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

Report No. 6945
BBN Systems and ~echnologies Corporation
Critical ratio data also allow us to estimate the received level at which a
narrow-band sound will be just detectable given a specified level of broad
band background noise. However, some man-made noises have strong tonal
components whose masking potential is not wholly predictable using critical
ratio data. Limited data on masking of one high-frequency pure tone by
another at various similar frequencies have been reported for Tursiops
(Bullock et al. 1968; Johnson 1971). No such data are available for masking
by low frequency tones, which are common components of industrial noise.
2.3 -6 Adaptations for reduced masking
Most masking studies present the signal and the masking noise from the
same direct ion. The sound localization abilities of marine mammals suggest
that, if signal and noise come from different directions, masking may not be
as severe as the existing critical ratio data suggest. In fish, the critical
ratio at any given frequency decreases as the angle of separation between
signal and masking noise increases (Chapman 1973). When the dominant
background noise comes from a small number of specific sources such as ships
or industrial sites, the background noise may be highly direct-onal. Even some
natural sources of background noise such as surf (Wilson et al. 1985) or ice
may be strongly directional in the horizontal plane. Wind-induced ambient
noise may exhibit significant variation in the vertical plane (Hamson 1985).
In these situations, directional hearing abilities could, in theory,
significantly reduce the masking effects of the noise. In the cases of the
bottlenose dolphin (~ursio~s) and the white whale, there is empirical evidence
that masking effects of a particular noise are indeed strongly dependent on
the relative directions.of arrival of the sound signal of interest vs. the
masking noise.
A study of directional masking at 80 kHz has been done using a bottlenose
dolphin exposed to 0.6 sec tone pulses (Zaitseva et al. 1975). While the
signal transducer was maintained at O0 relative to the animal's midline, a
noise transducer playing 50 to 100 kHz white noise could be moved to any
position around the.dolphin in the horizontal plane. At O0 azimuthal
separation the critical ratio was about 40.7 dB (Zaitseva et al. 1975), almost
identical to the figure obtained by Johnson (1968b). Moving the masking
signal away to angles of 7O to 180° separation caused decreases in critical
ratios from about 35 to 11 dB, respectively (Fig. 2.28). Thus, the masking
effect of background noise on Tursiops echolocation signals near 80 kHz will
be much reduced if the noise is coming from directions other than that of the
target of interest, This, coupled with the strongly directional nature of the
echolocation pulses emitted by toothed whales (e.g., Norris and Evans 1967;
Watkins 1980b; Au et al. 1986, 1987), is a very important adaptation for
improving echolocation range and performance in the presence of noise.
It has been demonstrated that the white whale takes advantage of its
directional sound emission and hearing capabilities while echolocating (Penner
et al'. 1986). When a noise source was placed in line between a white whale
and the echolocation target, the whale echolocated by bouncing its beam.off
the water surface. This allowed the whale to concentrate its echolocation
beam, and presumably its "receiving beam", in a direction slightly (-7")
different than that of the noise source. In this manner the white whale could