Laser microphone for audio surveillance via window panes--- make money online

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.

hey everyone I've been wanting to try

this laser surveillance or laser

microphone project for a while and

finally got some time tonight to give it

a shot so the idea is that you set up a

laser and bounce it off the window of a

house or a building and any sound

vibrations inside the room will cause

the window to bend very slightly and if

you can catch the laser beams reflection

you can demodulate the beams wavering

back into sound so you can listen in on

what's going on inside the room sounds

very simple and I gave it a shot I set

it up of five milliwatts helium neon

tube gone up tripod and aimed it at my

dining room window

which is about 125 feet away from the

shop and luckily the geometry here works

out pretty well so the returned laser

beam was reflected back towards the shop

and I probably only had an angle of

maybe 10 or 20 degrees input to output

angle on the window so one other

unfortunate setup for this set for this

experiment is that the windows are all

double paned in my house which is

actually very nice in terms of heating

bills and sound isolation but for this

project it's not so good so I take a

listen to what I got back

so to simulate voices inside the room I

used yeah like an audio commentary track

off the end of a jethro tull album and

used a sound meter to get about 70

decibels of average volume right near

the window where this was happening so

it sort of approximates actual

conversation in a room

rule is a dreadful Setzer we always got

there it's an office Abbas's the worst

traffic the worst journeys that people

or we try to scare us

and like you say the double pain effect

definitely causes some strangeness in

the return audio there's sort of a

whistling or a humming noise which I'm

betting is the resonant frequency in the

cavity between the two windows and you

know the way that my house is set up

here it's difficult to try a different

window although I think I'm going to

either set this up out at the street or

something or move to the side of the

house because I think I want to try a

much larger window the window that I was

using is only about you know twelve

inches on a side just about it's a

French door type window the receiver is

very simple this optical detector is a

Hama Matsu s7 eight one five and it's

just a photodiode with some integrated

amplification circuitry and all I have

is a 9-volt battery which I covered with

foil just for shielding and it's

connected by basically right to the

microphone input of my stereo so the

battery just provides a bias voltage for

the for the diode it's reverse biased

with about nine volts and it goes right

into the microphone jack on the stereo

so another problem I ran into is that

the bushes were kind of in the way

between here and the house so if my

neighbors didn't think I was weird

enough for traipsing back and forth in

the night with this laser I also had to

get out the hedge loppers and cut off

some branches that were in the way so

I'm not sure what they think of me now

but whatever so the system essentially

works and I'm fairly certain that with a

single pained window the audio fidelity

would be quite a bit better maybe I'll

try even just putting a sheet of glass

near the doorway or something just to

simulate a single pane window but anyway

what I really wanted to test was to sort

of improve the whole system so one of

the problems with this is that you have

to have the geometry if you really want

to perform surveillance on a window you

need to have geometry set up so that the

incoming laser beam and outgoing laser

beam are both accessible so if you have

a sort of a shallow angle like 10

degrees or something it works out okay

but then you don't really get as much

deflection in the audio in the beam so

obviously at zero degrees the beam would

just go straight in and straight

back out and I don't think that would

work at all although there are better

laser microphones that make use of

interferometry to make this work a

little bit better

it uses a reference beam and all kinds

of stuff to make a differential

measurement on the window and I'll put

some links in the description but anyway

my idea was to sign a laser beam into a

room and then reflect it off of like a

glass picture like a framed picture

that's hanging on the wall and then you

could use the telescope to look at the

reflection that's appearing on the wall

somewhere else in the room and by

watching through bauble in the

reflection you could recover the audio

based on how the audio is shaking the

the framed picture the glass picture on

the wall so I tried this out tonight and

really didn't get anywhere the dot was

quite large I don't sure why the spread

on that five mil elazer tube is so high

I seem to remember being much better but

anyway the laser dot was a good two

inches in diameter and it's only 150

feet away or something like that but in

the telescope it looked quite nice I

also tried just looking at the route the

10% you know glass is about 10 percent

reflective usually if it's uncoated and

if it's dusty it's it's quite a bit more

reflective so I tried looking at that

with the telescope to just the

reflection off the window and you know I

could see it in the telescope just fine

but with my optical sensor here I didn't

really pick up any recoverable audio so

I think what I'm going to try next is a

little bit more of a sensitive optical

detector maybe a photomultiplier tube

with a low voltage because it's it's

it's much too much light for a

photomultiplier tube maybe if I just

used a low drive voltage it would be

okay I think this has the potential to

work I probably should sit down and do

the geometry to see how much beam wobble

there actually is so in a typical setup

you know the path length between a

framed picture on the wall and the wall

itself maybe is only you know five or

ten feet or something and so the

deflection of the beam is probably gonna

be measured in you know well under a

millimeter probably a hundred microns or

something like that so maybe it's not

even feasible to do this but I thought

it was worth a

and I'll I'll be trying some of the more

some of the other interferometry

techniques I've got a friend that's

pretty interested in this too so

hopefully we can get something going

that isn't so dependent on beam angle

and we'll report back all right let me

know if you have any suggestions and I

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