Phase setting of Subwoofer Using AVIA..Odd Result.Cause?

Discussion in 'Archived Threads 2001-2004' started by Richard_s, Apr 25, 2002.

  1. Richard_s

    Richard_s Second Unit

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    I chose to start a different thread than put this into The current thread entitled “AVIA Note: Subwoofer Calibration” started by Guy Kuo”. That is an exceptionally good thread on how set up the subwoofer using AVIA so I felt it would be best to not post this question there. I am sure that what I have is probably unique to my set-up / room acoustics. I would like to understand what this all means (if that is ever possible).
    This has to do particularly with the Phase adjustment using AVIA so this is my setup story:
    Balanced all 5 channels first with the Radio Shack SPL meter pointing up at 45 degrees toward the ceiling. Did the main and center with the microphone pointing from the listening position to the speakers. I did the surround speakers by pointing the microphone towards the respective surround channel.
    Little about my system and room:
    Room 16x24, Cathedral to 14 feet, 150 sqft of glass (did setup with all shades open glass exposed)
    JBL N26 Mains on 31 inch stands (6 feet apart centered on the RPTV and 12 feet from the listener)
    N-center on top of my RPTV
    Surrounds on the walls facing each other both are 12 feet from listening position.
    JBL PB12 Subwoofer placed on a piece of plywood to keep it off the carpet and in a corner *8 inches off each wall
    Yamaha 5550 (no room size or delay adjustment except for a delay for the center channel)
    Now what I found:
    After balancing the mains, as stated above, they were all within +/- 1 db of each other based on AVIA.
    Balanced the subwoofer to the mains. All balanced very well. All five speaker positions had the same SPL reading for the respective speaker and the subwoofer to within +/-1 db. The LFE test also was within +/- 1db
    Did the phase adjustment using AVIA and did the one person method of maximizing the SPL reading (sub only has 0 & 180 settings for phase).
    Then went back to re-adjust the subwoofer to mains and found that, as expected, after the phase adjustment causing a higher SPL for the subwoofer I had to turn down the subwoofer volume. I am at about 1/3 volume on the PB12.
    Now here is what I do not understand:
    With the phase set to maximize the SPL reading my subwoofer balance at each speaker varied greatly. For example my Right surround was +2 db from the mains and my Left surround was –5db to the mains. I put the subwoofer phase back to the position where the SPL was lower and now the subwoofer was within +/- 1 db at all five speaker locations. The warble test and the low frequency sweep test had peaks and valleys with both phase settings but were larger with the phase set to maximize the SPL of the subwoofer using AVIA.
    The sound quality seems extremely good when the phase is set contrary to the correct way of setting the phase. The phase setting that gave the lower SPL reading for the subwoofer gives better overall sounding bass as well as better balance across the room. This is opposite to what is recommended so this does not seem to make sense. I also set the subwoofer to full volume and although the bass was extremely high it was tight (not boomy or bloated sounding).
    So why do I get this result?
     
  2. Saurav

    Saurav Cinematographer

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  3. Harold_C

    Harold_C Stunt Coordinator

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    You probably have a big room resonance somewhere close to the crossover point. Setting the subwoofer phase switch for electrical cancellation (less bass output), will reduce the levels in the crossover region and help with taming the room node, much like a parametric EQ band.

    This doesn't strike me as terribly surprising. Nor do I think there's a problem with it. Speaker designers use the same trick sometimes to smooth the frequency response at the crossover point between a midrange and a tweeter.
     
  4. Richard_s

    Richard_s Second Unit

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    Thanks. Just wanted to know if this is a set-up that makes sence. So far it seems to sound very good. Going to watch the "Men in Black" DVD later that should give me a good DD5.1 + LFE experience.

    Will get "The Haunting DTS" DVD soon to give a test of my DTS.
     
  5. raze

    raze Agent

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    Hi Guys,

    Here's something I came across while calibrating my new SVS sub using Avia.

    The alternating tones are much lower in volume from the center, right main, and surrounds than the left main.

    I would say the volume is about half of what is coming from the left main.

    Any ideas why this is?

    Thanks,

    R

    Equipment:

    Denon 3801, JVC 70BK(dvd player), Mitsu 65907, SVS 20-39CS+, Sampo 1000.
     
  6. Richard_s

    Richard_s Second Unit

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    Are all your speakers set to "small"?

    Did you check your Right/Left balance and adjust this first and then balance the center and surrounds to the Mains?

    Just a thought..
     

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