Phase

Phase

Postby modz » Thu Aug 18, 2011 8:25 pm

So it seems that a lot of people (including myself) are confused about speakers being out of phase with each other. Most of us know that JBL's are out of phase with other speakers. I wire my JBL cabinets backward just in case someone has an extension cab with non JBL's. I have talked to prominent sound guys that think one guitarist using JBL's wired normally and the other guitarist using JBL's wired backward are out of phase with each other. I hand delivered a cabinet to a top band and their sound guy immediately reversed my wiring and he said it would be out of phase with the rest of the instruments in the band. I didn't argue the fact with him. I figure if a guitarist could hit the exact note at the exact millisecond as another guitarist then maybe.
My belief is only program music from the same source can be out of phase with each other since no 2 guitarist can play the exact notes as the other.
Any thoughts?
I was in my studio and had 2 different guitar tracks and I reverse phased one track and I did hear phase cancellation. Have I been wrong in my thinking?????
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Re: Phase

Postby SarnoMusicSolutions » Thu Aug 18, 2011 10:09 pm

Tracy, I'd say you're spot on. It's a pretty good rule of thumb to wire JBL's with reversed polarity. You seem to have a much better grasp of phase and relative phase than this soundman you're referring to.

The thing that soundman said about one amp's speakers being out of phase with other players in the band is an example of a poor understanding of phase/polarity, and phase coherence. Phase coherence being the least understood of the issues. And the guy, by reversing Tracy's wiring actually did the opposite of what he was probably thinking he wanted.

BUT, it absolutely makes no difference if one guitar player's amp is out of phase with another guitar player's amp. No difference at all. The speakers are not going to be delivering the same exact signal and most definitely are not going to deliver the same signal from the very same point in space since the cabinets are in different locations and could never occupy the same point in space. This soundman need to consider that many of the effects pedals or amp channel switchings that happen in a guitar rig also reverse polarity (absolute phase) in the exact same manner that speaker polarity reversal does. If a guitar player is using a Fender amp, say, like a Twin or Pro, and that guitar player unplugs from the reverb channel and then goes over to the "normal" channel, he has just reversed his polarity since those two amp channels are 180 degrees out of phase with eachother. Is that a problem on stage with the other players? Of course not. If a guitar player has a Stratoblaster with a bypass switch and flips it on or off, he just reversed his polarity. Is that a problem, obviously not. And many amps with overdrive channels will also flip the phase as the drive channel is engaged. Obviously this doesn't suddenly make one's guitar disappear or cancel out anothers'. Relative speaker polarity from one player to another is simply and completely a non-issue.

Phase is an issue among speakers that are mounted in the same speaker cabinet and that are fed the same signal, or among multiple cabinets fed by the same amp rig. If one speaker WITHIN THE SAME RIG is going the opposite direction as another, then we have a problem.

Like Tracy said, no two players could ever play the very exact same note with the exact same amplitude and phase response and harmonic content at exactly the same time down to the microsecond thru the same speaker cab. Those are the conditions where this could even begin to be an issue, and that never would happen in the real world. Two different players with two different rigs that are naturally in two different locations in space will have such radically non-coherent signal sources that they will never cancel each other out as this soundman may have been talking about. Total and utter hogwash to suggest such a thing. I get the theory behind what he may have been thinking, but that conclusion comes from a limited understanding of all the physics at work and the realities of non-coherence and phase, and especially how it relates to guitar signals. Even when the two guitar amps get mic'd and mixed together at the board, perhaps even summed to mono, the two different guitar signals will naturally have plenty of time, phase, pitch, and harmonic differences that they will still both be heard and won't cancel eachother out, no matter which way ones speakers are wired relative to the other.

Now maybe this kind of thinking of the soundman would be better applied to the two kick drums in a two-drummer band where if both drummers happen to nail their kick hits at precisely the same time, having the two mic's in phase with each other would prevent the lowest frequencies from canceling each other out in that brief moment. I get that application of theory in dealing with transients and low frequencies. But the theory and physics behind that thinking fall apart as you go up in frequency and are dealing with guitar signals.

So I'd have to agree with Tracy sticking with the popular convention of wiring JBL's in reverse, just in case they ever are paired up with "normal" speakers. Since basically all other speakers are +Voltage-outward and -Voltage-inward, this rule of thumb helps us avoid situations of phase cancellation in multi-speaker cabinets or multi-cabinet amp rigs.


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Re: Phase

Postby modz » Fri Aug 19, 2011 12:37 am

Thanks for the in depth reply Brad. Wiring JBL cabs backward seems like a good rule of thumb to me. Anyone ever use a JBL and say an EV 12L together, wired the same? Not pretty. Kind of reminded me of a wah pedal half open. Interested to hear a counter view on this. Sound guys that are musicians for me please.
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Re: Phase

Postby JamminJommy » Fri Aug 19, 2011 2:36 am

Having run sound for a lot of heavily baffled pit orchestra sound as well as a LOT of jazz, I agree with Brad.

