Gas Turbine Handbook : Principles and Practices

Gas Turbine Acoustics and Noise Control 153
frequencies and begins to fade at high frequencies. Researchers developed
electronic weighting curves or filters for sound level meters to
process the sound as the ear would hear it. These weighting curves
are commonly known as A, B, C, D, and E and are applied depending
upon what is being measured. A and C weightings (or filtering)
are the most widely used in environmental acoustics. A sound level
meter set to measure A-weighted sound levels captures all the sound
energy from 10 Hz to about 12,000 Hz or more and processes the
measurement to arrive at a single sound level value. The following
figure illustrates the weighting curves as applied to the sound spectra
from 10 Hz through 10,000 Hz and the actual weighting values are
listed in Table 10-1. An example of performing an overall A-weighted
calculation is given in Appendix C-12.
Applying the A-weighting curve shown in Figure 10-2 to the
noise signature shown in Figure 10-1 results in an overall A-weighted
sound level of 120.5 dB. This is a much easier number to deal with
than having to always produce a spectrum and show all the discrete
sound pressure levels. To continue this simplification approach, the
Figure 10-2. Frequency response curves for A and C weighting
measurements