Laser microphone for audio surveillance via window panes

[hwy163]

AND IT MAKES ME WONDER WHY NO ONE HAS DUPLICATED THE DEVICE PLACED BY RUSSIA IN A WOOD CARVING. IT HAD NO BATTERY. PERIODICALLY A TRUCK OPERATED BY THE RUSSIANS WOULD PARK AND POINT A MICROWAVE AND IT WOULD HIT THE DEVICE AND SEND BACK THE VOICES OF WHOEVER WAS NEAR THE DEVICE.

[ninjastalker92]

@hwy163 Your caps lock is stuck.

[TheNinjinx]

@hwy163 Above your "shift" key there's a button called "caps lock", please press it.

[hwy163]

I wonder if a laser could pass through hypothetical "device X" on its way to the window pane and at the same time be deflected via "device x" to a receiver to unscramble what sound may be on the laser. I've seen music from a record player transmitted via laser to an amplifier. When the technician placed his clipboard in the laser path the music stopped.

[fisherdigital]

I saw a thing one time that the CIA had put a tiny tiny prism in the glass to do this

[skonkfactory]

2 inches / 150 feet ~= 1.1 milliradians, which is about right for a short HeNe tube.

[bur1t0]

To solve the angle issue you could use short pulses of light, and the vibrations of the window would be frequency modulations on the return pulses. My maths is telling me a clock source in the 1Thz range is needed, although it strikes me as a task with a much simpler analogue solution.

[steveBB30]

Would it help to measure how the double pane distorts the sound? I'm thinking if you can sync a sound played in the room, to a duplicated signal in your workshop, you 'd be able to compare the pure signal to the received signal. Difference should be the distortion. amplification or attenuation on a frequency dependent basis might allow correction

[KiloSierraAlpha]

Coolest piece of spy tech I've seen was the old Soviet "American Great Seal".. it had no moving parts or electronics... just a resonant cavity that Soviets 'activated' with radio waves. I think it was invented by Theramin... what a genius.

[scottrharris]

Also, try expanding the laser beam and focusing it to a small spot on the window. That way geometry might help you prevent the multiple reflections from overlapping on your detector.

[scottrharris]

There are a ton of fringes in the picture at 1:28. I'm guessing that these are due to multiple reflections from both surfaces of the two panes. If you collect a larger area with a lens you'll average over these fringes and perhaps cut down on the howling. However, you might also reduce your sensitivity to sound. Try getting a clean spot back onto the Rx wall and then placing the detector and a small lens at the edge of the beam so that you can average over fringes and do some slope detection.

[ntomata0002]

By the way I guess that a frame glass doesn't oscilate as a window because it has a smaller area so it absorbs less sound energy, it also has much greater stiffness because its small dimensions and it is in contact with a material that absorbs the energy (the frame itself and the picture)

[ntomata0002]

3:54 just point the laser to their window and listen by yourself

[Rskater3]

How about a small piece of a "retro reflector". From what I know of retro reflectors, they are designed to send the laser light in the same direction it came from (or at least that's what the ones on the moon are designed to do). I know they sell reflector tape at the hardware store, although I am not sure how well this tape would work for this aplication. It does take away the "stealth" factor a little bit, but just a small piece of a reflector should do it.

[bkraz333]

@Rskater3 The US gov't embedded tiny retroreflectors in the windows of the Russian embassy/consulate at some point in history for the express purpose of using laser microphones for espionage. It worked very well until the Russians somehow detected them.

[TheRealShadowspawn]

@Rskater3 ADT/security system stickers are reflective, and usually placed in the windows facing the street :)

[SuperDisco91]

So simple, and it works! Did I notice your FPGA plugged into a serial connector? Are you trying to make a UART?

[bkraz333]

@SuperDisco91 My first FPGA project will be a video display device that can scan image data slowly, store it in a framebuffer, and output VGA. I have the framebuffer and VGA working, but there is a pesky problem with signal artifacts. I may do a video asking for help on this one.

[narcoti]

where did you get that copper tape?

[bkraz333]

@narcoti It's 3M shielding tape, #1245. Try searching mouser or another electronics supplier.

[electrodacus]

Interesting thanks for sharing.

About 15 years ago I made a simple audio transmission using a small red key chain laser and a small solar cell from a calculator it worked great.

I got the idea from a presentation at my university of IR laser for data transmission.

Probably this can work even without an angle measuring the small change in distance but it must be quite precise.

Also mounting the laser few inch from the window can get you better resolution for tests only.

[googacct]

Using a computer, you could try generating single audio frequencies and build up a response curve of the system. It might be useful to find out the frequency response of the photodiode. It might be that the diode can not respond fast enough to the incoming signal and is producing a low pass filter.

[stuckinpants]

I once did something similar by putting a reflector on a speaker cone and a photoresistor close to it, audio quality was extremely low, dropping off almost completely above 400 Hz and requiring extensive filtering to recover anything understandable.

[superdau]

Yay for tripple glass window panes with IR reflection coating on the outer glass! (we are like this here in Europe ;) )

Hard to spy on me with a laser mike (not that there would be a reason to). I would see visible lasers and an IR laser won't get through to the inner glas.

[onefivefour]

I did this with a setup that was not as complex. I just used a light sensitive resistor (I got a pack of 5 or so from Radio Shack) for the laser receiver. My end results were similar to yours.

[NeitSotm]

Wouldn't the resonance frequencies in the direction perpendicular to the window be way above hearing range? Or did you mean resonant frequencies in the horizontal/vertical direction? Pretty cool project!

[superdau]

@NeitSotm

The space between insulating double paned windows is sealed and filled with argon at low pressure. This setup will have a resonant frequency at which the glas panes will easily vibrate, even with the smallest excitation from the outside (that's what resonance is all about). And the frequency? Well within hearing range! I got a window here that will "hum" (which it does at resonance frequency) for half a second after being knocked on.

[CaptainChaos]

Does your shop also have double paned glass?

[ScalesandGills]

Would it be possible to run an audio filter through the output, to clear up the received audio?

Also, would boosting the power on the receiver make much of a difference?

[iWinRar]

I have even done some FM transmitter based spy mics. All fun stuff imho

[CrudOMatic]

oh wait, it was

[CrudOMatic]

sounds like music

[iWinRar]

Well anything newer for the most part it is going to likely be dual pane glass, as for just mounting a pane of glass, id personally try finding a whole old frame with single plane glass or make your own and mounting that inside of a fake wall setup as i think that might change how everything will vibrate.

Also i have always been interested in homemade audio surveillance and have looked into parabolic mics made of many things and even using a stethoscope and holding it up to a wall and record.