Originally Posted by AV-NUT-99
Another question on Subwoofer phasing adjust prior to running ARC. You may recall I have two Velodyne SC-12 passive subs running on the Velodyne amplifier in my equipment rack (same signal is sent to both subs from sub out on the D2). I have four phase settings that I can apply, 0 degrees, 90 degrees, 270 degrees, and 360 degrees. My understanding of the need for phase adjustment is to not cancel out the output of one sub by having a second sub out of phase (worst case would be 180 degrees difference from one to the other) and thus cancelling output. Since I can't set these independently (the same setting goes to each sub) the only other area of concern would occurr if my subs were out of phase with my main L/R ( Sig 8s ). But, when trying to do the initial output level setting, as clearly laid out by Bob, I am not able to use the radio shack meter to determine if I am having any phasing issue between the subs and my mains because the test sigmals are sent out to each speaker independently. When the sub level adjust signal is output, I tried switching to different phase positions on the Velodyne amp, but could not detect any difference in output level between any of the settings (which is what I would expect) since each sub would always be in the same phase as the other. The possibility exists though, that my subs could be out of phase with my Sig 8s without my knowing it. I used the 0 degree setting and I don't see any real anomolies in my graphs, other than huge peaks at 30Hz, 70Hz, and 110Hz on my front L/R. I see the high peak at 30Hz in the sub graph, but not at 70Hz or at 110Hz, in fact the sub out put rolls off very quickly past about 60 Hz. I would think I would at least see the 70Hz peak replicated in the raw graph of the sub since the room should effect these frequencies equally. Only other posibility is that my Sub placement has them in a "null" area for this frequency and my front L/R positioning could be catching peak areas for the 70HZ and 110HZ room modes as 2 feet could make a lot of difference. I do recall that we put the front L/R speakers at this poistion due to perceived better base out put.
I may have answered my own questions on this, which is what I usually end up doing due to stressing my brain so much trying to get my thoughts written down
, but I would be interested in further discussion on this or at least hearing you thoughts or difficulties, if any, with sub phasing.
FYI - my subs are currently placed in the front L/R corners of my 12 X 19 X 10 enclosed, dedicated theater room, approximately 2 1/2 feet behind the front L/R speakers with each sub facing to the rear opposite corner. The subs are single driver sealed - no ports.
With multiple subwoofers you have two problems to address:
You want to make sure all the subwoofers are in phase with each other -- so that there is no cancellation of the output of one sub by any other sub.
In addition, you want to make sure your combination of subwoofers is in phase with the main speakers so that the output of the subwoofers does not cancel out any of the output from the main speakers in the vicinity of the crossover where they are both active.
The typical procedure is to use the Left Front speaker as the surrogate for all the main speakers and to adjust each subwoofer independently to be in phase with the Left Front speaker. When all subs are in phase with the same main speaker then they are, as a result, also in phase with each other.
But to do this you need two things:
1) A way to send pink noise (noise that's all in the low frequency range) through both the Left Front speaker and each individual subwoofer (all by itself) at the same time, and
2) A way to separately adjust the phase and polarity of each subwoofer separately from the other subwoofers.
You can accomplish (1) by powering down all but one subwoofer at a time and then playing the pink noise test (usually labeled as a subwoofer phase test) from any of the common video calibration DVD discs, such as Avia, or Digital Video Essentials. If you are getting 5.1 input from the player then leave the Anthem in "NONE" audio mode. If you are getting stereo from the player then leave the Anthem in Stereo audio mode. The Movie or Music bass manager will steer bass to the subwoofer as well. [Note that you can not use the Anthem's own internal test tones to do this test.] Once you have adjusted for one subwoofer, power it down and repeat for every other subwoofer.
Accomplishing (2) depends on your subwoofer equipment. If I understand you correctly, the subwoofer amp you are using does not give you the ability to alter the phase of one subwoofer with respect to the other, and there's no way to get around that in the subwoofer amp (i.e., there's no separate output which bypasses that phase control). And that your subwoofers themselves do not have their own phase controls because they are passive (have no internal amp).
If that's the case then you are stuck. There is no way for you to alter the phase of one subwoofer with respect to the other.
So until you change subwoofers, what you should do is ignore the potential issue of the subs being out of phase with each other and just get the pair of them in phase with your mains.
To do that, set the phase control on the subwoofer amp to any fixed value -- 0 degrees would be my choice. Then run the pink noise from the calibration DVD (so that BOTH subs and your Left Front speaker are playing simultaneously), and use the Polarity and Phase controls in the Anthem itself -- Setup / Speaker Calibration / Move and Music -- to adjust the Polarity and Phase of your pair of subs (together) against the Left Front speaker.
By the way, with both subs powered by the same amp, and with the two subs symmetrically positioned either side of, and equidistant from, the main listening position, and with both subs being of the same construction, the odds are they will be in phase with each other without you having to do anything.
The worst that can happen is that you have one sub's wires physically reversed compared to the other sub -- meaning they will be 180 degrees out of phase.
So make sure you haven't done that and then concentrate on getting the pair of them properly in phase with the Left Front speaker and you should be in good shape.
It is also the case that the phasing difference between a pair of subs like this and the Left Front speaker -- even when the phase is 180 degrees wrong -- can be harder to detect because the slight variations in positioning (one sub is closer to that Left Front speaker) will naturally tend to smooth out the cancellation from either sub.
That is, it may be hard for you to determine the right setting of the Anthem's Polarity and Phase control because you can't hear the difference between "best" and "worst" setting when playing the 2 subs against the LF speaker. This is not a bad thing. One of the advantages of having multiple subs is that phasing issues naturally blend away.
But if so, you might want to try this:
Power up only one sub and adjust it against the LF speaker using the Anthem's controls. Note the setting you end up with.
Now power up just the other sub and do the same thing. If the result is different, try splitting the difference in setting in the Anthem, and use that as your candidate "best" setting.
Now that you have an idea where the "best" setting most likely happens to be, power up BOTH subs again and try again. Start with the "best" setting you just determined and then compare that to the setting 180 degrees away from that to see if you can hear any difference. If you can, then see if you can refine your "best" setting at all. If that's too tough to hear, just go with that "best" setting you've already determined.
It is a good idea to get the volume levels for your main speakers and subs set at least ballpark close before adjusting Polarity and Phase. Cancellation can be a subtle effect and it is easier to hear if the two speakers are putting out about the same volume. When running the pink noise test from the calibration DVD, set the Anthem's Main Volume control to a level that yields about 75dB SPL reading. That's loud enough for you to hear what you need to hear without being so loud that it becomes too painful to listen long enough to hone in on the "best" setting. Trust your ears and adjust Main Volume up or down from that as necessary so that you can complete this task. It takes a while to do a careful listen.
Again what you are looking for is the Polarity and Phase setting that produces the "fullest" sounding bass -- the least cancellation. In particular when comparing a subwoofer against the Left Front speaker, choose the setting that seems to produce the maximum amount of volume in the higher frequencies of the pink noise test as that's the area near the crossover where cancellation will occur.