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Re: contact microphones - how?

At 7:48 PM -0800 2/10/02, eileen wrote:
>I'm trying to capture sounds from the inside of a "live" object. 
>Someone suggested using a contact microphone. Would this be the 
>correct type of mic to use?

It might be, but it depends on the object and what quality of sound 
you are after.

>Is a contact microphone able to capture audio? How exactly do the
>vibrations translate into audio? I'm just a little confused on what the
>contact microphone actually does.

A contact microphone is a particular kind of "transducer," just as 
any sort of microphone is a transducer. A loudspeaker is another kind 
of transducer.

"Transducer" simply means a device that turns one kind of energy or 
vibration into another. A contact mic turns the vibrations in a solid 
object into small electrical signals that can be amplified. The more 
familiar kinds of microphones (what I sometimes call "air" mics) turn 
the vibrations of the air into small electrical signals that likewise 
can be amplified. It isn't necessary (though it might be helpful) to 
know the technical details of these different types of microphones, 
but it is important to know to some degree how they behave.

The signal coming out of a microphone is not necessarily the same 
thing that you might hear when standing a few feet away from the 
object that is being amplified or recorded. Objects radiate different 
frequencies of sound in different directions, so if you listen to an 
instrument (such as an acoustic guitar) from one location it will 
sound differently than it does from another location. Similarly it 
will sound a lot different if you put your ear against it.

Putting your ear against a guitar is in many ways similar to putting 
a contact mic on a guitar. The quality of sound is very different 
from the quality of sound you'd hear from a distance. Another thing 
to consider is that putting your ear, or a contact mic) directly in 
contact with an object actually affects the sound of the object 
because it interferes with the free vibration of the object.

Though this may seem more complicated than you may have hoped, it's 
actually an opportunity to learn more about sound and amplification 
by listening carefully and thinking about the sound you are trying to 
capture and the technology you are using to do the job.

So the answer to your first question about whether to use a contact 
mic to capture sounds from the inside of an object is "it depends." 
I'm also not sure what you mean by a "live" object. If you mean 
"live" as in the case of a live cat, it don't think a contact mic 
would do you much good. If you mean "live" in the sense of a vibraing 
object (such  as a Martha waste basket), then the answer might be 
that you can use a contact mic but that you need to be careful where 
you attach it in order to avoid damping out the sound. Sometimes you 
need to use a combination of techniques to get the "whole" sound 
you're after.

Richard Zvonar, PhD
(818) 788-2202