Editing Atmospheric diffraction (section)

==Acoustical diffraction near the Earth's surface==

In the case of [[sound waves]] travelling near the Earth's surface, the waves are diffracted or bent as they traverse by a geometric edge, such as a wall or building.  This phenomenon leads to a very important practical effect:  that we can hear "around corners".  Because of the frequencies involved considerable amount of the sound energy (on the order of ten percent) actually travels into this would be sound "shadow zone".  Visible light exhibits a similar effect, but, due to its much shorter [[wavelength]], only a minute amount of light energy travels around a corner.

A useful branch of acoustics dealing with the design of [[noise barrier]]s examines this acoustical diffraction phenomenon in quantitative detail to calculate the optimum height and placement of a soundwall or berm adjacent to a highway.

This phenomenon is also inherent in calculating the sound levels from [[aircraft noise]], so that an accurate determination of topographic features may be understood.  In that way one can produce sound level [[isopleth]]s, or contour maps, which faithfully depict outcomes over variable terrain.