MadSci Network: Physics

Re: Can one sound wave distort another sound wave?

Date: Wed Dec 6 02:51:08 2000
Posted By: Bruno Putzeys, Staff, Electroacoustics and Analog Electronics, Philips ITCL
Area of science: Physics
ID: 975430685.Ph


A sound wave is a travelling pattern of increased and decreased pressure. 
Pressure can be increased indefinitely but cannot decrease below zero. 
This means that air is a non-linear medium and that intermodulation can 
definitely occur. It does take sound pressure levels in the order of air 
pressure to produce this effect discernably. It is probably noticeable 
from 150dB SPL onwards, a level lethal to human listeners.

Light is an electromagnetic wave (ie. a pair of perpendicular waves in 
magnetic and electrical fields that create and maintain eachother), and 
hence does not need a medium. There is no limit to electrical and 
magnetical field-strengths (in a vacuum at least), so no non-linear 
effects occur.

There do exist optically transmissive media that have non-linear behaviour 
and in which distortion and intermodulation between light-wave occur. Such 
media are used, for example, to double the frequency of near-infrared 
laser light to make it green. Similarly, they can be used to build 3d 
displays by allowing a pair of laser beams to cross inside such a medium 
where the intersection becomes a new source with frequencies equal to the 
sum and difference of the two source frequencies.
In such a medium, your red and green spotlights (given sufficient 
intensity) would probably produce violet and far infrared.

Some more on this at



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