Page 270 - The Master Handbook Of Acoustics

CHAPTER
11

Diffraction of Sound

t is well known that sound travels around corners and around obsta-
Icles. Music reproduced in one room of a home can be heard down
the hall and in other rooms. Diffraction is one of the mechanisms
involved in this. The character of the music heard in distant parts of
the house is different. In distant rooms the bass notes are more promi-
nent because their longer wavelengths are readily diffracted around
corners and obstacles.

Rectilinear Propagation

Wavefronts of sound travel in straight lines. Sound rays, a concept
applicable at mid/high audible frequencies, are considered to be pen-
cils of sound that travel in straight lines perpendicular to the wave-
front. Sound wavefronts and sound rays travel in straight lines, except
when something gets in the way. Obstacles can cause sound to be
changed in its direction from its original rectilinear path. The process
by which this change of direction takes place is called diffraction.
Alexander Wood, the early Cambridge acoustician, recalled New-
ton’s pondering over the relative merits of the corpuscular and wave
theories of light. Newton finally decided that the corpuscular theory
was the correct one because light is propagated rectilinearly. Later it
was demonstrated that light is not always propagated rectilinearly,
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