REFRACTION OF SOUND

Refraction of Sound in Air Over Long Distances

A well-known example of long-distance transmission of sound is from Shoreline Amphitheater in the City of Mountain View.  But this phenomenon is not new.  For the last 100 years the long distance transmission of train whistle sound has been clearly evident.  So there is nothing special about audio sound that permits long distance travel.  In fact, the sound of every truck travels a long distance too.  The problem with high-energy music is that the sound is high-power, detectable, identifiable and can be readily traced to a single source.

When sound travels long distances, sound travel skywards is refracted downwards under certain weather conditions.  The result is that, distant from the venue, sound will return to earth.  Some areas will receive the sound, some areas will be quiet.  The vertical temperature gradients and wind profiles and consequently strata of different air density, control the process.  As these change, so are the locations of the effected areas where the sound "touches down". 

The way to control this effect, is not with the use of sound barriers, these are much too low to be useful.  One general solution is to retain as much sound in the audience, and pump less sound skyward.  This can be achieved, in part these days, with the use of directional loudspeaker systems.

Compare refraction with diffraction

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