Lav mic phase cancellation?

[LPdan]

Hi,
I recently had a problem with a musical I was running sound for, and was wondering if anyone else has had this issue. I have run many shows in this theater, with the same lav mics (some countryman B3 and some old MiPro lavs), and never experienced it before. When two people would get close to each other (like singing a duet face to face), the sound would get quiet and very tinny sounding. The difference in this show is that some actors wore the mics on their foreheads, while we usually have all mics near the mouth. For shows we have had since the "problem" show, we have taped all mics behind the mouth again and not seen the issue since. Could the combination of forehead and near-mouth locations have caused phase cancellation? Any other ideas?

 

[BattsAVL]

The combination could certainly cause the cancellation. Countryman style headsets are just very good at handling this issue because they are so close to the mouth that they operate at a lower level of signal to the beltpac transmitter. The result in steps of amplification dramatically help this issue. A more 'omni directional' mic that is clipped in wigs or taped has the port of mic facing out so as to gather sound from many different directions. So, someone speaking to the person at 2' has a pretty good chance of being received at both performers microphone resulting in this cancellation. Hard problem to solve if you really need to use these two mic types in this fashion. But slight changes in blocking can really improve this. Face out just a little more, or wear the taped on mic slightly to one side that is generally away from the performer in this scenario. Hope this helps!

 

[rwhealey]

Dealing with this problem is a big part of being a musical theater sound tech. The best way to do it is to fade out one of the mics when two performers are close and pick up both people with a single mic. It requires practice and quick hands on the faders.

 

[JD]

No matter what the distance, when mics get close the half-wave frequency is going to cancel out. Simply take the distance, multiply by two and that is the frequency that is going to get killed. Now, if the mics are right next to each other and you are missing a broad band of frequencies, then your mics are out of phase and it is time for a short cable off the receiver that flips pins 2 and 3.
I find the cancel effect a blessing when mic'ing an ensemble! It's like the world's cheapest parametric notch filter. If there is a booming or prone frequency, just position the mics 1/2 wave apart at that wavelength.

 

[jkowtko]

This is what "mixing" means. You control the mix of all input signals for best resulting sound.

In a musical you should be riding faders all the time ... ideally line by line. it is pretty standard to drop all but one mic when multiple actors get close enough to each other than they can all be picked up on one mic. Usually it's only two actors but you might run across situations where it's three or more. And it is precisely for the purpose that you are experiencing.

If you drop all but one fader by at least 10db you will usually remove any discernable frequency interference ... you don't have to drop the fader all the way to zero. So this is probably not as difficult as you may think.

Of course, the cheap way out is to tape the mic close to the mouth so the actors will have to be much closer together before you hear the interference ... but that's avoiding the problem, not dealing with it And it doesn't look good either.

 

[themuzicman]

Hi,
I recently had a problem with a musical I was running sound for, and was wondering if anyone else has had this issue. I have run many shows in this theater, with the same lav mics (some countryman B3 and some old MiPro lavs), and never experienced it before. When two people would get close to each other (like singing a duet face to face), the sound would get quiet and very tinny sounding. The difference in this show is that some actors wore the mics on their foreheads, while we usually have all mics near the mouth. For shows we have had since the "problem" show, we have taped all mics behind the mouth again and not seen the issue since. Could the combination of forehead and near-mouth locations have caused phase cancellation? Any other ideas?

This is called comb filtering and it occurs when a signal is hit with a slightly delayed version of itself. It is called that because the frequency response ends up looking like a comb when graphed.

This all being said -- micing from the forehead is generally almost always better sounding than micing from the ear -- I would like to see some pictures of the micing by the mouth, for my money you aren't gaining that much having it closer to the mouth, get it out boomed out of the ear on an ear loop so it doesn't look like the actors have worms on their faces!

 

[LPdan]

Thanks for all the info and additional advice. This is a community theatre, with various levels of experience within the mixers. Personally I am working towards line by line mixing, but am certainly not there yet. Is it fair to say that in general, taping behind the mouth is "safer" in that you get much better gain before feedback, with the greatest disadvantage being cosmetic, and also not sounding quite as good as forehead worn? This theatre also has installed speakers with a 150 degree pattern which make feedback issues tough. Would both mic locations be considered "acceptable", depending on conditions and level of experience? We have always got pretty good sound behind the mouth, cosmetics aside.

 

[jkowtko]

When you say "behind the mouth", you mean only 1" or so back, correct? This is essentially the same as a boom mic placement, similar in quality to just using a handheld mic. Great GBF, usually loud, clear sound. But very visible. Boom mics are often cardioid on top of that, but even omni mics that close to the mouth will have the gain turned down enough to remove noticable problems from the band, etc.

Aside from the boom mic placement there is "over the ear" and forehead mounting. Over-the-ear generally means anywhere from 0-2" forward of the top of the ear lobe. Most professionals should tell you that forehead placement gives you much better, more natural, frequency response than over-the ear, and to use over-the-ear placement only when absolutely necessary (e.g. bald actor not wearing anything on their head where a mic can be hidden.)

