How to capture EM interference with a microphone

J

jh

Guest
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?


Thanks,
Josh

p.s.: I hope people don't mind that I'm not including my real email
address. It's probably bad etiquette, but I'm kinda paranoid about spam.
 
On Fri, 04 Nov 2005 01:59:05 -0500, jh <no@thanks.com> 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?
Click to expand...
---
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
 
Thanks for the tip. I'll try both ideas (when I get a chance; it might
be a while) and post anything interesting. I'm hoping to eventually
(maybe after I learn more about electronics) build a reasonably
sensitive EMI-only "microphone" to use as a sort of computer
stethoscope.

Here's my idea: could a computer technician, with this tool and some
practice, hear and immediately recognize the EMI signature of a dying
power supply or other bad component, the same way an expert pilot can
instantly diagnose engine troubles just from their noises? Certainly,
many technicians recognize the distinctive sound made by some dying hard
drives; but most components don't make any noise that we can hear
naturally.

-- Josh


In article <shpmm1p6g9himi3aq7f6rvcnhs1333n6h9@4ax.com>,
John Fields <jfields@austininstruments.com> wrote:

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.
Click to expand...
 
"jh" <no@thanks.com> wrote in message
news:no-D4AFD6.16513704112005@localhost...
Thanks for the tip. I'll try both ideas (when I get a chance; it might
be a while) and post anything interesting. I'm hoping to eventually
(maybe after I learn more about electronics) build a reasonably
sensitive EMI-only "microphone" to use as a sort of computer
stethoscope.

Here's my idea: could a computer technician, with this tool and some
practice, hear and immediately recognize the EMI signature of a dying
power supply or other bad component, the same way an expert pilot can
instantly diagnose engine troubles just from their noises? Certainly,
many technicians recognize the distinctive sound made by some dying hard
drives; but most components don't make any noise that we can hear
naturally.

-- Josh
Click to expand...

Yes, That is sort of the way technicians do it. the exception is that
rather than listening, most of the time, we use meters and a visual display
called an oscilloscope. There are many other signals used in your computer
and other electronic devices that occur above the range of hearing. So, it
is easier to observe the signals visually and almost always easier to make
accurate measurements visually than with your ear. However, In some audio
applications, signal tracing can be done in a way similar to what you are
doing.

Believe it or not, I never had the chance to meet this guy but legend has
it, here in the town I live, We had an old timer TV tech who was blind. He
was able to diagnose many TV problems via touch and hearing and limited
assistance, from time to time...
 
["Followup-To:" header set to alt.electronics.]
On 2005-11-04, jh <no@thanks.com> wrote:

Thanks for the tip. I'll try both ideas (when I get a chance; it might
be a while) and post anything interesting. I'm hoping to eventually
(maybe after I learn more about electronics) build a reasonably
sensitive EMI-only "microphone" to use as a sort of computer
stethoscope.

Here's my idea: could a computer technician, with this tool and some
practice, hear and immediately recognize the EMI signature of a dying
power supply or other bad component, the same way an expert pilot can
instantly diagnose engine troubles just from their noises? Certainly,
many technicians recognize the distinctive sound made by some dying hard
drives; but most components don't make any noise that we can hear
naturally.
Click to expand...
not as well as a mechanic can diagnose an automotive fault by ear.
most of the stuff that gooes on inside compputers happens at frequencies
well above the threshold of hearing.

It may be possible to diagnose developing faults with an oscilloscope but
it's generally cheaper to use a redundant system than to employ an expert
to maintain the system.


most of the noise your microphone was picking up was probably the switching
frequency of the fan and hard-drive motors.

Bye.
Jasen
 
jh wrote:

Thanks for the tip. I'll try both ideas (when I get a chance; it might
be a while) and post anything interesting. I'm hoping to eventually
(maybe after I learn more about electronics) build a reasonably
sensitive EMI-only "microphone" to use as a sort of computer
stethoscope.
Click to expand...
I'd second the coil idea for that.


Here's my idea: could a computer technician, with this tool and some
practice, hear and immediately recognize the EMI signature of a dying
power supply or other bad component, the same way an expert pilot can
instantly diagnose engine troubles just from their noises? Certainly,
many technicians recognize the distinctive sound made by some dying hard
drives; but most components don't make any noise that we can hear
naturally.

-- Josh
Click to expand...
To some extent. One very handy tool is a little hand held buffer plus
pizeo, apply it to various points and you hear whats going on. The info
you get is limited, but presented fast and easy. You can differentiate
data from no data, real data from fixed frequencies, dc from ac, steady
frequencies from unsteady, intermittent signals, and so on.

A coil picking up everything at once.. not sure how the total mishmash
of all those sources blended together would tell you that much, other
than go/no go.


NT
 
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