2. Cetacean Ears
47
airborne sound, the conventional reference is 20 IlPa rms derived from the
minimum level required for a normal human ear to detect 2kHz (typically
our most sensitive frequency), which is a diffuse field pressure of 20IlPa,
which has an acoustic power density of approximately 1 picowatt/m
2 • Using
this pressure as a reference standard, the normal minimum human threshold in air is OdB (re 20IlParms). For underwater sound, the conventional
reference was arbitrarily set at 11lPa (American National Standards
Institute, 1968; see Au 1993).
Therefore, with different reference pressure conventions for air and
water, a sound must have a measured pressure 61.5 dB higher in water (re
11lPa) than in air (re 20IlPa), to have an equivalent intensity.
One approach to the intensity versus pressure issue would be to look for
clues in the hearing thresholds of land versus aquatic mammals; that is,
are all the thresholds offset by 26dB or by 61.5 dB? Unfortunately, best
thresholds for marine mammals range anywhere from -25dB re 11lPa to
more than 60dB re 11lPa (Richardson et ai. 1995) with the most common
odontocete values being about 45 dB re 11lPa (Nachtigall et aI., Chapter 8),
which ironically is the most confounding value. Clearly, cetaceans are not
going to make it easy for us.
While these equations give a theoretical background for functional ear
analyses, they also point out that these numbers are only idealized comparisons. Broad spectrum, cross-species, cross-media, and cross-paradigm
studies have far more complex problems than using different referents; in
some cases, comparisons may prove to be impossible. Both subtle and gross
environmental effects (salinity, temperature, depth, ambient noise, surface
reflection, etc.) as well as individual state (motivation, age, pathology) influence results. In most land mammal hearing studies, the test animals are juveniles raised in minimal ambient noise and tested in anechoic conditions.
Marine mammal hearing data are commonly obtained underwater with
normal ambient noise and the test subject is an adult animal for which there
is no auditory history. Cross-media anatomical studies are somewhat less
problematic, but the possibility that the results are skewed by small sample
size or because of pathological or congenital abnormalities must still be
considered.
3. Cetacean Acoustic Divisions
To accurately interpret auditory systems, it is important to have some form
of control or external acoustic metric for categorizing ears. Two forms of
acoustic data are available for cetaceans: audiometric data, which are available for fewer than 12 odontocete species, and sound recordings, which are
available for 67 species of both odontocetes and mysticetes (see Tyack and
Clark, Chapter 4; Nachtigall et aI., Chapter 8). The consensus of these data
is that cetaceans divide grossly into high- and low-frequency sound pro-
47
airborne sound, the conventional reference is 20 IlPa rms derived from the
minimum level required for a normal human ear to detect 2kHz (typically
our most sensitive frequency), which is a diffuse field pressure of 20IlPa,
which has an acoustic power density of approximately 1 picowatt/m
2 • Using
this pressure as a reference standard, the normal minimum human threshold in air is OdB (re 20IlParms). For underwater sound, the conventional
reference was arbitrarily set at 11lPa (American National Standards
Institute, 1968; see Au 1993).
Therefore, with different reference pressure conventions for air and
water, a sound must have a measured pressure 61.5 dB higher in water (re
11lPa) than in air (re 20IlPa), to have an equivalent intensity.
One approach to the intensity versus pressure issue would be to look for
clues in the hearing thresholds of land versus aquatic mammals; that is,
are all the thresholds offset by 26dB or by 61.5 dB? Unfortunately, best
thresholds for marine mammals range anywhere from -25dB re 11lPa to
more than 60dB re 11lPa (Richardson et ai. 1995) with the most common
odontocete values being about 45 dB re 11lPa (Nachtigall et aI., Chapter 8),
which ironically is the most confounding value. Clearly, cetaceans are not
going to make it easy for us.
While these equations give a theoretical background for functional ear
analyses, they also point out that these numbers are only idealized comparisons. Broad spectrum, cross-species, cross-media, and cross-paradigm
studies have far more complex problems than using different referents; in
some cases, comparisons may prove to be impossible. Both subtle and gross
environmental effects (salinity, temperature, depth, ambient noise, surface
reflection, etc.) as well as individual state (motivation, age, pathology) influence results. In most land mammal hearing studies, the test animals are juveniles raised in minimal ambient noise and tested in anechoic conditions.
Marine mammal hearing data are commonly obtained underwater with
normal ambient noise and the test subject is an adult animal for which there
is no auditory history. Cross-media anatomical studies are somewhat less
problematic, but the possibility that the results are skewed by small sample
size or because of pathological or congenital abnormalities must still be
considered.
3. Cetacean Acoustic Divisions
To accurately interpret auditory systems, it is important to have some form
of control or external acoustic metric for categorizing ears. Two forms of
acoustic data are available for cetaceans: audiometric data, which are available for fewer than 12 odontocete species, and sound recordings, which are
available for 67 species of both odontocetes and mysticetes (see Tyack and
Clark, Chapter 4; Nachtigall et aI., Chapter 8). The consensus of these data
is that cetaceans divide grossly into high- and low-frequency sound pro-