Engineering Acoustics. Malcolm J. Crocker

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frequency. There are other masking effects that are not normally as important for sound quality evaluations as the frequency effects discussed so far but still need to be described [16–19, 21–30].

Graph depicts the masking effect of a narrow-band noise of bandwidth 160 Hz centered at 1000 Hz. The contours join sound pressure levels of pure tones that are just masked by the 1000-Hz narrow-band noise at the sound pressure level shown on each contour. Graph depicts postmasking at different masker sound pressure levels. Graph depicts postmasking of 5-ms, 2-kHz tones preceded by bursts of uniform masking noise are plotted as a function of the delay between masker and signal offsets.

      Source: Reprinted with permission from [31], American Institute of Physics.

      4.3.4 Pitch

      Masking noise may change the pitch of a tone. If the masking noise is of a higher frequency, the pitch of the masked tone is reduced slightly; if the masking noise is of a lower frequency the pitch is increased slightly. This can be explained [34, 35] by a signal/noise ratio argument. The locus of the position on the basilar membrane at which the tone is normally perceived could be changed by the masking noise [34].

      Example 4.4

      What is the equivalent frequency of 2595 mel?

      Solution

      4.3.5 Weighted Sound Pressure Levels

      Figure 4.6 in this chapter shows that the ear is most sensitive to sounds in the mid‐frequency range around 1000–4000 Hz. It has a particularly poor response to sound at low frequency. It became apparent to scientists in the 1930s that electrical filters could be designed and constructed with a frequency response approximately equal to the inverse of these equal loudness curves. Thus A‐, B‐, and C‐weighting filters were constructed to approximate the inverse of the 40‐, 70‐, and 90‐phon contours (i.e. for low‐level, moderate, and intense sounds), respectively (see Figure 4.6). In principle, then, these filters, if placed between the microphone and the meter display of an instrument such as a sound level meter, should give some indication of the loudness of a sound (but for pure tones only).

Graph depicts A-, B-, and C-weighting filter characteristics used with sound level meters.

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