Capturing EM interference with a microphone

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.

 

Any way you can wrap the mic in a "sound proofing" material, so you damp
out the acoustic pickup and leave the EMI sounds?

 

the reason for that is that the dynamic microphone has a voice coil (like in a
loudspeaker) anf that coil picks up all the electromagneic interferance (EMI)
inside your computer.

You'll need to shield it against EMI. while not blocking too much sound
that that won't be easy. basically you need to surround it with a conductive
shell, electret mikes (like the "ctrappy lapel mike") are constructed that
way.

Bye.
Jasen

 

Try something like this...

|---------------
----------------+---///////-------+--------|
| | |--------------
K r |
A | |
| | |
+-----------------+--------+

where the antenna on the left is maybe 6-12" of wire, K-A is a small
schottky diode, 1N5711 maybe, R is 100k, output is audio coax.

Oh, ////// is a small inductor, 100 uH or something. This should pick
up both baseband signals and detect RF. Drive a high-impedance audio
input.

John

 

.

Sure! Just duct tape a piece of foam rubber over the part where the
sound gets in.

No, that's perfectly fine netiquitte, precisely because of spam, and not
only that, but it's _bad_ netiquette to request private answers by email,
(which you haven't done, so no worries ) because the purpose of USENET
posts is to share with everybody.

Cheers!
Rich

 

Hi Josh:
The high frequency noise is from the cmos technology switching between 1 and
10 nanoseconds or 100MHz to 1 GHz. This noise passes thru the microphone
cable and into the amplifier circuitry. At these high frequencies, the
signal is rectified and what you hear is the peak detection of the noise.
The only other noise is the switching power supply that creates a 1 MHz
pulse every 20 to 40 useconds. Therefore, the best way to hear electronic
noise with the existing amplifier is to just use a piece of wire about 2"
long that connects the shied or ground wire to the signal wire. You don't
even need the microphone. This is basically a UHF antenna.
Enjoy
Harold

 

An easy way, use an in-ear earphone, or other earphone. They've got a
coil of wire inside to pick up EMI, and a jack plug on the end. Tho
they're comparitively lo-impedance but, it's an experiment innit?

Like others have said, put a bit of plasticine over the sound holes.

 

An easy way, use an in-ear earphone, or other earphone. They've got a

Using an earphone to capture EMI looks usefull to me, but I'm not sure I
understood the principle. Please, what to do exactly with an earphone?
Should I connect it to aditional power source (like batery) to drive the
earphone, or no need? Should I, on the plug side of a cable, make something
like antenna?

Nikola

 

Just connect it to the mic socket on your tape recorder (or an
amplifier or whatever), instead of a microphone.

It's a bit lo-impedance to be ideal, but should work.

No! Just plug an earphone into the mic socket. That's it!

Altho, as an experiment, you could try the antenna. Just a piece of
wire, connected to one of the contacts on the jack plug.

I'd like to hear about your results.

 

Ok, you'll know when I get result.

Basicly, the point is to examine EMI in navigation devices on ships, becouse
it can be the reason for some malfunctioning, sometimes. Using osciloscope
or other measuring devices is hard (hard to cary it all the time when going
to service). Once I saw a man wearing in-ear phones, probably driven with
batery, and touching the electronic circuits with conductor (from
earphones). He was using earphones instead of an osciloscope. So, what to do
to use same principle to measure EMI?

Nikola from Croatia

 

I thought I posted something, but here goes again.

A telephone pickup coil is an inductive pickup.
I think they still sell these.

I listen to noise via a portable handheld amplifier, just
an op-amp. I have a switch to either connect a coil or just a
rod antenna. The gain is adjustable. Noises from magnetic vs
electrostatic can sound very different. Magnetic also penetrate metal.
Static magnetic fields can be heard when positioning the coil,
but a Hall Effect sensor would be best at this.
I still need to configure to measure other noises. I need
to connect a photodiode to listen to light noise, 60 Hz etc. DC
light should be free of noise. I also need to measure vibration. A
vibration sensor will connect to bottom of handheld device, so
when its sitting on a table, the noise can be heard. I also need
to measure LF audio, as in ventillation noise and changes of
less than 10 HZ. This is not just a usefull device I carry, its
necessary!

Therte is a product which is basically my electrostatic sensor
It says inductive pickup, but I think its really just a rod.

http://www.electrical-contractor.net/The_Store/EX/40180.htm



greg

 

Here's a good link.

http://www.logic1.com/rf-engineering/rfdetector/

Also try and put a radio real close to your computer and see if it slows
down any processes. Start at 100Mhz transmitters and work up.

I guess the experiment would be like a counter for 10 second... see if it
counts the same number after 10 seconds has elapsed. Then try to scale the
distances.

Marc

 

Here's a good link.

http://www.logic1.com/rf-engineering/rfdetector/

Also try and put a radio real close to your computer and see if it slows
down any processes. Start at 100Mhz transmitters and work up.

I guess the experiment would be like a counter for 10 second... see if it
counts the same number after 10 seconds has elapsed. Then try to scale the
distances.

Marc