Thursday, November 24, 2011

Laser microphone for audio surveillance via window panes

I bounced a laser beam off of a window in my house and recovered the audio from inside the room via the beam deflection. I used a Hamamatsu S7815 amplified photodiode and connected it with a 9V battery to my stereo's microphone input jack. The audio quality was very low -- probably due to the double-pane windows in my house. Speech was just barely intelligible.

I also tested the procedure of bouncing a laser beam off of a framed picture that is hanging on the wall inside the room to be monitored. The reflected beam will hit a wall somewhere else in the room, and the dot can be monitored by a telescope from remote. The goal would be to measure the beam wobble via the telescope and recover the audio without needing a stringent geometric relation to the target room. This didn't work at all, but I think with a sensitive detector, it has potential.

More about laser microphones:


  1. Lucid Science have a nice description of a similar project:

  2. Looks like a good one for Mythbusters as well ... only they will never show us the interesting details that folks like us need. That's why we have to do it ourselves I guess. Funny how I knew it was Tull before you said it.

  3. I love this web page Mr. Ben. When I was a boy my friend Don and I taped a mirror to the back of a pie plate and talked into it. We listened to the suns reflection with a photo transistor put across the input of a radio shack amplifier that looked just like a transistor radio. Seemed to work pretty good, and was a big thrill for a couple of sixth graders. Good times back in the 1970's. Very best regards, Tom, AB9NZ,