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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.
Follow Ups:
I was thinking of one of those for my own room. A friend of mine picked one up and I got a tour of it. It worked great. If it is single channel, I guess it is off my list. I feed stereo input into the sub.After trying the Velodyne, I measured my family room's response just using Arta, and was surprised at the large 40Hz bump. My speakers, at the time, had an F3 of just about 40Hz in room. I am now experimenting with on-wall speakers but these only go down to about 55Hz.
For what it's worth, my active sealed B&W sub can be crossed over down to 40Hz which helps out some. I cross mine over at 45Hz at the moment but equalization is necessary here.
n/t
I like how it worked in my friend's setup. I was going to check it out more closely next time I came by anyway.
excellent post. the main speaks are running full bore, you are correct.I have thought about getting a Behringer eq but hated the idea of having a digital piece when i mainly listen to vinyl. But you're suggestion is to have it for the subs only...that is doable. is there a specific model?
i would not ever run them out of phase of one another. both switches are set one way or another. both give quite a difft effect.
The floorstanders are away from all walls, a benefit to having a G shaped room - the heel of the L is bumped back 6'...forming a G and the 4th corner in the room (where 1 sub is) is only 7' long and at a 45 deg angle. highrise condo.
they probably go below 50hz, but i can set them up with spikes & they growl much lower. with 'em unspiked, on the carpet & way out in into the room they don't play very low...but sound dope.
still have to read other responses, but thanks for the time spent.
Audio-video logic is selling the Velodyne product at a discount -- $600.00 Link: http://cls.audiogon.com/cgi-bin/cls.pl?misceqal&1179166638Other link below is to the current Behringer product, which lists at $380. You'll have to buy the calibrated microphone and a long cable; I believe Behringer sells both.
IIRC, I bought the predecessor product used on audiogon for about $200, including the microphone and cable.
The calibration is not perfect because, for calibration purposes, you have to insert the equalizer in the signal path between the preamp and main amp as well as between the preamp and the subs. Then, once you operate, you take the equalizer out of the path between the preamp and main amp. If you don't want to leave the equalizer in the signal path for your main amp, and you want to be more precise in your equalization, you can equalize maunally, using a CD with pink noise on it as a sound source and the equalizer's RTA to adjust for a flat output into the room.
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