subwoofer variable phase alighment - opinions

Discussion in 'Home Theater Projects' started by Matt_Doug, Jul 2, 2003.

  1. Matt_Doug

    Matt_Doug Stunt Coordinator

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    Does anything less than 180 deg shift mean anything audibile and is continually variable phase allignment on subs and receivers mean anything.
     
  2. Terry Montlick

    Terry Montlick Stunt Coordinator

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    Continuously variable phase alignment allows you to match the phase of your subwoofer with that of your main speakers at the crossover frequency, at least for one listening position.

    For a typical subwoofer crossover at 120 Hz, one wavelength of sound will be 9.4 feet long. So for every 9.4 feet of DIFFERENCE between main speaker distance and subwoofer distance, there will be a 360 degree (1 wavelength) phase shift. This gets you back to where you started, because the sound wave has turned a full 360 degrees around.

    But if the distance difference is half this, or 4.7 feet, then your main speakers and subwoofer will be 180 degrees out of phase. They would cancel at the crossover point, and there would be a nice notch cut out of the frequency response at the crossover frequency. Of course, if they were 180 degrees out of phase, than all you'd have to do would be to swap the subwoofer wires to flip the polarity, and things would be just fine.

    Continuously variable adjustment means that you can make finer adjustment than the 180 degrees you get my swapping wire polarity. Remember, though, that this is really only good for one listening position (or more precisely, a grid of listening positions)!

    Hope this helps.
     
  3. Matt_Doug

    Matt_Doug Stunt Coordinator

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    Thank you Terry. I asked this because i use an external crossover to cross my mains at a lower frequency than my receiver can go, and was never able to successfully use the variable phase on the crossover. Instead I bypass all subwoofer controls (what a waste) on the external crossover using only the receiver sub out which is set to main + LFE . There is some overlap but its the best sounding I could get. my room is only 20 x 12 X 9 and because of furniture...etc the most I could muster is 12 feet from the mains and 7 feet from the sub a difference of 5 feet. My trig is a little rusty so what is the phase angle for 5 feet difference with a 50hz hi & low pass - thanks in advance.
     
  4. Terry Montlick

    Terry Montlick Stunt Coordinator

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    It's actually easier than trig! Here's how you calculate it:

    The speed of sound is about 1130 feet/sec. At 50 Hz, one wavelength (360 degrees) is simply 1130/50, or 22.6 feet. so if 22.6 feet is 360 degrees, then 5 feet is (5/22.6)*360, which is 79.6 degrees.
     

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