October 13, 2010

Subwoofer transient response

Does transient response in subs really matter? Are sealed subs better for their superior stop on a dime bass? It turns out that the answer in most rooms is "no" in both cases.

Subs aren't as different as you think

When you look at actual subwoofer measurements, they aren't as different as one might expect. Most of them, whether high quality or cheapies, measure about the same except at the low end which is less critical. A vented subwoofer will ring around it's tuning point, but as with many cases, if tuned around 20 - 30 Hz, this will have no effect on music. It will tend to influence low frequency effects which are artificial in nature.

Here is one example:

JL Audio fathom >

Scroll down to the spectral decay. You might like to look at some of the other measurements and you will see that most of the subs measure the same above about 30 Hz.

Sub tests >

Look at the spectral decay in the first example. It's a top notch sealed sub, and at 100 Hz you get 6db of decay in the first 40 ms. That's about as good as it gets, and with low frequencies, things get worse.

Now lets look at a room measured by Ethan Winer:

Look at the first chart and you see that it takes about 100ms to achieve the same amount of decay due to modes in the untreated room. In the next chart with treatment, the subwoofer range is virtually the same and it's only about about 80 Hz that signficant improvement is seen. At this point the room now decays faster than the subwoofer, but only outside it's passband.

Of course, each room is different.

Looking back at those measurements, the best performing sub I found out of the lot seems to be an REL, one which measures very poorly in every other way:

REL storm >


So what does all this mean? There are a lot of things to worry about in setting up a sound system, but subwoofer transient response isn't one of them. Even a treated room will ring out longer than the sub itself, except for LFE content. The subwoofer transient response can't be considered in isolation from the room - the two are combined together and the ear can't distinguish between them. The ear is also not quite so sensitive to time domain issues in the sub range, certainly not as much as many of us have been told.

The lower midrange deserves more attention, but gets strangely ignored. Room modes cause ringing there and this can be treated effectively with bass traps. EQ will have no effect on ringing there, but we can flatten the inroom response with multiple bass sources.

The key point I'd like to make is this - ignore what you can't fix and what doesn't need to be fixed - in this case subwoofer transient response. Instead, focus on what you can fix and what does need to be fixed. This is the reverse of what many choose to do. Those that haven't figured out what needs to be done have a habit of tail chasing. Rather than find real problems to fix, they chase after imaginary ones.

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