Civil Engineering Reference
In-Depth Information
10
PERPENDICULAR
5
RANDOM
0
GRAZING
PERPENDICULAR
-5
GRAZING
-10
-15
-20
20
50
100
200
500
1k
2k
5k
10k
20k
Frequency in Hertz
FIGURE 31.5 Frequency response of an hypothetical microphone for three angles of incidence. (Adapted from
Peterson, A.P.G. (1979). Noise measurements: Instruments. In Harris, C. M. (Ed.), Handbook of Noise Control
[pp. 5-1-5-19]. New York: McGraw-Hill. With permission.)
incidence of approximately 70
. This will produce a measurement most closely corresponding to the
random-incidence response. Care must be taken to avoid shielding the microphone with the body or
other structures. The response of microphones can also vary with temperature, atmospheric pressure,
and humidity, with temperature being the most critical factor. Correction factors for variations in
decibel readout due to temperature effects are supplied by most microphone manufacturers. Atmos-
pheric effects are generally only significant when measurements are made in aircraft or at very high alti-
tudes, and humidity has a negligible effect except at very high levels. In any case, microphones must not
be exposed to moisture or large magnetic fields, such as those produced by transformers. When used in
windy conditions, a foam windscreen should be placed over the microphone. This will reduce the con-
taminating effects of wind noise, while only slightly influencing the frequency response of the micro-
phone at primarily high frequencies. In an industrial setting, the windscreen offers the additional
benefit of protection of the microphone from damage due to striking and/or airborne foreign matter.
8
31.4.1.1.3 Root Mean Square
Because sound consists of pressure fluctuations above and below ambient air pressure for which the
arithmetic average is zero, a root mean square (rms) averaging procedure is applied within the SLM
when FAST, SLOW, or IMPULSE measurements are taken. In effect, each pressure (or converted
voltage) value is squared, the arithmetic sum of all squared values is then obtained, and finally the
square root of the sum is computed to provide the rms value. The rms value is what appears on the
meter's display.
31.4.1.1.4 TRUE PEAK SLM
Some SLMs include an unweighted TRUE PEAK setting that does not utilize the rms measurement
averaging technique, but instead provides an indication of the actual peak SPL reached during a
pressure impulse. This measurement mode is necessary for determining if the OSHA limit of 140 dB
for impulsive exposure is exceeded. A Type 1 or 2 meter must be capable of measuring a 50-msec
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