# Why do two microphones sound funny?



## mbenonis

Haven't done a QotD in a while so I thought I'd post one. Here goes.

Why do two microphones sound funny when they are placed near each other and fed through the same speaker?


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## shiben

If they are out of phase, this might be a problem. I believe if they are exactly out of phase, they cancel eachother out, and you get no sound at all. I am assuming that both mics have the same source, ie, 1 person talking into 2 mics spaced a bit apart and then fed thru? I know that there is a switch on most consoles to fix the phase issue, not quite sure how it works, seeing as I am not primarily a sound guy.


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## museav

shiben said:


> If they are out of phase, this might be a problem. I believe if they are exactly out of phase, they cancel eachother out, and you get no sound at all. I am assuming that both mics have the same source, ie, 1 person talking into 2 mics spaced a bit apart and then fed thru? I know that there is a switch on most consoles to fix the phase issue, not quite sure how it works, seeing as I am not primarily a sound guy.


Heading in the right direction, just need to take it a bit further...


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## Van

Is it cause they're comedians ? 
'though I have to say I've known some comedians who couldn't make a mic sound funny, no matter what......

Happy day Everyone ?


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## DaveySimps

Due to comb filtering? They add a delayed version of the original signal since there are two elements picking it up in a slightly different manner.

~Dave


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## JChenault

I hesitated answering because I am a lighting guy - but what the heck - let me give it a go.

First - definition of layout to see if I have it right.

You have two mics. You put them ( say) a foot or two apart. You connect both to an amp and through to a speaker. When you have someone speaking the voice does not sound correct. Is this the situation you have in your mind?

<If this is not the problem you are asking - you can stop reading now>

If so I believe the problem is that some of the high frequencies are canceling each other out. And others are reinforcing themselves so they are louder. It has to do with basic physics.

For example - consider the note middle C. it is about 261HZ. Speed of sound in air is (if I remember my high school physics) about 1100 feet/second. So when I am hearing a middle C, there are (1100/261) = 4.2 waves in a foot of air. So the wavelength of a middle C is about three inches. This means that if the distance from one microphone to the source of sound is exactly 1/2 wavelength ( IE 1.5 inches) the middle C will be suppressed in the amplifier.

We would also suppress any frequencies whose half frequency is a multiple of 1.5 inches so things would sound hollow.

This is why we use floor mikes. If you put a mike a foot or two above the stage, it would hear the direct sound, and the sound reflected off the stage floor. This destructive interference suppresses certain frequencies. By putting the mic close to the floor, you get better sound quality.

John


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## DaveySimps

When might we see confirmation of the correct answer?

~Dave


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## Studio

No clue but I do know if they are close enough they cause feedback and mess with the eq.


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## Ric

mbenonis said:


> Haven't done a QotD in a while so I thought I'd post one. Here goes.
> 
> Why do two microphones sound funny when they are placed near each other and fed through the same speaker?



Hmmm..
Microphones on their own make no sound!

However I'm assuming that you mean the sound they pick up, in which case has already been answered with the response Comb Filtering.

2 different 'versions' of the same sound, caused by microphone differences and distance from the orginal sound source, played back through a single sound source (same speaker in this case) will combine with reinforcements at some frequencies and cancellations at others, changing the 'tone' of the original sound.


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## theatremowat

Ok, well i'm defiantly more sound than lighting, and this is a really common problem. Feedback will never be the result of simply having two mics close to each other, to who ever said that. The only time that becomes an issues is when you have to many open mics in a condensed area. 

I've run into the problem you speak of many times, having 30-40 mics in a 45ft by 26ft stage. yes the correct analysis was one of the first i believe, the phase is simply incorrect. if you have two identicals sounds (its a sin wave) and they are not correctly in phase they will completely cancel each other out. so if your board has a phase button, all digital boards do, and some higher end analog, but most don't. Simply press the phase switch on just one of the mics. 

