Page 26 - The Master Handbook Of Acoustics

CHAPTER
1

Fundamentals of Sound

ound can be defined as a wave motion in air or other elastic
Smedia (stimulus) or as that excitation of the hearing mechanism
that results in the perception of sound (sensation). Which definition
applies depends on whether the approach is physical or psy-
chophysical. The type of problem dictates the approach to sound. If
the interest is in the disturbance in air created by a loudspeaker, it is
a problem in physics. If the interest is how it sounds to a person near
the loudspeaker, psychophysical methods must be used. Because
this book addresses acoustics in relation to people, both aspects of
sound will be treated.
These two views of sound are presented in terms familiar to those
interested in audio and music. Frequency is a characteristic of peri-
odic waves measured in hertz (cycles per second), readily observable
on a cathode-ray oscilloscope or countable by a frequency counter.
The ear perceives a different pitch for a soft 100 Hz tone than a loud
one. The pitch of a low-frequency tone goes down, while the pitch of
a high-frequency tone goes up as intensity increases. A famous
acoustician, Harvey Fletcher, found that playing pure tones of 168 and
318 Hz at a modest level produces a very discordant sound. At a high
intensity, however, the ear hears the pure tones in the 150-300 Hz
octave relationship as a pleasant sound. We cannot equate frequency
and pitch, but they are analogous.
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