342
P.E. Nachtigall et al.
between 25 and 70kHz, and peaks in sensitivity occurred at 25 and 50kHz,
at levels of 47 and 46dB re 11lPa, respectively.
An audiogram was collected for a false killer whale (Pseudorca
crassidens) by Thomas et al. (1988). Results from this study are shown
in Figure 8.2. Thresholds were reported from 2 to 115 kHz, with the
range of best sensitivity between 16 and 64kHz, and peak sensitivity at
64kHz.
Baseline audiometric work was also conducted with a Risso's dolphin
(Grampus griseus) by Nachtigall et al. (1995). This study was unique in
that it was conducted in a natural setting: Kaneohe Bay, Hawaii,
with natural background noise, including snapping shrimp (Au and Banks
1998). Because of the natural background noise, it was not possible to determine peak sensitivity in the species, as the background noise apparently
masked sensitivity to frequencies from 5 to 80 kHz. Ambient noise levels
throughout this range were approximately 65 dB re 1IlPa2/Hz, and measured, thresholds for these frequencies ranged from 64 to 74 dB re 11lPa.
The threshold at 1.6kHz was 124dB, while at 100 and 110kHz the thresholds were 124.2 and 122.9dB, respectively. The data indicate that the audiogram was similar to the values reported for P crassidens (Thomas et al.
1988) if the masking effect of the background noise was considered (see
Fig. 8.2).
In summary, the auditory threshold research conducted with cetaceans
reviewed above shows a consistent V-shaped audiogram, typical of a mammalian hearing curve. In all species tested, maximum sensitivity was at frequencies at or above 15 kHz, and all except the killer whale studied by Hall
and Johnson (1972) reported good sensitivity at much higher frequencies,
up to the high of 115 kHz for T. truncatus (Johnson 1966). The signal levels
at the most sensitive range were also relatively consistent, between 40 to
50 dB re 11lPa, the exception, again, being the killer whale measured by
Hall and Johnson (1972), which had a sensitivity level of 30dB re 11lPa at
15 kHz. Below the range of most acute hearing, all species showed gradual
decreases in sensitivity, while above this range a sharp drop in highfrequency sensitivity was observed. The frequency where this drop off
occurred seems to be a species-specific trait (Thomas et al. 1988), although
for all species except 0. orca sensitivity did not begin to drop off until above
100kHz.
2.2 Temporal Auditory Summation
The detectability of an auditory stimulus generally increases with increasing duration of the signal up to some maximum duration, called the "time
constant." Durations longer than the time constant have no further benefit
for detection. There is typically a 3 dB improvement in threshold for each
doubling of the signal duration (Plomp and Bouman 1959; Zwislocki 1960).
This pattern is consistent with the idea that detection employs a temporal
P.E. Nachtigall et al.
between 25 and 70kHz, and peaks in sensitivity occurred at 25 and 50kHz,
at levels of 47 and 46dB re 11lPa, respectively.
An audiogram was collected for a false killer whale (Pseudorca
crassidens) by Thomas et al. (1988). Results from this study are shown
in Figure 8.2. Thresholds were reported from 2 to 115 kHz, with the
range of best sensitivity between 16 and 64kHz, and peak sensitivity at
64kHz.
Baseline audiometric work was also conducted with a Risso's dolphin
(Grampus griseus) by Nachtigall et al. (1995). This study was unique in
that it was conducted in a natural setting: Kaneohe Bay, Hawaii,
with natural background noise, including snapping shrimp (Au and Banks
1998). Because of the natural background noise, it was not possible to determine peak sensitivity in the species, as the background noise apparently
masked sensitivity to frequencies from 5 to 80 kHz. Ambient noise levels
throughout this range were approximately 65 dB re 1IlPa2/Hz, and measured, thresholds for these frequencies ranged from 64 to 74 dB re 11lPa.
The threshold at 1.6kHz was 124dB, while at 100 and 110kHz the thresholds were 124.2 and 122.9dB, respectively. The data indicate that the audiogram was similar to the values reported for P crassidens (Thomas et al.
1988) if the masking effect of the background noise was considered (see
Fig. 8.2).
In summary, the auditory threshold research conducted with cetaceans
reviewed above shows a consistent V-shaped audiogram, typical of a mammalian hearing curve. In all species tested, maximum sensitivity was at frequencies at or above 15 kHz, and all except the killer whale studied by Hall
and Johnson (1972) reported good sensitivity at much higher frequencies,
up to the high of 115 kHz for T. truncatus (Johnson 1966). The signal levels
at the most sensitive range were also relatively consistent, between 40 to
50 dB re 11lPa, the exception, again, being the killer whale measured by
Hall and Johnson (1972), which had a sensitivity level of 30dB re 11lPa at
15 kHz. Below the range of most acute hearing, all species showed gradual
decreases in sensitivity, while above this range a sharp drop in highfrequency sensitivity was observed. The frequency where this drop off
occurred seems to be a species-specific trait (Thomas et al. 1988), although
for all species except 0. orca sensitivity did not begin to drop off until above
100kHz.
2.2 Temporal Auditory Summation
The detectability of an auditory stimulus generally increases with increasing duration of the signal up to some maximum duration, called the "time
constant." Durations longer than the time constant have no further benefit
for detection. There is typically a 3 dB improvement in threshold for each
doubling of the signal duration (Plomp and Bouman 1959; Zwislocki 1960).
This pattern is consistent with the idea that detection employs a temporal