Analysis and Ranking of the Acoustic Disturbance Potential of ...

Report No. 6945
, BBN Systems and Technologies Cor'poration
noise in the masking band, contrary to the assumption of Fletcher (1940). In
this instance, the threshold signal-to-noise ratio would be negative, i.e.,
< 0 dB.
Direct measurements of critical bandwidth in a marine mammal were
obtained by Moore and Au (1983). They reported that a bottlenose dolphin's
critical bandwidths at 30 and 60 kHz were, respectively, about 10 and 8 times
wider than expected based on the equal power assumption. At 120 kHz the two
methods gave similar results.
Threshold Signal/Noise Ratio. The above-mentioned results of Moore and
Au (1983) show that, at 30 and 60 kHz, the bottlenose dolphin apparently can
detect sounds 10 dB and 9 dB below the level of the noise in the corresponding
critical band, i.e., at S/N = -10 and -9 dB. At 120 kHz the threshold S/N is
near 0 dB.
Critical ratios of 20 dB or more are not incompatible with negative
values of threshold S/N ratios; they are merely different ways of expressing
the same phenomenon. Critical ratios relate total signal level in a narrow
band to spectrum noise level on a Mper HzM basis. The negative S/N ratios
represent signal level in a band to total noise level across that same band.
Though the conclusion that threshold S/N ratios may be negative is
somewhat startling, it has been shown that human subjects can detect signals
such as tones and speech at negative S/N ratios (Miller et al. 1951; Scharf
1970). Structured signals such as speech may be especially well detected due
to differences between their frequency content and that of the noise, and also
due to factors such as redundancy and context that give clues about the type
of sound to expect next.
Payne and Webb (1971) discussed many of the human signal detection data
in relation to the signals propagated by baleen whales, and suggested that
baleen whales may also be capable of detecting sounds at negative S/N ratios.
Hearing abilities of baleen whales are unknown, but some other groups of
marine mammals (especially toothed whales) can discriminate intensities,
frequencies and directions at levels comparable to those of humans. Bearing
this in mind, the hypothesis of Payne and Webb (1971) on the hearing abilities
of baleen whales is in line with data on marine mammal hearing abilities
presented earlier in this section.
Laboratory tests of masking may really be tests of intensity
discrimination, the task being to distinguish between the critical band of
noise alone and the band of noise plus a signal. If a noise band has a
certain intensity, there is a discrete increase in noise intensity that will
cause the noise to be perceived as being more intense. Similarly, if a signal
is added to noise, the signal will be perceived when the sum of the
intensities of signal and noise cause a perceived increase in loudness over
the noise alone;
Even in the absence of much detailed information about intensity discrimination
by marine mammals, critical ratio data give valuable information,
including an indication of the frequencies that are least prone to masking.