Cutting through the mix- Phase Cancellation and Inversion -

[billyvitale]

I would like know if anybody has insight on a "quest" I have in terms of understanding guitar sound in a live setting s.. and that is the question of "Phase"... My experience teaches that in certain live playing situations I get a good sound and can focus on playing.. in others the sound is just not there and I spend most of the engagement struggling with tweaking nobs .. if I can (sometimes playing through a PA for example simply means you are confined to trying to tweek some effects).. In general my Fender Strat seems to be better at "cutting" through .. but even with that sometimes I struggle.. I have been told that it is a "Phase" issue.. just like with speakers.. if you guitars phase is not in sync with something else you will get cancellation which screws up your sound.. and no matter what you do it will be very hard to fix this. Even playing louder .. which doesn't help anyway cause if you do that the others will follow (:-)!)... anyway looking forward to your response.

I do know there is a phase inverter circuit that is part of Tube amps basic inner workings.. I also know that expensive Sound ports include a phase inversion capability.. is this something that can be built so that the guitarist can try modifying the phase or have a sound tech do it.. such that cutting through the mix becomes something that can be better controlled?

Any ideas suggestions or insight is really appreciated..

[Alan0354]

There is always an easy to invert the signal, like adding a tube. BUT, I don't think that will help. The reason is because there is a lot of different frequency from the guitar. The sound you hear is from the amp plus the reflection of the sound from the surrounding......that you have no control in a live gig. Sound travel very slow, say the speed of sound is 600 miles per hour, it's 10,560" per second. If you look at a 1KHz signal, the wave length is 10560/1000=10.56"!!!! So you move 5" you invert the phase of the 1KHz sound. For 2KHz, it is 5.28", so you move 2.6", you invert the phase.

And different frequency has different wavelength and the phase is all different when reflected back. What you say might work a little better if you are in a recording room where the reflection is minimized or tightly controlled. But then you still have to find a sweet spot for you amp to make the inverter helps.

[Austin]

Open back cabs have less phase issue than closed back. A closed back cab could be out of phase with your bass player or another closed back cab and that would cause some decrease in volume of low frequencies because of phase cancelation. I always thought it would be cool to have a pedal that would switch the two leads of the guitar cord to be able to try both and see which sounds better.

[Enzo]

The phase inverter inside yoour amp has nothing to do with the phase of sound leaving the speaker. The phase inverter takes the preamp signal and passes it along to one power tube as is, then also to the other power tube but "out of phase", meaning in reverse polarity. This allows the two tubes to take turns, like a two-man lumberjack saw cutting down a tree. When one guy pulls, the other guy pushes. If they both pulled, nothing would move.

I could be wrong, Alan, but I learned in my physics class that 60 miles per hour was 88 feet per second, meaning 600 miles per hour ought to be 880 feet per second. And even that sounds low to me for sound in air. But even so, a 1kHz note will have a wavelength of about a foot. So your point is well taken about moving around.

In fact when I used to run sound, I would set up some music then walk across the sound field and I could hear phasing as I walked along - the distance to one speaker versus the other of course varied as I walked. I even hear it on TV shows like sitcoms, where the actor is wearing a hidden wireless and there is also a booom mic. As he moves and bends over, I can hear phasing effects. SUbtle but audible. Civilians would never notice.

Billy, it takes two sound sources for you to have a phase problem. Two out of phase sources can cancel at your ear. A simple experiment you can do is this: Play some music through your stereo system. Listen long enough to get a good idea what it sounds like. Now reverse the wires on just ONE of the speakers. Left or right, I don;t care, but just one. Now play the same music. You should hear an obvious difference. The speakers are now out of phase and they cancel. But unplug one or the other and each sounds OK by itself. MY usual description is that in good stereo, the sound SEEMS to come from center between the speakers and has a full sound. Out of phase speakers tend to seem like nothing is in the middle and all the sound seems to come from "somewhere else." Not only that, most of the bottom end will be missing. Point of the experiment is to demonstrate speaker phasing effects.

For you to have a phase problem on stage, I would think you'd either have to have two cabs, one out of phase with the other. OR you have a PA monitor in addition to your amp. If the monitor sound is out of phase with your amp speaker, you could get some cancellation. Have the sound guy hit the invert button on that mixer channel for a quick check.

The absolute phase of the guitar speaker doesn't matter so much. SOme golden ears say they have a preference, but most of us couldn't tell you which way ONE speaker was wired. But when more than one speaker is involved, then they all have to be in agreement.

[Alan0354]

Haha, I found my mistake and I covered it up already. I have the new numbers in the post already. Yes, it is really to show the point, it's Saturday!!! Acoustic is a totally different animal because of the speed of the sound. It is almost like microwave transmitter and receiver where the wave length is so short you have all the doppler effect and phase shift. I think you might need an EM antenna program to find the effect. Doing it by switching EM to sound wave and change from light speed to sound speed to simulate!!!!

