# Single vs. dual mics?



## kieran777 (Mar 15, 2009)

Why is there a difference in sound quality between a single microphone and a lectern with two microphones, when the acoustics are constant and the mircophones are the same type?


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## DaveySimps (Mar 15, 2009)

The biggest advantage is to have a backup mic in place ready to go should something happen to one of the mics. Most of the folks who I know who use dual mics, only actually use one at a time. The second one is there as a redundancy. If for some reason you do decide to use both at the same time, you will probably want to throw one out of phase to prevent a canceling effect or comb filtering. 

~Dave


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## Eboy87 (Mar 15, 2009)

What Dave said. When you have two mics like that in close proximity picking up the same source, it takes time for the source, or waveform, to go from the speaker's (not loudspeaker) mouth to the microphone. It takes a different amount of time for that same signal to reach the second microphone. When those two signals combine further down the signal path, and cancel out certain frequencies. The result is that strange sound you hear, comb filtering. 

Now, like Dave said, the second mic is usually a backup in case the first one fails, or the second mic is feeding a record truck for broadcast. It's not really a good idea to use the two at the same time.

This is the same thing that happens on stage when two actors with lav's are delivering lines to each other's face with both mic's live. You get cancelations which makes it sound odd.


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## rwhealey (Mar 15, 2009)

The reason is because of "microphone interference" or "acoustic phase cancellation". If a podium had two mics at each back corner, pointing inwards, there would be no change in sound ONLY if the speaker stood EXACTLY in the center. However, since that never occurs in practice, the sound will arrive at slightly different times at each microphone, which, when mixed together, will cause both constructive and destructive interference and totally screw up the frequency response.

Thus, only one mic should be turned on at a time. You can place the two mics in an XY configuration which will lessen the effects of the interference, but I can't think of a good reason to do that when micing a podium -- one mic will get the job done.

My source (_Microphones: Design and Appliication_ by Lou Burroughs) has some diagrams that I could post as soon as I can get to my scanner.


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## Stookeybrd (Mar 15, 2009)

Like everyone else has said, having two of the same mics won't help too much.

What you classically see is(are?) an omni mic and a hypercardioid mic above and below each other. This way you have a very directional mic that sounds great but when the Speaker shifts side to side you have the omni mic to jump in and provide a little help.


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## Chris15 (Mar 16, 2009)

But folks, we're forgetting the most important reason to have 2 mics on a lectern. People will fiddle with a single mic but for some reason seldom will touch a pair that look like they are nicely adjusted...

Now I'm not saying that the second should be connected anywhere or even work


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## museav (Mar 16, 2009)

I have seen two mics used off to either side for people who will not use a lav or headset mic but who turn a lot or are addressing a very wide audience. It tales a good person behind the console to ride the faders and use the relative levels to minimize any interference.

When two mics are closely spaced there could be several reasons for it. As noted, one common reason is for redundancy, the same reason many broadcasters are seen wearing two lavs and why many broadcast audio mixers have two mic inputs per channel. Another common reason is to have separate mics for separate mixes, be it for two broadcasters or house and recording or whatever. The third reason goes back to the old Grateful Dead "Wall of Sound" days where they used two mics slightly spaced apart with the person singing into one of the pair. By wiring the mics out of polarity and sound common to both mics such as sounds on stage or ambient noise was canceled out while only differential signals such as the person singing directly into the one were reproduced. This potentially lost a bit of gain on the vocals but allowed the sound system to also serve as the monitor system and be located behind the performers on stage. In other uses it allows canceling out ambient noises.


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## TheDonkey (Mar 16, 2009)

Now, as nobody else has mentioned it, I would assume the canceling out would be in effect, but wouldn't you pan the 2 mics to hard left and right so you have a stereo type feed going? That's what I always thought there were two for.


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## museav (Mar 17, 2009)

TheDonkey said:


> Now, as nobody else has mentioned it, I would assume the canceling out would be in effect, but wouldn't you pan the 2 mics to hard left and right so you have a stereo type feed going? That's what I always thought there were two for.


The person speaking is a mono source and is not physically moving, so having the sound move extreme left to right as they turn their head would seem undesired in most applications.


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## Andy_Leviss (Mar 21, 2009)

DaveySimps said:


> . If for some reason you do decide to use both at the same time, you will probably want to throw one out of phase to prevent a canceling effect or comb filtering.



Three things...first, there's no reason to use both at the same time. It will cause you nothing but trouble, for all the reasons mentioned. It's a backup, or, depending on design choices, sometimes one will be aimed down, one up, to account for dramatic height differences between speakers. 

Second, to border on being pedantic, you're flipping it out of POLARITY, not out of phase. The one has nothing to do with the other. Polarity is an either/or thing that can be flipped and is electronic in nature and independent of time; phase is a continuous shifting scale related to time, measured in degrees. When a perfect sine wave is 180 (or some multiple) degrees out of phase, it will produce the same effect as being of reversed polarity, but they are NOT the same thing, and this is NOT the case for complex non-sine signals.

Third, flipping polarity will in most cases not do a thing to help you here. If your source were in a fixed position, DELAY would help, to time (phase) align the two mics), but since your source moves in relation to both mics, delay won't help you here.

Just use one mic at a time


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