Some people think that all men should wear over-the-ear because they have lower frequency vocals and therefore the side of the face has better pickup. In my experience over-the-ear never sounds anywhere near as good as forehead mount, and I personally would avoid it whenever possible. Over-the-ear loses a lot of the high frequency vocal range. I've heard the sound effect called "oatmeal box" ... try talking or singing into an oatmeal box and see if you like it. Very hard to EQ over-the-ear ... I usually have to drop off bass and mid frequencies severely. So I stick with forehead mount whenever possible.

 

[Dustin Schisler]

No matter what the distance, when mics get close the half-wave frequency is going to cancel out. Simply take the distance, multiply by two and that is the frequency that is going to get killed. Now, if the mics are right next to each other and you are missing a broad band of frequencies, then your mics are out of phase and it is time for a short cable off the receiver that flips pins 2 and 3.
I find the cancel effect a blessing when mic'ing an ensemble! It's like the world's cheapest parametric notch filter. If there is a booming or prone frequency, just position the mics 1/2 wave apart at that wavelength.

JD, I would love to understand this a little more as I think this is the exact issue I am having right now. We are 17 mics in a pretty small space and I have three female actors who I think may be cancelling each other out when they group up to speak. Can you explain a little more, or point me in the direction where to learn about a "short cable that flips pins 2 and 3"? Thank you if advance!

 

[JD]

JD, I would love to understand this a little more as I think this is the exact issue I am having right now. We are 17 mics in a pretty small space and I have three female actors who I think may be cancelling each other out when they group up to speak. Can you explain a little more, or point me in the direction where to learn about a "short cable that flips pins 2 and 3"? Thank you if advance!

Ok, below is a chart that shows how frequency converts to feet. If we pick 400hz, we see that the wavelength is 2.5 feet. So a half-wave would be 1.25 feet. If we placed two microphones 1.25 feet apart, and someone was speaking in the first microphone, whatever was received by the second microphone would pick up out-of-phase. If the mics were adjusted so that the output was the same, everything around 400hz would be missing. 800hz would also be diminished.

The "rule of 3" helps here in that you microphones should never be closer together than three times the distance between the microphone and the sound source. In live performance, this may end up being impossible. In those cases, you would try to give one of the mics enough gain over the other that the effect gets washed out.

Out of phase wiring can also be a problem. Most mic lines are balanced 3 conductors; +, -, and shield. On an XLR connector you can reverse phase by making a short male to female cable where pin 2 on the male goes to pin 3 on the female. Pin 3 on the male goes to pin two on the female. For TRS connectors, you would do the same thing so the the ring and tip contact reverses. If two microphones somehow end up in reverse phase with each other, and they are close enough, their outputs will cancel each other out. Reversing the phase on one of them will fix that.

 

[jkowtko]

JD, I don't dispute your explanations and suggestions for fixed mic placement ... in the OP's case though I think we're talking about body mics, in which case there is no fixed distance between microphones and therefore no frequencies you can really target with something like polarity reversal. As the actors move in closer to or away from each other the frequencies that would interfere are constantly changing.

I was told in a Meyer Sound seminar that the human ear stops discerning quieter sounds that are 6db or more below the main sound -- this was one of the fundamental theories behind speaker clustering. So if you have two mics mounted on actors heads and they are close to each other, you are probably going to have to count on the fact that a given actor's voice will pick up louder through one mic than the other, rendering the potential interference negligible. Therefore the trick of pulling one mic down a bit on the sound board.

I too have experienced the tinny sound issue, but it also may be sound reflections off the actor's faces. Have you even had an actor with a cheek-mounted mic cup their hands over the mic when they were pretending to talk on the phone? Ouch!

There are two things I would do first in this situation that won't cost you anything can be done immediately: (1) try dropping volume of all but one actor in the group to how it affects the sound, and (2) solo monitor each mic in the group with headphones to hear what each mic is picking up.

 

[JD]

JD, I don't dispute your explanations and suggestions for fixed mic placement ... in the OP's case though I think we're talking about body mics, in which case there is no fixed distance between microphones and therefore no frequencies you can really target with something like polarity reversal. As the actors move in closer to or away from each other the frequencies that would interfere are constantly changing.

Yes, I am aware that he is talking about body packs and face mounted microphones. Still, the same rules of physics apply. His statement was that when the actors/actresses came close together the sound went south. Those mics are Omni which tend to be far more prone to cancellation as they are not directional and are perfectly happy picking up the person next to the person you are trying to mic. I have actually used this to an advantage when I run short on body packs. A simple clip on like a Sennheiser ME2 can be shared by two performers who are going to be very close to each other, with surprisingly little need to ride the gain! Just finished up a Passion Play where we used 12 body packs to cover 16 performers. Requires a little planning, but all came through clear as a bell.
I would agree that in his case phase reversal would not achieve anything, but leaving this factor out of a conversation on phase cancellation would be remiss.
As for cupping the mic, I have one musical director who loves to do that, thinking she is shielding her conversation, while creating a huge single-point notch resonator that causes me to have to dive to kill her channel before we are all treated to a 2khz full volume solo!

 

[TimMc]

Wrong thread... oops.