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

Report No. 6945
BBN Systems arid Technologies Corporation
otariids have slightly greater sensitivity and,a more elevated high frequency
cutoff than do phocids (Bullock et al. 1971; Schusterman 1981; Moore and
Schusterman 1987; Fig. 2.27). The cutoff frequency of otariid hearing in air
is about 32 to 36 kHz, not much lower than the underwater cutoff of 36-40 kHz
(Schusterman 1981). In contrast, the in-air cutoff of the harbor seal is
around 20 kHz, considerably lower than its underwater cutoff around 60 kHz.
Based on behavioral experiments, both otariids and the harbor seal are most
sensitive at 2 kHz and at 8-16 kHz and notably less sensitive at the
intermediate 4 kHz frequency (Fig. 2.27). These animals are also similar to
one another in that all suffer some loss of hearing sensitivity in air
relative to water when results are expressed in directly comparable units,
i.e., in dB re 1 pw/cm2 (Mdhl 1968a; Moore and Schusterman 1987).
2.3.4 Effects of sound duration
Signal duration influences the hearing threshold, at least under some
circumstances. Almost all behavioral studies on hearing sensitivity have
employed pure tones that were played to the test animals for at least 1/2 s,
and in some cases the animals were allowed to control signal duration.
However, Johnson (1968a) used tones of variable duration, including some that
were much shorter than those generally employed. Frequencies ranged from
Figure 2.27.
In-Air Audiograms of Several Pinnipeds: California Sea Lion
(Moore and Schusterman 1987); Average of Two Fur Seals (Moore
and Schusterman 1987); Harp Seal (Terhune and Ronald 1971); and
Harbor Seal (M6hl 1968a).