[Chuck H]

billyvitale, I don't think that phase is the problem. There will always be small phase cancellations in a room with sound happening inside and these acoustic anomolies are percieved as natural. The exception being as Enzo described above. If you have two identical, but out of phase waveforms, there will be considerable phase cancellation. This, also as Enzo described, may occur with a stage monitor setup, if your using PA support and the PA and your amp are both playing, if you have multiple cabinets or a single cabinet with multiple speakers. In these circumstances you need to be sure that everything is in phase. To extrapolate one scenario... If you mic your guitar cab and have that PA channel feeding a stage monitor and there is no inversion, there will be phase cancellation because the monitor is facing one way and your amp is facing the other. OK... Next...

Cutting through, IMHE, is a matter of trying to occupy part of the frequency spectrum that the other musicians aren't crowding. For example, if the singer is bumped at 1K, the snare is bumped at 1K and the bass player is pushing high mids for his tone at 1K, you don't want to be anywhere near 1K. Which is difficult because 1K would be about right for guitar. I've never liked the term "cutting through the mix". To me this implies undeniable top end. Which isn't exactly what players want. From this we expound to "bright, but not harsh" or "clear but not shrill". Guitarists want to sound fat and rich with a little top end for definition. Why should we be pushed into and isolated to occupy only frequencies above 1.5k and below 4k? Any good "mix" has each instrument occupying frequency points different enough that the percieved result is clarity. Listen to any good recording with this in mind and you'll hear what I'm talking about. This same affect needs to be achieved on stage for a good live mix. Several venue based factors complicate the issue too. Which is why a good sound guy, or learning to be a good sound guy yourself is the difference between mush and a good show. But once you have all band members in their own spaces (in the mix) it should be a lot easier to tweak for a given venue. If you change rigs a lot, like using an amp for one gig and a POD into the PA for the next and a different amp that needs mic'ing for another you will never nail down your own space in the mix with this particular show and band.

Hope this helps.

[Steve Conner]

+1. The only thing I would add is:

Phase cancellation (interference) can only occur between multiple copies of the same audio signal. So, if you had multiple speaker cabinets hooked up to your amp. Or, the direct sound from a single speaker could interfere with the same sound reflected from a nearby wall. Or again, the sound from the front of an open-backed cabinet could interfere with the sound coming from the back, which is what gives an open-backed cabinet its distinctive sound.

You can't get phase effects between two different sounds, like the lead and rhythm guitars, as there's no phase relationship between them at all. In signal processing math speak, the two signals are "uncorrelated". Any issues you have there are best analysed in terms of frequency bands as Chuck explained.

Executive summary: Use a good tube amp with a single 12" speaker pointed in the general direction of your ears, and you shouldn't go far wrong on stage. Think twice before accepting a gig in a disused swimming pool.

[Austin]

You can't get phase effects between two different sounds, like the lead and rhythm guitars, as there's no phase relationship between them at all. In signal processing math speak, the two signals are "uncorrelated". Any issues you have there are best analysed in terms of frequency bands as Chuck explained.

What if the lead and rhythm guitars are playing the same notes both using closed back cabs out of phase with each other?

[Chuck H]

What if the lead and rhythm guitars are playing the same notes both using closed back cabs out of phase with each other?

There are simply too many finite differences for cancellation even in this scenario. Imagine two people with shotguns facing off. Both using the same gun, same ammo. And both miraculously pull their triggers at exactly the same time. And miraculously each shotgun barrel is aimed directly at the other one. How much reduction in injury do you think we might achieve? Probably not much.

Now, more than likely it would be impossible to exact all these parameters. So lets have the two shotguns of differing choke pattern. Slightly different ammo, trigger pull is off by a couple of ms, and one shooter is aiming for the kneecaps while the other is shooting center mass. Much more likely circumstances and virtually no damage control.

But it is fair to mention that if both gunmen were aiming in the same direction, at the same target, that the end result, with all the energy focused at one target instead of two, could be even more horrifying.

So think of your playing as a horrifying shotgun blast.

[Steve Conner]

I say thank you for this useful post. Even with two guitars playing the same note, I still wouldn't consider it the same signal. Now, if the guitars and amp rigs were identical, and the controls all set the same, and you spent an hour tuning them together, and used a machine to pick the strings for identical pick attacks, things might get interesting. But that's not really relevant to a gig scenario.

[Austin]

So think of your playing as a horrifying shotgun blast.

Bwahaha.... Gee I never thought of it like this before..... Tooo funny!!

[GTOBoy]

Ah 2 thoughts: mic placement and comb filtering due to all anomalous variables already cited here by all

[Enzo]

Ah, thanks. Comb filtering is the term I couldn't bring to mind when I described walking across the sound field.

[Chuck H]

It's definitely the best way to get everyone sounding their best and accupying their own space without isolating each instrument to stand out in a single bandwidth area.