3. Impulse Sound Sources
117
can be used both in echolocation and in social interactions (Norris et al.
1994). These two components are probably phase linked because they both
have their genesis in the same event, pulse production (although in most
odontocetes there may be more than one pulse generator).
Any reader who has delved into the literature on this subject may have
already encountered some confusion regarding the frequency composition
of odontocete clicks. For example, in addition to the high-frequency (HF)
component (between 120 and 150kHz) in the signals recorded from the
harbor porpoise, Phocoena phocoena (Dubrovskiy et al. 1971; M~hl and
Andersen 1973), there are also reports of a low-frequency (LF) component
(at 2kHz) (Kamminga and Wiersma 1981; Amundin 1991b). The supposition is that this 2 kHz component represents LF energy from the airborne
vibrations. It is probably the same sort of LF (airborne) component that
can be found in early reports for Steno bredanensis, the rough-toothed
dolpin (Norris and Evans 1967), Globicephala scammoni, the North Pacific
pilot whale (Norris 1969), and T. truncatus (Turner and Norris 1966) among
others. It is, however, apparently fundamentally different from the LF
component in the signals from more recent reports as, for example, in
T. truncatus (Au et al. 1974; Moore and Patterson 1983; Sigurdson 1997a)
or Pseudorca crassidens, the false killer whale (Au et al. 1995), which is
presumably a secondary tissue-borne HF peak, perhaps from a second
set of generator tissues.
If the airborne component is detectable, it tends to be lower in frequency
(often falling within our audible range) and lower in intensity than
the tissue-borne component(s). Therefore, the LF airborne component
could be missed, easily ignored, or routinely filtered out, especially if
broad-bandwidth recording equipment were adjusted to be less sensitive
in order to prevent overloading by the more intense HF tissue-borne
component(s). When investigators use, for example, audio range equipment, which is insensitive to the HF tissue-borne components, the airborne
component becomes more prominent, audible, and reported. The airborne
component also tends to be reported in situations where animals
produce overall low-intensity clicks. This can occur because of illness, as in
stranded animals, or when they are housed and recorded in highly reverberant tanks. The airborne component might also be more perceptible in
situations where recordings are made near the animal, as is the case when
contact hydrophones are used. Sorting out these frequency components in
sounds recorded opportunistically from free-ranging animals can also prove
difficult. One contributing factor is that the airborne (LF) component
is more omnidirectional when compared to the tissue-borne (HF) component. In addition, recordings of free-ranging animals cannot control for
the orientation or relative positions of the animals with respect to the
hydrophone, and the equipment necessary to record ultrasonic frequencies
and broad bandwidths is often incompatible with field conditions in the
marine environment.
117
can be used both in echolocation and in social interactions (Norris et al.
1994). These two components are probably phase linked because they both
have their genesis in the same event, pulse production (although in most
odontocetes there may be more than one pulse generator).
Any reader who has delved into the literature on this subject may have
already encountered some confusion regarding the frequency composition
of odontocete clicks. For example, in addition to the high-frequency (HF)
component (between 120 and 150kHz) in the signals recorded from the
harbor porpoise, Phocoena phocoena (Dubrovskiy et al. 1971; M~hl and
Andersen 1973), there are also reports of a low-frequency (LF) component
(at 2kHz) (Kamminga and Wiersma 1981; Amundin 1991b). The supposition is that this 2 kHz component represents LF energy from the airborne
vibrations. It is probably the same sort of LF (airborne) component that
can be found in early reports for Steno bredanensis, the rough-toothed
dolpin (Norris and Evans 1967), Globicephala scammoni, the North Pacific
pilot whale (Norris 1969), and T. truncatus (Turner and Norris 1966) among
others. It is, however, apparently fundamentally different from the LF
component in the signals from more recent reports as, for example, in
T. truncatus (Au et al. 1974; Moore and Patterson 1983; Sigurdson 1997a)
or Pseudorca crassidens, the false killer whale (Au et al. 1995), which is
presumably a secondary tissue-borne HF peak, perhaps from a second
set of generator tissues.
If the airborne component is detectable, it tends to be lower in frequency
(often falling within our audible range) and lower in intensity than
the tissue-borne component(s). Therefore, the LF airborne component
could be missed, easily ignored, or routinely filtered out, especially if
broad-bandwidth recording equipment were adjusted to be less sensitive
in order to prevent overloading by the more intense HF tissue-borne
component(s). When investigators use, for example, audio range equipment, which is insensitive to the HF tissue-borne components, the airborne
component becomes more prominent, audible, and reported. The airborne
component also tends to be reported in situations where animals
produce overall low-intensity clicks. This can occur because of illness, as in
stranded animals, or when they are housed and recorded in highly reverberant tanks. The airborne component might also be more perceptible in
situations where recordings are made near the animal, as is the case when
contact hydrophones are used. Sorting out these frequency components in
sounds recorded opportunistically from free-ranging animals can also prove
difficult. One contributing factor is that the airborne (LF) component
is more omnidirectional when compared to the tissue-borne (HF) component. In addition, recordings of free-ranging animals cannot control for
the orientation or relative positions of the animals with respect to the
hydrophone, and the equipment necessary to record ultrasonic frequencies
and broad bandwidths is often incompatible with field conditions in the
marine environment.