Environmental Engineering Reference
In-Depth Information
Figure 7-41. Calculated effect of microphone height on the measured noise spectra for
point and distributed sources.
to results in free-field conditions (
i.e.
away from all reflecting surfaces). The solid curve
represents a calculated spectrum from a point source (such as a gearbox) located 20 m
above hard ground, as received at a microphone position 1.2 m above ground and 40 m
from the source. The peaks and valleys represent interference patterns caused by differ-
ences between the distances traveled by sounds coming directly from the source to the
receiver and those reflected from the ground surface. Under ideal conditions (with no mean
wind or turbulence and perfect ground reflection), the levels vary alternately from 6 dB
above free-field values to very low values. For an assumed incoherent ring source with a
20-m diameter (which represents the broadband aerodynamic noises) the distributed-source
curve applies. For a microphone height of 1.2 m, the variation of sound pressure level with
frequency from a distributed source is less than that for a point source, but it is still
significant.
A measurement at the ground surface, however, gives a constant enhancement above
free-field values that is 6 dB over the entire frequency range, as indicated by the horizontal
line in Figure 7-41. Thus, it is common practice to place microphones at ground level on
a hard, reflecting surface (such as plywood) and then to deduct 6 dB from all measured
sound pressure levels. When there are very low-frequency components to be measured, cal-
culations suggest that microphone placement is not critical. The first dip in the spectrum
occurs at a frequency well above that associated with low-frequency rotational harmonics,
as shown in Figure 7-5.
Acoustic Instrumentation
The requirements for acoustic instrumentation are derived from the type of measure-
ments to be performed and most directly from the frequency range of concern. For the
frequency range of 20 to 10,000 Hz, standardized equipment is available for detect-
ing, recording, and analyzing acoustic signals. A number of different microphones with
flat frequency responses are available. Likewise, sound-level meters that meet existing
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