Civil Engineering Reference
In-Depth Information
120
90 dBA
100
90
60
40
120
100
90 dBA
90
60
40
125
1000
8000
1/3 octave band center, Hz
FIGURE 31.6
Spectral differences for two different noises that have the same dBA value.
be of equal hazard by the OSHA-required dBA measurements (since they both are 90 dBA), but the 1
3-
octave analysis demonstrates that the lowermost noise is more hazardous as evidenced by the heavy con-
centration of energy in the midrange and high frequencies.
Perhaps the most important application of the spectrum analyzer is to obtain data that will provide the
basis for engineering noise control solutions. For instance, in order to select an absorption material for
lining interior surfaces of a workplace, the spectral content of the noise must be known so that the appro-
priate density and thickness of material may be identified. If the noise is found to be primarily of low
frequency, the absorption techniques may not provide adequate reduction because low frequencies are
more difficult to absorb than high frequencies.
Lacking a spectrum analyzer, the hearing conservationist can obtain a very rough indication of the
dominant spectral content of a noise by using an SLM and taking measurements in both dBA and
dBC for the same noise. If the (dBC
/
dBA) value is large, that is, about 5 dB or more, then it can be
2
concluded that
the noise has considerable low frequency content. If, on the other hand,
the
(dBC
dBA) value is negative, then the noise clearly has strong midrange components, since the A-
weighting curve exhibits slight amplification in the 2000 to 4000 Hz range. Such rules-of-thumb rely
on the differences in the C- and A-weighting curves shown in Figure 31.2. However, they should not
be relied upon in lieu of a spectrum analysis if the noise is believed to have high frequency or narrow
band components that need noise control attention.
2
31.4.1.4 Acoustical Calibrator and Microphone Calibration
Each of the instruments described previously contains a microphone that transduces the changes in
pressure and inputs this signal into the electronics. While modern sound measurement equipment is gen-
erally stable and reliable, calibration is necessary to match the microphone to the instrument so that the
accuracy of the measurement is assured. Because of its susceptibility to varying environmental conditions
and damage due to rough handling, moisture, and magnetic fields, the microphone is the weakest link in
the measurement equipment chain. Therefore, an acoustical calibrator should be applied before and after
each measurement with an SLM. The pretest calibration insures that the instrument is indicating the
Search WWH ::
Custom Search