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Nowadays the “A”-weighting network is the most widely used since the “B” and “C” weightings do not correlate well with subjective tests. This does not weight the signal, but it enables the signal to pass through unmodified. In addition to one or more of these weighting networks, sound level meters usually have a linear or “Lin.” network. A specialized characteristic, the “D”-weighting, has also been standardized for aircraft noise measurements. The “A”-weighting network weights a signal in a manner, which approximates an inverted equal loudness contour at low Sound Pressure Levels (SPLs) the “B”-weighting network corresponds to a contour at medium SPLs and the “C”-weighting network corresponds to an equal loudness contour at high SPLs.
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This has resulted in three different internationally standardized characteristics termed “A,” “B,” and “C” weightings. The signal may pass through a weighting network, which is an electronic circuit whose sensitivity varies with frequency in the same way as the human ear, thus simulating the equal loudness contours ( ISO:226:2003). Therefore, the sound pressure filter is used to filter the sound signal according to different coefficients for evaluation at different frequencies. The human ear does not represent a linear system, i.e., the “sensing” level is not the same for different frequencies (e.g., 100 Hz and 1 kHz).