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Report No'. 6945 BBN Systems and Technologies Corporation where CR represents the critical ratio in terms of signal level relative to spectrum level noise. This gives the bandwidth in Hz of the band of masking noise that contains power equal to that of the signal tone (see Scharf 1970). Johnson ( 1968b) , Terhune ( 198 1 ) and others have used equation (2) to calculate masking bandwidths in Hz for marine mammals based on the assumption that signal power equals masking power. Figure 2.29 shows the results of such calculations, expressed as a percentage of the center frequency of the masking band. Based on the available critical ratio data and the equal power assumption, masking bands often appear to be on the order of 1/6th to 1/3rd of an octave in width, i.e., bandwidth equa1.s 11.6 to 23.2% of the center frequency (Fig. 2.29). If one of these "rules of the thumbt1 were spictly true, the critical ratios at several frequencies would be as follows: 100 Hz 1 10 kHz 100 kHz 1/3 octave 13.6 dB 23.6 d~ 33.6 dB 43.6 dB 1/6 octave 10.6 20.6 30.6 $0.6 As evident from Figure 2.29, the critical ratios at low frequencies (human, cat) exceed those expected if the masking bandwidth is 1/3 octave. In contrast, critical ratios for marine mammals listening at most higher frequencies are somewhat lower than those expected if the masking bandwidth were 1/3 octave, or even 1/6 octave, particularly if one ignores the harp and ringed seal data that have been questioned by Moore and Schusterman ( 1987). When attempting to calculate the radius of audibility of marine mammal calls or industrial noise in the presence of background noise, several workers have assumed that masking bands are 1/3 octave wide (e.g., Payne and Webb 1971; Gales 1982; Miles et al. 1987). Gales (1982) also considered the possibility that, at frequencies below 450 Hz, the masking bandwidth exceeds 11'3 octave. As evident from Fig. 2.29, masking bandwidth may indeed exceed 1/3 octave at low frequencies if marine mammals listening in water are similar to terrestrial mammals listening in air. If so, noise power in the masking band will be higher than calculated from the 1/3 octave assumption, and the radius of audibility of low frequency sound would be less than that calculated. Conversely, for higher frequencies where the masking bandwidth seems to be less than 1/3 octave based on critical ratio data for marine mammals, the radius of audibility could be somewhat greater than calculated assuming a masking bandwidth of 1/3 octave. All of these estimates depend on the validity of the equal power assumption, i.e., that a narrowband sound signal is masked when total noise power in the masking band equals or exceeds the power of the signal. The equal-power assumption may not accurately represent the width of the masking band (Scharf 1970; Kryter 1985). Other methods, measure the masking band directly by manipulating the bandwidth of sounds masking a signal. The term "critical bandf1 is used for direct empirical measures of the masking band (Scharf 1970). In humans, the critical band in Hz is about 2.5 times wider than the critical ratio equal-power band at the same center frequency. This means that humans can detect a signal whose level is somewhat less than the band level of
Frequency (Hz) Figure 2.29. Estimated masking bandwidths of several marine and terrestrial mammals, expressed as a percentage of center frequency. Dashed horizontal lines show selected standard bandwidths. Assumes that signal power must equal or exceed noise power in masking band in order to be detected, i.e., masking bandwidth = (antilog Critical Ratio)/lO. Based on same sources as previous figure.
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FIG. 4.1 CORRECTION TO 300 M REFERE
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