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T.w. Cranford
ently produce pulses with the oligocyclic waveform (and have the potential
to contain a bimodal high-frequency spectrum). A few other anatomic
details, which are beyond the scope of this chapter, also correlate with pulse
type, such that we may now begin to predict signal type from anatomic
configuration (Cranford 1992a; Cranford et al. 1996).
For the uninitiated student there is some potential for confusion regarding acoustic frequency composition reported in the literature on odontocete sounds and their generation. In the last few paragraphs, all references
to frequency refer to the tissue-born, primarily ultrasonic, components
implicated in echolocation behavior. In the literature, various statements
about low- and high-frequency components in the spectrum of acoustic
pulse waveforms may not always refer to the same components. Since these
terms are relative, they can pertain to different components depending
upon the species and recording situation. In my estimation, the key to
understanding and the potential for confusion arise because a distinction
has not always been made between the "airborne" and "tissue-borne" components of the clicks these animals generate. This distinction is important
to the issues raised in this chapter.
The distinction being made here is between two distinct but intimately
related components. The airborne component is the result of vibrations
that occur primarily in the nasal air spaces. These vibrations will not be
effectively coupled to the aquatic environment (in the absence of special
circumstances, such as resonance), because of the great impedance mismatch between air and tissue. The tissue-borne component is comprised
of vibrations that occur primarily within the tissues of the head, which
are impedance matched to the environment, primarily through special
"acoustic fats" (Norris 1968). Consequently, these tissue-borne vibrations
will be comparatively higher in frequency, have more directionality, and be
more effectively coupled to the aquatic environment than the airborne
component. This distinction is subtle since it is likely that the bulk flow of
air causes or actuates both vibrational components during the pulse generation process, suggesting a phase-linked relationship. The distinction
becomes more complicated when we realize that the tissue-borne component can be composed of one or two (perhaps more) distinct frequency
peaks. The important point is that the tissue-borne vibrations are acoustically coupled to the aquatic medium and should be relatively more intense
than the airborne component when they are both recorded in the water.
The impedance mismatch between the air and tissue (or water) means that
airborne vibrations are mostly internally reflected, absent some special
effect like resonance.
The airborne component may represent "generator noise," a by-product
of the functioning mechanism, while the tissue-borne component is the
propagated sound used for sonar. There is evidence that both components
are functional, the more omnidirectional airborne component in social circumstances (Amundin 1991a), and the directional tissue-borne component
T.w. Cranford
ently produce pulses with the oligocyclic waveform (and have the potential
to contain a bimodal high-frequency spectrum). A few other anatomic
details, which are beyond the scope of this chapter, also correlate with pulse
type, such that we may now begin to predict signal type from anatomic
configuration (Cranford 1992a; Cranford et al. 1996).
For the uninitiated student there is some potential for confusion regarding acoustic frequency composition reported in the literature on odontocete sounds and their generation. In the last few paragraphs, all references
to frequency refer to the tissue-born, primarily ultrasonic, components
implicated in echolocation behavior. In the literature, various statements
about low- and high-frequency components in the spectrum of acoustic
pulse waveforms may not always refer to the same components. Since these
terms are relative, they can pertain to different components depending
upon the species and recording situation. In my estimation, the key to
understanding and the potential for confusion arise because a distinction
has not always been made between the "airborne" and "tissue-borne" components of the clicks these animals generate. This distinction is important
to the issues raised in this chapter.
The distinction being made here is between two distinct but intimately
related components. The airborne component is the result of vibrations
that occur primarily in the nasal air spaces. These vibrations will not be
effectively coupled to the aquatic environment (in the absence of special
circumstances, such as resonance), because of the great impedance mismatch between air and tissue. The tissue-borne component is comprised
of vibrations that occur primarily within the tissues of the head, which
are impedance matched to the environment, primarily through special
"acoustic fats" (Norris 1968). Consequently, these tissue-borne vibrations
will be comparatively higher in frequency, have more directionality, and be
more effectively coupled to the aquatic environment than the airborne
component. This distinction is subtle since it is likely that the bulk flow of
air causes or actuates both vibrational components during the pulse generation process, suggesting a phase-linked relationship. The distinction
becomes more complicated when we realize that the tissue-borne component can be composed of one or two (perhaps more) distinct frequency
peaks. The important point is that the tissue-borne vibrations are acoustically coupled to the aquatic medium and should be relatively more intense
than the airborne component when they are both recorded in the water.
The impedance mismatch between the air and tissue (or water) means that
airborne vibrations are mostly internally reflected, absent some special
effect like resonance.
The airborne component may represent "generator noise," a by-product
of the functioning mechanism, while the tissue-borne component is the
propagated sound used for sonar. There is evidence that both components
are functional, the more omnidirectional airborne component in social circumstances (Amundin 1991a), and the directional tissue-borne component