Page 184 - Master Handbook of Acoustics
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disturbances in the ripple tank.
For example, at a point in space, assume an air particle responds to a passing disturbance with a
given amplitude and 0° direction. At the same instant another disturbance requires the same
amplitude, but with a 180° direction. This air particle satisfies both disturbances at that instant by not
moving at all.
Tonal Signals and Comb Filters
A microphone is a passive instrument. Its diaphragm responds to fluctuations in air pressure that
occur at its surface. If the rate of such fluctuations (frequency) falls within its operating range, it
generates an output voltage proportional to the magnitude of the pressure. For example, if a 100-Hz
acoustical tone actuates the diaphragm of a microphone in free space, a 100-Hz voltage appears at the
microphone terminals. If a second 100-Hz tone, identical in pressure but 180° out of phase with the
first signal, strikes the microphone diaphragm, one acoustically cancels the other and the microphone
voltage falls to zero. If an adjustment is made so that the two 100-Hz acoustical signals of identical
amplitude are in phase, the signals reinforce each other, and the microphone delivers twice the output
voltage, an increase of 6 dB. The microphone responds to the pressures acting on its diaphragm. That
is, the microphone simply responds to the vector sum of air pressure fluctuations impinging upon it.
This characteristic of the microphone can help us understand acoustical comb-filter effects.
A 500-Hz sine tone is shown as a frequency component in Fig. 10-1A. All of the energy in this
pure tone is located at this frequency. Figure 10-1B shows an identical signal except it is delayed by
0.5 msec with respect to the signal of A. The signal has the same frequency and amplitude, but the
timing is different. Consider both A and B as acoustical signals combining at the diaphragm of a
microphone. Signal A could be a direct signal and B a reflection of A from a nearby side wall. What
is the nature of the combined signal output by the microphone?