HELP me with dual subwoofer phase controls - AVS Forum
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Old 12-18-2012, 05:11 PM - Thread Starter
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For those with dual subwoofers placed at the same distance to your couch, do you set the phase for both of them at zero?

Here's a guide from Brian (Rythmik):
Quote:
How to set up multiple subs?
One cool feature of phase/delay control on the plate amp is it allows you to compensate the phase difference arise from difference in physical distance. To use this feature, one needs to use line in, instead of LFE in. The purpose is to make sure that direct sound multiple subs arrive at the listening seat at the same time so there is no cancellation in sound. To do that, use the farthest sub as the reference. Then measure the physical distance difference in ft. Note the sound speed is about 1 ft per milli-second. As an example, say the difference is 5ft, then set the phase/delay knob on the closer sub at 5ms. Then one can use room EQ program to calibrate both subs at the same time.

Here's a feedback from forumer neutro:
Quote:
I finally put my subs in a symmetric position each side of the TV, so equidistant from the main listening location. Strangely I found that using 180 deg. phase difference between the subs both sounded the best and managed to get rid of a nasty null in the crossover region. So phase sometimes have unexpected consequences. The best is to try phase difference from 0 to 180 degs to see what sounds better, simply. In theory, with equidistant subs, setting one at 0 degrees and the other at 180 will create a dipole, and the listener will be exactly on the dipole's null boundary (perfect cancellation). In practice it's not necessarily the case. First, woofers can be wired in opposite polarity in the two subs -- which would require setting the phase to 180 degrees immediately (or changing the woofer's polarity). Second, this is only true in an anechoic setting. Inside a room, interactions with the room will create standing waves and room modes will be dominating the frequency response. What reaches the listener is a combination of sound directly emitted from the subs and sound reflected by the room boundaries. With subs the first component is generally not the strongest unless you sit beside the sub.
Regarding the "anechoic setting", is it true?

Here's Brian's reply regarding setting different phase controls (such as sub1 at 0 degree and sub2 at 180 degree) for subs located equidistant:
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Don't listen to those. Advise like that can work in a particular room and sub placement combination. It will cause cancellation of direct wave at your listening position. So what is left is the room reverberation (or in other words, echos in your room). The sound stage can completely collapse. That is not what we want.

You can try with your front speakers, if you connect them in phase, the sound stage is between your speakers. If you connect them out of phase, the sound stage becomes blurry. If your front speakers have excellent phase response, you will again hear very clear sound stage, but this time, the sound stage appears to be on your left and right sides with very little between the speakers. Now the sub is out of phase, but the front speakers are not, so what do we get? It is something in between. I for one think correct phase reponse is above everything. I have heard people wiring woofers out of phase with tweeters in order to fix an out of phase problem. This even happened to well known speaker designers. But those "fixes" have never become standard practices. In another words, you will never hear some speakers become outstanding because they wire the tweeter out of phase with woofers. They are....merely fixes.

Here's forumer LarryU's comment:
Quote:
If the subs are equal distances from the listening position both subs need the phase set to Zero.

Considering this diagram as an example of a non-symmetry room:

... sub1 is next to a large window but sub2 is next to a brick wall. Both phase controls at zero? ...or may be different? Please help.
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Old 12-18-2012, 06:06 PM
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I have always left the subwoofer phase on "0". I left it up to the reciever or subwoofers EQ to handle the subwoofer setup. Some subwoofer manufactuers do not put phase controls on their subs.
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Old 12-19-2012, 04:49 AM - Thread Starter
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Originally Posted by bsoko2 View Post

I have always left the subwoofer phase on "0". I left it up to the reciever or subwoofers EQ to handle the subwoofer setup. Some subwoofer manufactuers do not put phase controls on their subs.
My AVR does not have SubEQ HT for dual sub tuning... hence the need to tune manually.
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Old 12-19-2012, 10:40 AM
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Quote:
Originally Posted by Skylinestar View Post

My AVR does not have SubEQ HT for dual sub tuning... hence the need to tune manually.

Then I would get the Anti Mode and let it EQ your subs. I have used the Anti Mode in the past with success and I gave it to Todd for his subs http://www.avsforum.com/t/1441048/quad-subwoofers-in-real-world-living-room
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Old 12-20-2012, 08:34 AM
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Just for the record: my subs are currently set at 0 and 180 degrees, but I didn't notice much change in SPL at any phase settings (max 2 dB difference on the whole range). The reason why I used 180 degrees was that it improved a null I had in the crossover region quite a bit. Now Snowmannick and Ed Mullen from SVS both suggested that I revert the phase to 0 for both subs and tweak the sub distance setting in my AVR to fix the null instead. I'll be doing that shortly.

