Introduction to Acoustics

odontocetes does not necessarily mean that the same or
a similar propagation process is occurring with baleen
whales.
20.1.1 Hearing Sensitivity of Odontocetes
Almost all our knowledge of hearing in cetaceans comes
from studies performed with small odontocetes. The
most studied species is the Atlantic bottlenose dolphin
(tursiops truncatus). Despite the amount of research performed
with the bottlenose dolphin, our understanding
of auditory processes in these animals lags considerably
behind that for humans and other terrestrial mammals.
There are still many large gaps in our knowledge of
various auditory processes occurring within the most
studied odontocetes. The first audiogram for a cetacean
was measured by Johnson [20.9] foratursiops truncatus.
Since then, audiograms have been determined for
the harbor porpoise (phocoena phocoena) byAndersen
[20.10]andKastelein et al. [20.11], the killer whale
(orcinus orca) byHall and Johnson [20.12] andSzymanski
et al. [20.13], the beluga whale (delphinapterus
leucas) byWhite et al. [20.14], the Pacific bottlenose
dolphin (tursiops gilli) byLjungblad et al. [20.15], the
false killer whale (pseudorca crassidens) byThomas
et al. [20.16], the Chinese river dolphin (lipotes vexillifer)byWang
et al. [20.17], Risso’s dolphins (grampus
griseus) byNachtigall et al. [20.18], the tucuxi (sotalia
fluviatilis) bySauerland and Dehnhardt [20.19], and
the striped dolphin (stenella coeruleoalba)byKastelein
Threshold (dB re 1µPa)
140
120
100
80
60
40
20
0
10 100
Frequency (kHz)
Fig. 20.2 Audiogram for different odontocetes species (after [20.7, 8])
Cetacean Acoustics 20.1 Hearing in Cetaceans 807
et al. [20.20]. The audiograms of these odontocetes are
shown in Fig. 20.2. It is relatively striking to see how
similar the audiograms are between species considering
the vastly different habitats and areas of the world
where some of these animals are found and the large
differences in body size. All the audiograms suggest
high-frequency hearing capabilities, with the smallest
animal, phocoena phocoena having the highest hearing
limit close to 180 kHz. However, the orcinus orca,
which is over 95 times heavier and about six times longer
can hear up to about 105 kHz. The actual threshold values
shown in Fig. 20.2 should not be compared between
species because the different methods of determining the
threshold can lead to different results and because of the
difficulties of obtaining good sound pressure level measurements
in a reverberant environment. For example,
Kastelein et al. [20.11] used a narrow-band frequencymodulated
(FM) signal to avoid multi-path problems
and the FM signals may provide additional cues not
present in a pure-tone signal. Nevertheless, the audiograms
shown in Fig. 20.2 suggest that all the animals
had similar thresholds of 10–15 dB.
A summary of some important properties of the
different audiograms depicted in Fig. 20.2 is given in
Table 20.1. In the table, the frequency of best hearing
is arbitrary defined as the frequency region in which
the auditory sensitivity is within 10 dB of the maximum
sensitivity depicted in each audiogram of Fig. 20.2. With
the exception of the Orcinus and the Lipotes, the maximum
sensitivity of the rest of the species represented
Phocoena p.
Stenella c.
Tursiops t.
Sotalia f.
Delphinapterus l.
Orcinus o.
Pseudorca c.
Inia g.
Tursiops g.
Grampus g.
Lipotes v.
Part F 20.1