MadSci Network: Physics

Re: Can a laser be used as a long distance microphone?

Date: Mon Aug 2 18:36:40 2004
Posted By: Kenneth Beck, Senior Research Scientist, Chemistry and Physics of Complex Systems, Pacific Northwest National Laboratory
Area of science: Physics
ID: 1091081043.Ph

Dear Stewart,

Yes, you are correct.  This does work as a means of detecting audio 
vibrations from distant objects, such as windows.  The optics train for 
such a sensitive device requires the use of a technique known as 
interferometry.  This aids in the elimination of background noise.

An advanced version of this type laser microphone is describe at this 


So, who originally came up with the idea of a microphone which would 
modulate light with sound?

On June 3, 1880, Alexander Graham Bell transmitted the first wireless 
telephone message on his newly-invented "photophone" over 213 meters 
(~700 feet) using sunlight. The device allowed for the transmission of 
sound on a beam of light.  

Bell's photophone worked by projecting voice through an instrument toward 
a mirror. Vibrations in the voice caused similar vibrations in the 
mirror. Bell directed sunlight into the mirror, which captured and 
projected the mirror's vibrations. The vibrations were transformed back 
into sound at the receiving end of the projection.  However, until the 
development of modern fiber optics, technology for the secure transport 
of light inhibited use of Bell's invention. 

For details of Bell's patent, read this URL...


If you're more curious, you can easily make your own laser microphone and 
receiver to tranmsit voice or music.  Check out this URL...


Good Luck!

---* Dr. Ken Beck

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