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Old 12-20-2012, 02:55 PM
 
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Quote:
Originally Posted by neutro View Post

Just for the record: my subs are currently set at 0 and 180 degrees, but I didn't notice much change in SPL at any phase settings (max 2 dB difference on the whole range). The reason why I used 180 degrees was that it improved a null I had in the crossover region quite a bit. Now Snowmannick and Ed Mullen from SVS both suggested that I revert the phase to 0 for both subs and tweak the sub distance setting in my AVR to fix the null instead. I'll be doing that shortly.

As phase is changed, your ear won't necessarily pick up the difference but as the phase is changed, so will change your SPL/dB graph. If one has a continuous phase control, it's quite interesting to see what happens to a graph as the phase is slowing changed.
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Old 12-20-2012, 03:28 PM
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I have 2 subs, an svs pb12-nsd/2 and pb12-plus and an onkyo 5010 receiver. I had the plus set at 0 phase and the nsd/2 at 180 and receiver had them set at -4 and -4.5 volume. I then changed the nsd/2 to 0 phase and reran audessy and the receiver set them both to +3 I believe. Also when I turned off the nsd/2 it didn't seem to make a difference after setting to 0 phase. I then reversed the wiring in the nsd/2 as per svs and set both subs to 0 phase and the receiver has set them to the -6 range each now. That tells me the output is maybe louder now and the polarity must have been reversed. Also the volume is noticeably lower after turning the nsd/2 off. Hope I did things correctly here. I AMA newbie and know nothing about this really so hopefully doing the right things here.
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Old 12-20-2012, 03:38 PM
 
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Quote:
Originally Posted by TommmyJ View Post

I AMA newbie and know nothing about this really so hopefully doing the right things here.

In honesty, without a real time analyzer (RTA) program, you're SOL regarding how phase affects the acoustics of a room. The good news, as long as you don't smoke your sub's amplifier, you're good to go.

Basically, what's happening with phase and the act of changing the phase, the in/out of phase drivers or timing of the push of the cone is being changed and if two cones are playing simultaneously, the frequencies being produced can cancel each other out. Not good. Some people can hear the difference but my ears are not good enough to hear a difference but if running a RTA program, you can see the changes taking place on the screen, in realtime.

It actually is quite interesting to see peaks and valleys in your graph develop as the phase control on one of the subwoofers is changed.
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Old 12-20-2012, 03:49 PM
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I knew there was a difference and something was wrong as per Ed Mullen's comments in the SVS thread when I changed my NSD/2 to 0 phase and it had bad effects on the sound. Like I said I could turn the NSD/2 off and it didn't even seem like it was on at all. So by turning my NSD/2 to 180 before it was partially fixing the polarity issue but not 100%. When I have reversed the wiring now it seems to have a boost so it seems to be alright. When you say as long as I don't smoke the amp do you mean just the process of messing with the wires or do you mean sometimes the amps can go due to switching the wiring around like this? Thanks
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Old 12-20-2012, 03:49 PM
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As requested by the OP, I'm posting my thoughts here also.

First, I always recommend that dual subs be checked to ensure they are both the same absolute polarity. Two subs with opposite polarity is actually more common than one might suspect, particularly if they are different brands, vintage, use different amp platforms, or possibly one has had an amp replaced by the owner.

Second, both subwoofers should be verified to have positive absolute polarity. This means the woofer cone will move outward when the test signal waveform swings positive. This is important because if the subwoofer is exhibiting negative absolute polarity, it will be out of phase with the front stage loudspeakers at the selected speaker/subwoofer crossover frequency. This will manifest itself as a notch-out in the combined FR and also disembodied sounding bass with soundstage collapse. The more basic AVR auto-set-up systems on the market will typically not detect if a subwoofer is negative absolute polarity. It will simply measure the time it takes for the signal pulse to reach the mic and then set the sub distance accordingly. The more sophisticated auto-set-up systems do have an absolute polarity check, but it's not always completely reliable due to room reflections. The easiest way to confirm a self-powered subwoofer has positive absolute polarity is with an acoustic polarity tester. Failing that, there is aother method which takes longer, but is equally valid.

Third, if both subs are the symmetrically located in the room and the same distance to the LP, generally I recommend setting the phase to 0 on both subs and allowing the AVR to set the subwoofer distance. If the owner has the ability to measure the combined FR of the LCR front stage speakers and the subwoofer(s), the sub distance can be tweaked at this point to obtain the smoothest FR with the least amount of notch-out at the speaker/sub crossover frequency. Sometimes the AVR auto-set-up sub distance isn't always perfectly optimal in this respect, and small tweaks can pay dividends to minimize phase/FR issues in the speaker/sub transition band.

Fourth, if the two subs are a different distance to the LP, as Brian recommends above - if the subs are equipped with a variable phase control, it can be adjusted on the closer subwoofer to better integrate/reinforce with the farther subwoofer, and this will then make the single AVR sub distance setting more valid for both subs.

