subwoofer distortion or not?

[BC Dave]

I know what a powered subwoofer sounds like when it is coming apart at the seams from being pushed too hard, but other than that, how does one tell when a powered sub is starting to audibly distort? My daughter had Freaky Friday on last night and during the first earthquake scene my couch was shaking, but the quite loud rumble sounded like the sub was stressing (but not on the verge of blowing up). That got me thinking: how clean should this kind of event sound on a sub? Was I hearing distortion or simply what the producers put on that DVD? Maybe subs should have clipping indicators so one knows when to turn the volume down!

[Geoffcin]

I know what a powered subwoofer sounds like when it is coming apart at the seams from being pushed too hard, but other than that, how does one tell when a powered sub is starting to audibly distort? My daughter had Freaky Friday on last night and during the first earthquake scene my couch was shaking, but the quite loud rumble sounded like the sub was stressing (but not on the verge of blowing up). That got me thinking: how clean should this kind of event sound on a sub? Was I hearing distortion or simply what the producers put on that DVD? Maybe subs should have clipping indicators so one knows when to turn the volume down!

Most subs distort to the tune of 5% to 30% when pushed. This might sound bad, but for LF effects it is not a problem. One way to tell if your at the limit is if there's a clattering sound. This is the sound of the driver banging against the limits. What is the make and model of your sub?

[Chuck]

I know what a powered subwoofer sounds like when it is coming apart at the seams from being pushed too hard, but other than that, how does one tell when a powered sub is starting to audibly distort? My daughter had Freaky Friday on last night and during the first earthquake scene my couch was shaking, but the quite loud rumble sounded like the sub was stressing (but not on the verge of blowing up). That got me thinking: how clean should this kind of event sound on a sub? Was I hearing distortion or simply what the producers put on that DVD? Maybe subs should have clipping indicators so one knows when to turn the volume down!

Hi Dave,

It's impossible to say what you're hearing without a lot more information, and probably can't be sure without being there. I think about the best I can do is give you some general information that might be helpful.

To begin with, it is probably important to realize that most woofers and sub-woofers actually have very high distortion even at modest levels. By the time it becomes obvious it's at horrendous levels (at least with a good sub). You're actually hearing lots of bass distortion all the time, but it usually doesn't rise to the level where it's a bother, until you start overdriving things.

Secondly, unless you've done something to moderate your rooms nodes, the really powerful bass that shakes the room is probably the result of standing waves rather than lots of bass in the soundtracks. Most people really miss the excess energy of their standing waves when they first eliminate them, because room shaking bass becomes much less common (but much more rewarding when it's there for real).

Also, if you're using a ported sub, the port may be mis-tuned slightly to "enhance" bass output at the expense of accuracy and control. This can result in uncontrolled cone movement at certain frequencies, and will often result in the driver exceeding it's linear stroke. In some cases the distortion will be all too obvious.

Finally (actually there is more that could be said, but this will have to do for now), most powered subs have some kind of peak limiting to prevent damage to the amp or driver. When these circuits come into play their effects can sometimes be pretty obvious.

If the distortion isn't on the recording it should vanish if you lower the volume a little. If it goes away, then you might want to consider a bigger sub. Think in terms of upgrades that will give you at least twice the output of the current sub or you may hardly notice any difference at all. If you want really clean bass go for a large TL or servo-sub. IMHO, too much is better than too little, but deep clean bass, like horsepower, cost money.

Hope this helps,

Chuck

[BC Dave]

Thanks for your reply. There's no sound like the woofer bottoming out. What I was probably hearing is port noise. There's a huge jump in the volume of the movie at that point. It would be interesting to know if anyone else has the movie and notices a similar burbly sound during that scene.

As for the make and model of the sub I don't think you could have heard of it. It is a Quest QS10 sold by Future Shop in Canada. Mission subs built in Canada used the identical woofer and digital amp about three years ago. Tom Nousaine said that driver was able to pump out "bass with a vengeance" (something like 111 db at two meters at 32 hertz.) The Mission's cabinet was slightly bigger than my Quest's. Admittedly the room is quite big for this sub (24 by 12) but it seems to do a great job on most stuff. Maybe this scene pushed it near its limits (as does the opening scene with the waves in Die Another Day). But it holds up really well in Terminator 3 and Finding Nemo.

[Chuck]

Thanks for your reply. There's no sound like the woofer bottoming out. What I was probably hearing is port noise. There's a huge jump in the volume of the movie at that point. It would be interesting to know if anyone else has the movie and notices a similar burbly sound during that scene.

For sure, if the woofer was bottoming out, you'd know it. That sound is unrelated to the music, and sounds like the woofer basket is being smacked with a hammer. Port noise, an under-damped resonance, a room node, or some combination of the three, is certainly a possibility. It could also be that you're simply exceeding the linear excursion range of the drive, or triggering the limiting circuits in the subs amplifier.

It might be helpful if you could determine the frequency where the problem occurs. If you can get a signal generator (download a shareware software signal generator and use your PC and sound card) you can run a sweep through the bass and see if the output peaks at some point. Unless you've done a lot of room treatment and/or design you'll find at least one very obvious peak in the bass, that being at the lowest major room node. You may also find that the sub goes a little crazy at a particular frequency. Such things can be caused by under-damping, loose baffles, box talk, and so on, but once you have a frequency that will trigger the event it's a lot easier to investigate.

If we had "Freaky Friday" I'd check the levels of that earthquake, but we don't have it, so that's not an option.

Does the distortion go away when you lower the volume, or does it sound the same, just at a lower level? If it sounds the same at lower levels then it is unlikely that it is the result of overdriving anything.

Hope this helps,

Chuck