
If the subs are not located in the same horizontal plane as the main speakers (and close to them) as is usually the case when they are placed in corners or close to the front wall, then unless you have subs with fully variable phase controls of at least 0 to 180 degrees, you'll need to do extensive trial and error to get the phase relationship correct between the subs and the sats. And yes, that's pretty much as hard as it sounds.

Hi Frank - I agree with you that finding the optimal phase setting is a major challenge of subwoofer setup. But I would add that, IME, phase adjustment alone is often not an adequate way to time align the sub with the mains. The reason was stated by Cbw723:

I'm not sure the automatic setup system work any better. They are primarily EQ programs, and do nothing for the transient response. And, frankly, it would be very hard for them to fix transient response because that requires playing the mains and sub(s) at the same time and (usually, because of sub placement and processing delays) delaying the MAINS. The phase control on a sub won't do this either and, to the extent that it is useful at all, can only work when the sub is closer to the listener than the mains.

I agree with Cbw723 that neither phase adjustment nor EQ (whether manual or automated) will adequately address the time alignment problems created by placing the sub farther from the listener than the mains, which is probably the most common setup in the typical audiophile listening room. The reason phase adjustment cannot adequately address the time alignment problems of this setup is that, by placing the sub farther from the listener than the mains, the speakers that need to be delayed for proper time alignment are the MAINS, and NOT the sub. Obviously, the phase adjustment on the sub can do nothing for this. The common use of digital EQ on the sub just makes the time alignment problem worse (even though it can dramatically help frequency response), because it introduces a processing latency that further delays the sub's output relative to the mains.

IME, for setups in which the sub is farther from the listener than the mains, the only way to time align the system is to be able to DELAY THE MAINS. But, judging from the systems here on A'gon, very few audiophile systems have this capability. For systems that do not have the capability of delaying the mains, time aligning the sub with the mains requires that the sub be placed more or less coplanar with the mains. But placing the sub coplanar with the mains might not result in the best frequency response. Which brings me back to my initial observation in the OP that...

Under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game.

IMO, the way to defeat the zero sum nature of this game is to:

1. Place the sub(s) to get the best frequency response (varies from room to room) and fix transient response problems with DELAY. This assumes you can independently delay the sub(s) and the mains, which as I mentioned above, doesn't seem to be a common capability in audiophile systems.

-OR-

2. Place the sub(s) to get the best transient response (i.e. roughly coplanar with the mains) and fix frequency response problems with EQ. But to the extent that the EQ introduces processing latency, you will have to move the sub(s) CLOSER to the listener than the mains. Again, this doesn't seem to be a common arrangement in audiophile systems.

-OR-

3. In light of (1) and (2), I have recently come to the conclusion that the most effective way to optimize both frequency response and transient response is to be able to independently control BOTH THE EQ AND THE DELAY of both the sub(s) and the mains. That allows you to correct for room modes (better frequency response) and time align the various speakers (better transient response).

Of course, I could be wrong about this. And...

This all this assumes that the time alignment of the sub(s) with the mains affects the transient response of the system in ways that are (a) audible, and (b) not reducible to changes in frequency response. Duke (Audiokinesis) has expressed doubts about that assumption, raising the question: What are the limits to the temporal resolution of human hearing at low frequencies? As I understand it, that is a subject about which there is some controversy.

Bryon

Spatial issues are not that important in the LF as we are talking only a few milliseconds with waveforms that are 50 milliseconds long at 20 Hz. A PEQ will delay only a few milliseconds also so no big deal.

Group delay can be a lot more than a few milliseconds and can be a problem as it can add delays that are equivalent to moving your sub 30 feet....

Transient response is important but spatial positioning of a sub is not the big deal you make it out to be - as long as sub and speakers are within a few feet of being the same distance to the listener then no big deal. Of course this argument only holds for very low frequencies and a mere 2 milliseconds can be important at 1 KHz

The answer is simple. Its because people stick there ultra powerful subs in a corner, which throws massive boom, and is often far away from the mains as well. I have been saying the same thing here, and nobody seams to get it. Put the sub under the center channel, between the mains.....put your electronics in the corner! If you do this all the problems are gone. In the older days subs stunk IMO, even the good ones cant compare to subs now. Back then corner placement was about added db gain(they still never blended right then either). These days, putting these huge, powerful subs in the corner is rediculous. The ONLY way I would try a corner placement is if you were running the sub under 40HZ. Otherwise it never sounds right to me.

Why dont you put your main speakers against a wall, or corner? -Bloated, muddy, heavy bass, right? So why do it to your sub? I ran my sub at 100HZ with my Magnepan 1.6's and in the center position you would NEVER have known it was coming from the mains. Phase/distance issues are GONE!

I dont like any subs sound placed out of center.

01-24-11: Shadorne
Spatial issues are not that important in the LF as we are talking only a few milliseconds with waveforms that are 50 milliseconds long at 20 Hz.

Shadorne - I agree that, at 20Hz, which is a wavelength of over 50 feet, and which corresponds to a time scale of around 50 ms, a few milliseconds of time misalignment is probably no big deal. But what about at 80Hz, which is a wavelength of about 14 feet, and which corresponds to a time scale of around 14 ms?

Let's say the delay introduced by EQ is something like 2-4 ms, and that the delay introduced by placing a sub in the corner, when the mains are out in the room, is something like 3-6 ms. In systems that combine corner placement with EQ, you might have a delay between 5-10 ms. All of these numbers are an appreciable fraction of 14, the number of milliseconds that corresponds to a wavelength of 14 feet and a frequency of 80Hz.

IMO, this math illustrates that the use of EQ and/or the use of corner sub placement could introduce delays that result in significant time misalignments. As to whether those time misalignments are audible, I can only say that, from my experiences with sub setups, differences of LESS THAN A FOOT are easily audible, even in circumstances where there is little or no difference in frequency response.

Transient response is important but spatial positioning of a sub is not the big deal you make it out to be - as long as sub and speakers are within a few feet of being the same distance to the listener then no big deal.

You may be right that I am exaggerating the importance of time alignment at low frequencies, though it is not my intent to exaggerate. My view about the importance of time alignment at low frequencies resulted from about 2 years of periodic experimentation with placement, EQ, and delay. But I suppose the view I'm advancing might be considered unconventional. Though, if it is unconventional, then why do several different manufacturers of full range speakers make an effort to time align their drivers, INCLUDING the woofers?

It seems to me that the importance (or lack thereof) of time alignment at low frequencies essentially depends on the human limits of temporal resolution at low frequencies. The limits of temporal resolution has been studied in depth, where estimates place it on the order of magnitude of MICROseconds, with some estimates placing it as low as 5 μs. But these studies tend to measure higher frequencies. I do not know the human limits of temporal resolution at lower frequencies. If they are anything even vaguely approaching those of higher frequencies, then it would lend credence to the idea that time alignment is audible at low frequencies, making time alignment an important consideration in subwoofer setup.

Bryon

"Many subs have group delays of up to or more than 1 cycle. Of course this is audible."

I don't remember the exact group delay figures used in the study I read, but they were consistent with what one would expect from a well-designed vented system. Using digital signal processing, group delay was isolated from frequency response (don't ask me how they did it). On test signals group delay was slightly audible, but on music it was statistically inaudible. The audible effects normally attributed to group delay were replicated by a frequency response curve mimicing the high group-delay system, even if the group delay is eliminated by DSP. So while large group delay correlates very well with perceived "slowness" in the bass region, it is not the actual cause.