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Old 12-20-2012, 05:02 PM - Thread Starter
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Ed, thank you for the great info and tips.
So the comment about anechoic environment is not true?
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Old 12-20-2012, 05:04 PM
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Quote:
Originally Posted by Ed Mullen View Post

As requested by the OP, I'm posting my thoughts here also.
First, I always recommend that dual subs be checked to ensure they are both the same absolute polarity. Two subs with opposite polarity is actually more common than one might suspect, particularly if they are different brands, vintage, use different amp platforms, or possibly one has had an amp replaced by the owner.
Second, both subwoofers should be verified to have positive absolute polarity. This means the woofer cone will move outward when the test signal waveform swings positive. This is important because if the subwoofer is exhibiting negative absolute polarity, it will be out of phase with the front stage loudspeakers at the selected speaker/subwoofer crossover frequency. This will manifest itself as a notch-out in the combined FR and also disembodied sounding bass with soundstage collapse. The more basic AVR auto-set-up systems on the market will typically not detect if a subwoofer is negative absolute polarity. It will simply measure the time it takes for the signal pulse to reach the mic and then set the sub distance accordingly. The more sophisticated auto-set-up systems do have an absolute polarity check, but it's not always completely reliable due to room reflections. The easiest way to confirm a self-powered subwoofer has positive absolute polarity is with an acoustic polarity tester. Failing that, there is aother method which takes longer, but is equally valid.
Third, if both subs are the symmetrically located in the room and the same distance to the LP, generally I recommend setting the phase to 0 on both subs and allowing the AVR to set the subwoofer distance. If the owner has the ability to measure the combined FR of the LCR front stage speakers and the subwoofer(s), the sub distance can be tweaked at this point to obtain the smoothest FR with the least amount of notch-out at the speaker/sub crossover frequency. Sometimes the AVR auto-set-up sub distance isn't always perfectly optimal in this respect, and small tweaks can pay dividends to minimize phase/FR issues in the speaker/sub transition band.
Fourth, if the two subs are a different distance to the LP, as Brian recommends above - if the subs are equipped with a variable phase control, it can be adjusted on the closer subwoofer to better integrate/reinforce with the farther subwoofer, and this will then make the single AVR sub distance setting more valid for both subs.

Does it follow that if you also use your main speakers as a bass source, in addition to dual subs, that it would be important to verify their polarity as well?

 


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Old 12-20-2012, 05:09 PM
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Quote:
Originally Posted by Spanglo View Post

Does it follow that if you also use your main speakers as a bass source, in addition to dual subs, that it would be important to verify their polarity as well?

Yes. This can be done with a battery. Once you verify the speakers are positive absolute polarity, they can be used as a tool to help determine if the subwoofers are also.

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Old 12-20-2012, 05:17 PM
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Very good, thank you.

 


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Old 12-20-2012, 07:46 PM
 
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Quote:
Originally Posted by TommmyJ View Post

When you say as long as I don't smoke the amp do you mean just the process of messing with the wires or do you mean sometimes the amps can go due to switching the wiring around like this? Thanks

I was teasing you in reference to your comment when you posted"

Quote:
I AMA newbie and know nothing about this really so hopefully doing the right things here.

So, as long as you don't see any smoke coming out of your subwoofer, you're good. tongue.gif
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Old 12-21-2012, 07:45 AM
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Quote:
Originally Posted by BeeMan458 View Post

In honesty, without a real time analyzer (RTA) program, you're SOL regarding how phase affects the acoustics of a room.

I did use REW's RTA tool while playing pink noise at it was indeed quite interesting. However phase adjustments, for me, had little impact on the overall frequency response I got at the listening position. It did change the null in the crossover region but as Ed said, it may be better tackled by adjusting the sub distance in the AVR settings. I'll do that as soon as I can have some quality time alone with my HT setup biggrin.gif

Another interesting observation: sometimes the overall SPL was going up a bit playing with phase but this was apparently only due to a room mode that was boosted. So max SPL is not necessarily the best way to adjust phase. It also shows that I could not obtain any important cancellation of the two subs whatever the phase setting was. This is why I said that room interactions / reflections / modal resonances were dominating. If they weren't, obviously I should have strong cancellation around 80 Hz setting the phases at 0 and 180 if the subs are equidistant.
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Originally Posted by Ed Mullen View Post

As requested by the OP, I'm posting my thoughts here also.

A big thanks -- your opinions are enlightening as always.

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Old 12-21-2012, 07:57 AM
 
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Quote:
Originally Posted by neutro View Post

Another interesting observation: sometimes the overall SPL was going up a bit playing with phase but this was apparently only due to a room mode that was boosted

Your above highlights the need for a RTA as your above shows change took place and each room has it's unique foibles. With a RTA, one can see these changes as they take place in realtime as opposed to guessing, based on what they hear.

