Subwoofer fase problem !

Discussion in 'Archived Threads 2001-2004' started by Michael Eriksen, Nov 11, 2001.

  1. Michael Eriksen

    Michael Eriksen Stunt Coordinator

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    I have the Atlantic 452 THX sub stacked in a corner along with a Servo-15. I just got my X-30 controller to the servo and im right now trying to calibrate the system.
    The AT has 2 fase settings. Normal and invert.
    The servo has a variable fase setting from 0 to 180 degree.
    When At is set at normal and servo at 180 the output is about 5-6 db higher.
    Same thing happends when the AT is set to invert and the servo to 0 degree.
    I presume that means that the 2 subwoofers are in fase in these 2 situations and that the subs use a different standard to define normal and 0 degree.
    That it all well. But now I dont know if I should set them at invert/0 degree or normal/180 degree.
    I guess that depends on which solution is in fase with the rest of the speakers.
    BUT, i cant for the life of me spot any difference nor can I meassure any increase in output with an spl meter regardsless of which sub solution I use.
    The Avc-a1se (avr-5800) is set to THX subwoofer mode and all speakers are set to small. That means the subs get the LFE signal along with all signals below 80 hz. Signals below 80 hz are not sent to the speakers.
    With that setup, does it matter whether the subs are set to normal/180 degree or invert/0 degree, because they will never produce a sound that is produced at the same time by a speaker. I just kinda figured that the subs playing in or out of fase with the rest of the speakers dosent matter with these settings...or am I wrong ? That could explain why I cant hear nor meassure any difference.
    If there is an difference I will ofcourse prefer to set it up properly, but I really dont know how to find out which solution is in fase with the speakers then.
    Any feedback is appreciated...
    Thanks in advance
    Michael Eriksen
     
  2. Guy Kuo

    Guy Kuo Supporting Actor

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    What signal are you using while measuring subwoofer + main speaker bass output? It must be a signal in the overlap frequencies and be present in both the sub and main speakers simultaneously for you to measure the SPL difference of in and out of phase conditions.
    If the above are met and you stil don't measure any differences, it is possible that the subs are at 90 degrees out of phase from the mains. That would probably mean the wrong delay or distance has been set for the subwoofers in the processor.
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    Guy Kuo
    Ovation Software, The Calibration Tool Source
     
  3. Michael Eriksen

    Michael Eriksen Stunt Coordinator

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    I havent used a testsignal that is produced by both speakers and subs since they dont get the same signal. If its below 80 hz, the subs get it, and if over 80 hz the speakers gets it.
    I just tried with lots of movieclips and music.
    But why does fase matter if the subs dosent produce the same signal as the speakers and vice versa ?
    I could try to play a 80 hz sinus wave...will that damage the speakers if the signal is 0 db. Its the 450 set from AT.
    Michael
    [Edited last by Michael Eriksen on November 11, 2001 at 05:32 AM]
     
  4. Guy Kuo

    Guy Kuo Supporting Actor

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    Unless you are running a brickwall crossover with very steep rolloff's, the 80 Hz point isn't an instant barrier between sound going to the mains vs the subwoofer. The mains actually continue to reproduce sound below 80 Hz and the sub reproduces sounds above 80 Hz.
    What you've managed thus far is to find how to operate your two subwoofers in phase with each other (higher SPL), but they also need to be brought into phase with the mains speakers. Otherwise, the overlap frequencies (and there is overlap above and below 80 Hz), will be a region in which the subs and the mains will interfere with each other. In order to test the phasing of the subs with the mains, one must apply a signal which is centered at or near the crossover point (in the overlap). This can be done with a main channel test signal if the mains are defined as "small" in size. You can use a warble test which passes through the overlap frequencies and adjust phase for the smallest variation on the SPL meter. Alternatively a shaped noise in the overlap frequency range could be played while you attempt to maximize measured SPL at the listening position.
    Without a test signal which is routed to both the mains and the subs, you have no means by which to test their phasing relative to each other.
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    Guy Kuo
    Ovation Software, The Calibration Tool Source
     
  5. Michael Eriksen

    Michael Eriksen Stunt Coordinator

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    Thanks Guy,I will try that then.
    I have a testsignal on a cd. 80 hz sinus wave 0 db.
    Will the speakers be ok with playing such a signal or is there a possibility for damage ?
    That signal should be good to test the fase I think.
    Michael
     
  6. Guy Kuo

    Guy Kuo Supporting Actor

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    Turn the volume way down if the 0 dB means it is 0 dB full scale. That would be very loud. If you volume is turned down so you are at a normal volume (80 dB SPL) you won't risk any speaker damage. Be sure to mount the SPL meter rather than hand holding it. A change in meter position will alter the results.
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    Guy Kuo
    Ovation Software, The Calibration Tool Source
     
  7. Michael Eriksen

    Michael Eriksen Stunt Coordinator

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    I think we have a winner. I played the 80 hz signal.
    With the AT set to normal phase I got a 5 db higher output. Same results with the servo set to 180 degree.
    So normal/180 is in phase with the speakers and also the subs are in phase with each other.
    I was the same sets i used, but know I just know its right.
    Thanks alot for your help Guy.
    regards Michael Eriksen
     
  8. Guy Kuo

    Guy Kuo Supporting Actor

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  9. Jim A. Banville

    Jim A. Banville Supporting Actor

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    To find the "best" phase position, you need a test CD with several bass tones above, at and below the crossover point, like the Test CD from Stereophile magazine. You should first note the frequency response of the main speakers alone playing the bass trst tones. Then you should note the frequency response of the subwoofer alone. Now, note the response of the mains and sub together. After comparing these three frequency response "charts" you should have a good idea of how the mains and sub are interacting. You should then make logical selections of the phase controls, noting changes in the combined response of the mains and sub. You may find one phase position that gives high output at the crossover frequency, but causes a reduction in adjacent frequncies. This is where you are lucky by having a variable phase setting. I would leave the sub's phase selector on "0", and adjust from the X-30.
     

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