So I discovered that my HP laptop leaks/transmits its built-in mic audio somewhere around 24Mhz

[Eugen Leitl]

http://www.reddit.com/r/RTLSDR/comments/1le3if/so_i_discovered_that_my_hp_laptop_leakstransmits/

So I discovered that my HP laptop leaks/transmits its built-in mic audio
somewhere around 24Mhz (self.RTLSDR)

submitted 23 hours ago by cronek

I accidentally stumbled upon a signal in the 24MHz range, appearing to be 4
carriers. I tuned to it and heard silence, then someone came into my office
and started talking and I could hear them speak. The signal appeared to be
coming from my other laptop (not the one running the SDR) and was pretty weak
(my antenna, the crappy one that comes with the dongle, stuck to a metal
stapler was right next to the HP laptop). Here's a picture

Both mics transmit independently, in the picture I rubbed one mic. The signal
appears to be mirrored.

When I tap the microphone, or make a loud noise that would clip the preamp,
the signal drifts off and then slowly comes back to its original frequency,
as illustrated here (only one of the two mics drifted, if I hit it harder or
clip both mics, both will drift).

I'm pretty sure that if I build a nice high-gain antenna optimized for 24Mhz
I would be able to pick up the sound from some distance away. The laptop is
an EliteBook 8460p. I have checked identical laptops and they do not transmit
at this frequency. I didn't have the time to scan the full spectrum though.
I'm guessing the preamp is really crappy and somehow ends up transmitting FM
at HF freqs.

Anyone has any ideas about this? I work in a high security setting and having
laptops transmitting audio from everyone's office/meeting room etc is a
really big deal. I somewhat doubt it to be an intentional listening device
due to the weird frequency drifting. For now I guess I'll just disconnect the
mic preamp pcb.