In our case, I used the phase control to eliminate or reduce nulls and used Anti-Mode to attenuate peaks.

Still working on the above as currently this is an in session exercise.

"Day three.....had time to sleep on the why of the room null as the phase seemed to not be phasing me."

"Day four.......the loneliness of driving the family crazy with test tones is taking it's toll."
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Old 12-21-2012, 09:04 AM - Thread Starter
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Quote:
Originally Posted by BeeMan458 View Post

I used the phase control to eliminate or reduce nulls
Is it possible that by adjusting phase control to eliminate a null is a wrong thing to do?

According to Brian:
Quote:
So what is left is the room reverberation (or in other words, echos in your room). The sound stage can completely collapse. That is not what we want.

???
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Old 12-21-2012, 10:00 AM
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I have 3 subs. One on channel 1 and 2 on channel 2. I have audyssey sub eq. It will set the distance for each channel correctly, but if you have 2 on one channel, it obvioulsy can't set the distance accurately. What I did was adjust the phase on the closer of the 2 subs so that the SPL as measured by audyssey was at it's maximum. I figured this was the point where the 2 subs were most in phase. After that, I ran audyssey setup as usual.
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Old 12-21-2012, 11:00 AM
 
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Quote:
Originally Posted by Skylinestar View Post

Is it possible that by adjusting phase control to eliminate a null is a wrong thing to do?

Not if the resulting mix is an improvement.

After having gotten REW up and running, I'm a firm believer that having RTA capabilities is a must have for those who care about this type of stuff.
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Old 12-21-2012, 11:14 AM
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Quote:
Originally Posted by Skylinestar View Post

Is it possible that by adjusting phase control to eliminate a null is a wrong thing to do?
Quote:
Originally Posted by BeeMan458 View Post

Not if the resulting mix is an improvement..

Depends if it is a improvement over which situation. If tweaking the sub distance setting on the AVR while leaving the phase settings on the subs alone (at 0) also eliminates the same null (which remains to be seen in my case), then yes, tweaking the phase instead could be the wrong thing to do. It may be an improvement over no tweak at all but the correct tweak can be better. And I speak as someone who tweaked the phase to eliminate a null. As soon as I can try the alternative method I'll report back but that may have to wait a week or two considering the upcoming holidays.

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Old 12-28-2012, 07:53 AM - Thread Starter
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I have an experience about this before (equidistant subs):

When both phase controls are at zero, I got a null at 30Hz. When one is at zero & the other is at 90 degree, the null is removed, producing a flatter response (measured on REW software).

Now...the tough question is: Am I doing it correctly or wrongly? confused.gif
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Old 12-28-2012, 09:44 AM
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Quote:
Originally Posted by Skylinestar View Post

I have an experience about this before (equidistant subs):
When both phase controls are at zero, I got a null at 30Hz. When one is at zero & the other is at 90 degree, the null is removed, producing a flatter response (measured on REW software).
Now...the tough question is: Am I doing it correctly or wrongly? confused.gif

Sounds like you did something good. Multiple subwoofers in a room will, more often than not, produce results that are unpredictable. The best way to get the best sound is through random trial and error.
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Old 12-28-2012, 09:15 PM - Thread Starter
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Quote:
Originally Posted by KidHorn View Post

Sounds like you did something good. Multiple subwoofers in a room will, more often than not, produce results that are unpredictable. The best way to get the best sound is through random trial and error.
I thought the hard rule is that phase gotta be zero when the distant is the same confused.gif
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Old 12-28-2012, 09:47 PM
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IMHO, there are no hard rules. Measure and adjust accordingly.
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Old 12-29-2012, 07:52 AM
 
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Originally Posted by Stereodude View Post

IMHO, there are no hard rules. Measure and adjust accordingly.

The above sounds like a hard rule to me. tongue.gif

Agreeing with you when I post, a recommendation to all is to get some measuring gear so they can physically see the differences, different settings make on frequency sweep curves. Without measuring gear, everybody, despite their God given abilities are wishing and hoping and guessing. I have our continuous phase settings, set opposite to each other. The left sub's continuous phase setting control is set to two o'clock and the right sub is set to ten o'clock and this best setting is verified by a RTA, REW provided frequency sweep reading..

Getting OmniMic is the easiest route. Three hundred bucks, yes. But the improvement and personal satisfaction one gains in subwoofer performance, makes it money well spent. The hard route is downloading REW and buying all the necessary recording gear. Having the ability to set the time domain to correct for placement distance differences, at main listening position (~1ms/foot), is also a plus.

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Old 12-29-2012, 06:29 PM - Thread Starter
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Thanks for the info.
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Old 12-29-2012, 07:37 PM
 
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