Can anyone shed some light on my xo ?

Discussion in 'Archived Threads 2001-2004' started by RobP, Aug 26, 2001.

  1. RobP

    RobP Stunt Coordinator

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    Funny thing just happened. Well let me first say I am using an svs 20-39 and my receiver has a 100Hz fixed xo. Ok, so anyway I was doing a frequency sweep with THX optimode's 200-20Hz sweep and an spl meter, and I discovered a drop off around the point in which the sub is taking over. It was only a couple decibels but it was there. Well out of curiosity I turned the subs xo on and set it at 80Hz and reran the sweep expecting to find a loss of information between 80-100Hz. Instead I was shocked to find the drop off was gone it seemed much flatter through out. Although still a little variance it was not as drastic as it had been with the sub xo bypassed. I was kind of baffled so I messed around with it some more and kept finding the same results.
    One other thing I'd like to mention is, as the sweep neared 20Hz. There was a 4-6 db increase. This is because I run my sub 4-6 db "hot" correct?
    If anyone will enlighten me I will be much appreciative.
    Rob
     
  2. RobP

    RobP Stunt Coordinator

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    Come on guys some one has to have some idea why this is happening. Help me out here. I was thinking maybe the way I was testing it was inacurate? But my ear seems to agree with the spl meter, I don't know.
     
  3. John Chow

    John Chow Second Unit

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    Ok, here's maybe a try, apologies if you did everything correctly. Now when you say you turned the Sub's xover on, did you flip the cross-over bypass switch to on or off? Flipping the switch to on turns the crossover off (since it's bypassing it), and vice versa. This would only make sense if you had the crossover frequency at 80Hz the whole time. If you had it higher than that, it shouldn't matter. In fact, that's another test. Try turning the crossover frequency to the highest, and rerun the switches w/ and w/o the crossover. Another thing to consider, maybe your sub/speaker placement is causing weird cancellation effects, so that if the cross-over is not on, then the sub is outputting some information (due to the presumed 2nd-order nature of your receiver's crossover) and your speakers are sending out information that my be cancelling with the subwoofer. When you turn the cross-over on, perhaps the subwoofer is no longer outputting sound so no cancellation occurs.
    For your other question, I would say that it is 4-6dB higher due to room issues. I assume the 4-6dB hot should be happening across the subwoofer frequency range, not just at 20Hz. Also, are you compensating for the Radio Shack SPL meter at lower frequencies? At 20Hz, if the meter says 70dB, in actuality it's around 73-75dB, not sure of the precise number since i don't have the correction chart in front of me.
     
  4. RobP

    RobP Stunt Coordinator

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    Thanks for the reply John. I figured it out. I was indeed having some bass cancelation issues. I started playing with the subs phase control knob (which I had never really played with before). After taking some measurements, I discovered I had had the phase control knob set in the worst possible spot. After adjusting to a more appropriate spot the sweep did indeed flatten out. Thanks again for pointing me in the proper direction.
    Rob
     
  5. John Chow

    John Chow Second Unit

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    No prob. Looks like I forgot to actually mention phase though [​IMG] We did a couple of frequency sweeps at my friend's place who also has an SVS and found that changing that phase dial can definitely make a world of difference.
     

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