On Fri, 04 Nov 2005 01:59:05 -0500, jh wrote:

Hi,

I was hoping somebody here might be able to help me with a question.
First, some background. A couple of months ago I was trying to record
the sounds of the insides of my computer for an experimental sound
project. I first tried it with a cheap, crappy lapel mic that came with
a pocket voice recorder. It worked just fine.

Then I borrowed a fairly nice, high quality microphone and tried it
again. Sure enough, this microphone picked up a lot more sounds... in
fact, it recorded all sorts of beeps, buzzes, and hums that weren't even
there, apparently some sort of electromagnetic interference. I was
amused to find that this high-quality microphone was much more prone to
picking up this interference than the cheap one I tried earlier.

The thing is, the interference sounds were much more interesting than
the real sounds. Holding the microphone near the graphics card, it
recorded different noises depending on what was being displayed on
screen. The fans sounded like something out of a science fiction movie.
My personal favorite sound came from the power cord while the computer
was asleep: it made a bizarre sequence of changing pitches that repeated
every couple of seconds.

The only problem is, all of these great interference-caused phantom
sounds were almost drowned out by the actual normal sound produced by
the fans, hard drive, etc. in the computer. Needless to say, the
microphone was quite adept at recording these sounds.

So my question is this: is it possible to build a device, or modify a
microphone, so that it picks up ONLY the electromagnetic interference,
but no actual sound?

---
If your mic was picking up EMI and also acoustic sound you could
probably get rid of a large amount of the acoustic sound by
shielding the mic's diaphagm. That is, cover the places where the
sound enters the microphone in order to keep sound out of there.
Maybe cotton balls and Scotch tape?


The other thing you might want to try would be to wind a coil and
substitute it for the mic. I'd start with a couple of hundred turns
of 24 gauge telephone wire wound on an air core just to see what
would happen, and then go on from there.

--
John Fields
Professional Circuit Designer