More basic, just move them farther apart, why do you have 2 mics that close together, if one just just a redundancy or backup, simply turn it off if not needed.

That help?


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## mbenonis

Good discussion thus far. Those who said comb filtering are exactly correct, and I thought the above description was quite well written, JChenault.


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## DaveySimps

theatremowat said:


> Ok, well i'm defiantly more sound than lighting, and this is a really common problem. Feedback will never be the result of simply having two mics close to each other, to who ever said that. The only time that becomes an issues is when you have to many open mics in a condensed area.
> 
> I've run into the problem you speak of many times, having 30-40 mics in a 45ft by 26ft stage. yes the correct analysis was one of the first i believe, the phase is simply incorrect. if you have two identicals sounds (its a sin wave) and they are not correctly in phase they will completely cancel each other out. so if your board has a phase button, all digital boards do, and some higher end analog, but most don't. Simply press the phase switch on just one of the mics.
> 
> More basic, just move them farther apart, why do you have 2 mics that close together, if one just just a redundancy or backup, simply turn it off if not needed.
> 
> That help?



You often have this issue when you have two lav mics on actors. It is not at all uncommon to have them this close together, especially when singing a romantic duet or something of the like. In this instance, moving them farther apart would not be realistic. Turning one off and on may get difficult very quickly, and often yields undesirable sound balance. This is why you stills see many A/B type speaker systems in use for musicals.

Also, not all digital boards have polarity reverse options (it is not phase polarity is the correct term here). I can think of several smaller boards that do not. Although for just a few bucks, you can buy adapters that swap pins two and three. A few of these should be in every sound persons adapter kit. 

~Dave


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## museav

I think the important aspect that differentiated some responses is the recognition that relative phase is frequency dependent. Because wavelengths differ with frequency (high frequency = shorter wavelength) the effects of the differences in the distance from a source to two mics (or from two sources to a receiver) results in the relative phase of the two signals also differing with frequency. So the idea that cancellation (180 degrees out of phase) and summation (in phase) can occur due to the differences in distance to the two mics is correct but whether it is cancellation, summation or somewhere in between happening varies with frequency. Engaging a 'phase', or more technically correct 'polarity', switch for one of the channels will change the resulting response, frequencies that were canceling will then sum and vice versa, but will not actually eliminate the problem.


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## BillESC

When you see two mics on a podium there are two reasons for it.

1. Balance of the image to the public

2. Back up mic in position

One of the two mics will be muted.


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## museav

BillESC said:


> When you see two mics on a podium there are two reasons for it.
> 
> 1. Balance of the image to the public
> 
> 2. Back up mic in position
> 
> One of the two mics will be muted.


There is a third common reason for multiple mics and that is to provide dedicated feeds, as in a local reinforcement feed and a separate broadcast or recording feed. In that case both mics are live but feed two different, isolated systems.


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## BillESC

True, good catch.


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## Chris15

Someone noted the frequency of middle C earlier and it doesn't match what I remember... I understand middle C to be 512 Hz. A below middle C is 440 Hz, hence standard tuning for a concert piano is A440.

Reason 4 for double mics is because a single mic speakers will fiddle with, dual mics have some untold effect that means they don't.

Sometimes for awards shows etc they will install dual mics so they have options in terms of dual speakers so they can pick the closer one to get more level. In this case the other mic will be off or significantly ducked.

Double mics CAN increase the likelihood of feedback because of the constructive feedback as particular frequencies and hence you have a frequency dependant gain and so you run closer to the feedback threshold.

Let's not get into specifically designed mic setups for cancellation as used say for the podium in the middle of a football field where background noise is a problem...


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## museav

Chris15 said:


> Let's not get into specifically designed mic setups for cancellation as used say for the podium in the middle of a football field where background noise is a problem...


Or the Grateful Dead's Wall of Sound that used the same speaker system for house and monitors along with dual closely spaced, polarity inverted mics summed together and signing into only one of them.


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