I don't think I've ever run into a phase issue that wasn't room related. The instruments and PA have never been the culprit (with the exception of the time I ended up with a reverse-phase wired XLR cable. Don't ask.) Stereo, mono, it makes no difference. Take Jazz Choirs, even. The best jazz singers have impeccable intonation, but no matter how well the blend, no matter how identically they sing into identical mic's, they still possess a different voices and therefore... no phase problems. Same goes for any and all instruments when mic'd. At this point I believe I'm just restating what Brad has said. Phase/Shmase. Just pink the room when you get there and you can go from there.

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Re: Phase

Postby strumminsix » Fri Aug 19, 2011 6:54 am

Can speakers not in the same circuit be out of phase with each other? I don't think so.

Personally, I wire my cabs so that if I hooked up a battery to test the speaker pushes outward.
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Re: Phase

Postby modz » Fri Aug 19, 2011 11:45 pm

strumminsix wrote:Can speakers not in the same circuit be out of phase with each other? I don't think so.

Personally, I wire my cabs so that if I hooked up a battery to test the speaker pushes outward.


Right this is how I have always understood it too. I'm not the type of guy to call a sound guy out but I'm glad to hear others with the same beliefs. He wasn't a musician though:) The best sound guys are musicians too and understand guitar TONE comes at high volume. ha ha ha.... As long as both speakers (from one amp) push out with 9 volt battery pos to pos pole of speaker, it's all good.
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Re: Phase

Postby strumminsix » Sat Aug 20, 2011 12:47 pm

modz wrote:
strumminsix wrote:Can speakers not in the same circuit be out of phase with each other? I don't think so.

Personally, I wire my cabs so that if I hooked up a battery to test the speaker pushes outward.


Right this is how I have always understood it too. I'm not the type of guy to call a sound guy out but I'm glad to hear others with the same beliefs. He wasn't a musician though:) The best sound guys are musicians too and understand guitar TONE comes at high volume. ha ha ha.... As long as both speakers (from one amp) push out with 9 volt battery pos to pos pole of speaker, it's all good.

Right on, Tracy!!!! :hd:
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Re: Phase

Postby jeffm725 » Tue Aug 23, 2011 2:51 pm

I understand Phase issues to be pretty much exactly how Tracy and Brad describe them. It is a simple concept backed by some actually pretty involve theories.

Go to Steve Kimocks site, (message board) or seek out some of his posts on the Gear Page. He is crazy in tune with Phase issues. He is very detail oriented. He knows which pedals "invert phase" knows which buffers "invert phase" (The CB2 being one of them) and all the way down the line. I have never seen someone so actually involved with every minute detail of their signal path.
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Re: Phase

Postby zoooombiex » Wed Aug 31, 2011 8:20 am

jeffm725 wrote:I understand Phase issues to be pretty much exactly how Tracy and Brad describe them. It is a simple concept backed by some actually pretty involve theories.

Go to Steve Kimocks site, (message board) or seek out some of his posts on the Gear Page. He is crazy in tune with Phase issues. He is very detail oriented. He knows which pedals "invert phase" knows which buffers "invert phase" (The CB2 being one of them) and all the way down the line. I have never seen someone so actually involved with every minute detail of their signal path.


I was going to suggest the same thing. Kimock often focuses on absolute phase, i.e., the phase relationship between the guitar string and the speaker output, which is a bit different than phase of multiple speakers or phase between different players. Good reading though.
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Re: Phase

Postby SarnoMusicSolutions » Fri Sep 02, 2011 6:13 am

Great point. The absolute phase within the closed loop of a guitar rig argument is well demonstrated in Kimock's world as well as the far more common situation with an amplified acoustic guitar. Many acoustic preamps have a phase switch to help find the best setting that doesn't induce feedback. Kimock has found that he can sense and feel if the speaker polarity is jiving with the pickup polarity and helping reinforce the string and pickup energy. In this case of absolute phase (180 degrees) reversals, there's something to be argued regarding the way the acoustic energy from a speaker reacts back into a guitar. Kimock likes a polarity toggle switch on his electric rigs so he can choose the one that feels and sounds better to him.

But that is a fundamentally different argument, as you pointed out. The big confusion started this thread when a soundman wanted one guitarist's speaker cabinet polarity to be similar to the polarity of another's speaker cabinet across the stage. That's where the whole thing broke down into bad science.



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Re: Phase

Postby randy112 » Sun Aug 19, 2012 1:22 pm

Yea well keep working on it, the internet is grate for wannabe musicians, you should consider opening a YouTube account and posing your music on there. Many famous artists today were discovered on there.




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