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Re: Subwoofers in phase or out? a quick question

Posted by Bruce from DC (B) on March 28, 2007 at 16:30:50

In Reply to: Subwoofers in phase or out? a quick question posted by mattcecil1@yahoo.com on March 28, 2007 at 11:02:30:

Having read both of your posts and assuming that you are not feeding your main speakers through a high-pass filter (i.e. they are allowed to reproduce the full spectrum of the input signal), I would say you are attempting the impossible.

The problem is that the Thiels I'm sure reproduce flat to at least 40 Hz and somewhere between 40 and 50 Hz is your primary room resonance, which can add 10 dB or even more to the response. And the low pass on your subs is limited to 50 hz and above. The reason that REL gets away with this kind of a setup (i.e. running the mains full range, rather than through a high pass filter) is that the REL low pass adjusts all the way down into the 20Hz range. Most other subs can only be used this way with small standmount speakers, almost none of which develop significant output below 50 Hz.

There is no other brand of sub that I know of that does this, other than the Velodyne digital drives, which have a built-in parametric equalizer.

So, by flipping the polarity switches on your subs, you are alternating between destructive interference in the 40-50 Hz bandwidth, which will suppress the perceived bass level in that frequency band, or additive bass there which will probably be overwhelming (unless you throttle the subs way back).

Having the subs 180 degrees out of phase with each other is a real bad idea; so don't even consider it. Better to give them away than do that.

My suggestion would be to buy a Behringer digital RTA/Equalizer (they're only a few hundred dollars) including the calibrated microphone. (The Bass Nut gave me that idea several years ago.) If you're patient, one will come up for sale on Audiogon (which is where I got mine). This will allow you to match your subs (in-phase with the mains, please) to the bass output of your main speakers and it will compensate for the room resonance. The most that you may have to do is move the Thiels farther away from walls, etc. to reduce their bass output in the critical 40-50Hz region, since the Behringer will not be equalizing the drive to the Thiels, but only to the subs.

The Bheringer unit is a stereo unit, so you can equalize for each subwoofer separately, to account for the difference in placement. With the calibrated microphone, the whole operation is automatic and takes just a few minutes.

I have one of these, and it is money very well spent.

Velodyne sells a similar unit that is much easier to use than the Behringer, but also is more expensive (around $700) and is single-channel.

I hope this is helpful.

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