3. Impulse Sound Sources
137
cyclic) is multiplied by a Gabor function, the stable portion of each waveform remains, while the variable component of the functional signal will be
filtered out. It turns out that this mathematically stable portion of the signal
is the same as the stable portion demonstrated by the Kamminga's cinematic technique. This stable part of the signal also accounts for the overwhelming majority of the total energy contained in the pulse waveform. It
is tempting to conclude that the stable portions of each waveform represent the actual generator signal or the "signature" of the pulse generator,
but that is probably an overly simplistic interpretation. It may, however, be
safe to infer that the stable portions of each functional pulse waveform
represents the basic interaction between the initial generator signal and
the most significant functional components of the entire signal generation/propagation system.
Once odontocete sonar signals are generated, they are propagated
forward into a beam, one of the earliest recognized features of odontocete
echolocation (Schevill and Lawrence 1956; Norris et al. 1961; Schevill and
Watkins 1966; Norris and Evans 1967). Biosonar beam formation has been
the focus of substantial work ever since (see reviews in Norris 1968,1969,
1975; Morris 1986; Cranford 1992a; Au 1993). There has been considerable
evidence gathered about the dimensions and characteristics of the beam
for T. truncatus, and somewhat less for D. leucas and P crassidens; unfortunately, precious little is known about the sound beam in most other
odontocete species.
The mechanisms of beam formation are less well known than the beam
characteristics, but it is generally accepted that focusing and formation of
the sonar beam occurs in a series of stages, as suggested by Norris (1964).
Each of the stages is, more or less, associated with particular anatomic structures thought to be responsible for a portion of the beam formation process.
The bony skull, inflatable nasal diverticula, adipose topography of the
melon, and connective tissue theca, along with the nasal and rostral muscles,
all probably have a cumulative effect on the formation of the sonar beam
(Cranford 1992a).
5.1 Skull
The skull has long been suspected of playing a significant role in beam
formation (Norris 1964; Romanenko 1973, 1974; Schenkkan 1973;
Dubrovskiy and Zaslavskiy 1975). Curiously, the asymmetric quality of the
skull is often cited as an indicator of its functional importance, possibly
contributing to the formation of an asymmetric sound field. Asymmetry
in the sound field is known to help reduce ambiguity in solving for target
location.
Research implicates the geometry of the skull as an important facet of
the forward beam projection (Evans et al. 1964; Schenkkan 1972;
Dubrovskiy and Zaslavskiy 1975; Litchfield et al. 1979; Alcuri 1980;
137
cyclic) is multiplied by a Gabor function, the stable portion of each waveform remains, while the variable component of the functional signal will be
filtered out. It turns out that this mathematically stable portion of the signal
is the same as the stable portion demonstrated by the Kamminga's cinematic technique. This stable part of the signal also accounts for the overwhelming majority of the total energy contained in the pulse waveform. It
is tempting to conclude that the stable portions of each waveform represent the actual generator signal or the "signature" of the pulse generator,
but that is probably an overly simplistic interpretation. It may, however, be
safe to infer that the stable portions of each functional pulse waveform
represents the basic interaction between the initial generator signal and
the most significant functional components of the entire signal generation/propagation system.
Once odontocete sonar signals are generated, they are propagated
forward into a beam, one of the earliest recognized features of odontocete
echolocation (Schevill and Lawrence 1956; Norris et al. 1961; Schevill and
Watkins 1966; Norris and Evans 1967). Biosonar beam formation has been
the focus of substantial work ever since (see reviews in Norris 1968,1969,
1975; Morris 1986; Cranford 1992a; Au 1993). There has been considerable
evidence gathered about the dimensions and characteristics of the beam
for T. truncatus, and somewhat less for D. leucas and P crassidens; unfortunately, precious little is known about the sound beam in most other
odontocete species.
The mechanisms of beam formation are less well known than the beam
characteristics, but it is generally accepted that focusing and formation of
the sonar beam occurs in a series of stages, as suggested by Norris (1964).
Each of the stages is, more or less, associated with particular anatomic structures thought to be responsible for a portion of the beam formation process.
The bony skull, inflatable nasal diverticula, adipose topography of the
melon, and connective tissue theca, along with the nasal and rostral muscles,
all probably have a cumulative effect on the formation of the sonar beam
(Cranford 1992a).
5.1 Skull
The skull has long been suspected of playing a significant role in beam
formation (Norris 1964; Romanenko 1973, 1974; Schenkkan 1973;
Dubrovskiy and Zaslavskiy 1975). Curiously, the asymmetric quality of the
skull is often cited as an indicator of its functional importance, possibly
contributing to the formation of an asymmetric sound field. Asymmetry
in the sound field is known to help reduce ambiguity in solving for target
location.
Research implicates the geometry of the skull as an important facet of
the forward beam projection (Evans et al. 1964; Schenkkan 1972;
Dubrovskiy and Zaslavskiy 1975; Litchfield et al. 1979; Alcuri 1980;