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TRG-TRC007-EN 3
period one
Fundamentals of Sound
notes
Airborne sound is transmitted away from a vibrating body through the transfer
of energy from one air molecule to the next. The vibrating body alternately
compresses and rarefies (expands) the air molecules. The pressure fluctuations
that result from the displacement of these air molecules take the form of a
harmonic, or sine, wave. The amplitude of the wave depicts pressure. The
higher the amplitude, the louder the sound.
This transfer of energy takes time. Each complete sequence of motion
(compression and rarefaction) constitutes a cycle, and the time required to
complete one cycle is the cycle period. The frequency of the periodic motion is
the number of cycles that occur in a second. The unit of measure for frequency
is the hertz (Hz). One hertz is equal to one cycle per second.
The terms pitch and frequency are often (incorrectly) used interchangeably.
Frequency is an objective quantity that is independent of sound-pressure level.
Pitch, however, is a subjective quantity that is primarily based on frequency,
but is also dependent on sound-pressure level and composition. Pitch is not
measured, but is described with terms like bass, tenor, and soprano.
Sound Wave and Frequency
amplitude
amplitude
+
+
-
-
one cycle
one cycle
time
time
cycles
cycles
seconds
seconds
frequency, Hz
frequency, Hz
=
=
Figure 4
frequency, Hz
cycles
seconds
-----------------------
=