Page 149 - Master Handbook of Acoustics

CHAPTER 7

Diffraction

F rom observation, we know that sound travels around obstacles and corners. For example,

music reproduced in one room of a home can be heard down the hall and in other rooms. In

part, this is due to reflections from walls and other surfaces, but it is also due to diffraction.
Diffraction causes sound, which normally travels in straight lines, to bend and travel in other
directions; diffraction occurs even in a free field without reflecting surfaces. However, the character
of the music heard in distant parts of the home is different from the sound at the source. In particular,
the bass notes are more prominent than treble notes. This is partly because their longer wavelengths
more readily diffract (bend) around corners, past obstacles, and through openings. In contrast, treble
notes, with short wavelengths, exhibit comparatively less diffraction. Diffraction thus varies

according to the frequency of the sound in relation to the physical dimensions of the objects causing it.

Diffraction and Wavefront Propagation

Sound wavefronts normally travel rectilinearly, that is, in straight lines. Sound rays, a concept
applicable to mid and high audible frequencies, can be considered as beams of sound that travel in
straight lines perpendicular to the wavefront. Sound wavefronts and sound rays travel in straight
lines, except when something gets in the way. Obstacles can cause sound to change direction from its
original rectilinear path. The mechanism by which this change of direction takes place is called
diffraction. Incidentally, the word diffract is from the Latin word diffringere, which means to break
into pieces.

Isaac Newton pondered the relative merits of the corpuscular and wave theories of light. He
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, that diffraction can cause
light to change its direction of travel. In fact, all types of wave motion, including sound, are subject to
diffraction, a phenomenon caused by phase interference.

Christiaan Huygens formulated a principle that is the basis of the mathematical analysis of
diffraction. The same principle also gives a simple explanation of how sound energy is diverted from
a main beam into a shadow zone. Huygens’ principle can be paraphrased: Every point on the
wavefront of sound that has passed through an aperture or passed a diffracting edge is considered a
new point source radiating energy into the shadow zone. The sound energy at any point in the shadow
zone can be obtained mathematically by summing the contributions of all of these point sources on the

wavefront.