6. SOUND PRODUCTION AND DETECTION
153
of radius 3 cm at atmospheric pressure would be about 100 Hz. If we
treat the swim bladders of fishes as if they were spherical air bubbles,
the bladder dimensions in most individuals of squirrelfish, red hind, etc.,
would give resonant frequencies in the 50-100-H~ range. Even such
rough calculations indicate that the fundamental frequencies of most fish
sounds are in the same order of magnitude as the resonant frequencies of
the swim bladders. The resonance peak of fish swim bladders was found
to be flat, with a maximum rise of about 3-4 dB (Tavolga, 1964b). Conceivably, this would enable the fish to gain some efficiency from
resonance, but this gain would be quite small.
Harris (1964) investigated the properties of mathematical and physical models of vibrating swim bladders. He concluded that the pulsating
air bubble, such as a swim bladder, would have the elastic and resonant
qualities for a highly efficient underwater sound producer in the low frequency range that fishes usually produce. The toadfish, for example, develops sound pressures of up to 35 dB pb at distances within 5 meters,
and many species produce sounds powerful enough to be heard distinctly
above the sea surface. An underwater loudspeaker was constructed along
the physical principles of a toadfish swim bladder and was found to be
quite efficient in the 100-2000-Hz range (Tavolga and Wodinsky, 1963).
The mechanics of swim bladder sound production can be summarized
as follows: The fundamental frequency of the sound emitted is a direct
translation of the contraction frequency of the sonic muscles. The contractions of the sonic muscles produce volume and pressure changes in
the swim bladder, and, therefore, the entire surface of the bladder pulsates. The fact that the natural frequency of the swim bladder as an air
bubble is in the same order of magnitude as the vibration frequency of
the sonic muscles undoubtedly aids the efficiency of the system, and the
mechanical energy of the moving muscle is transmitted by way of air
pressure changes to the entire bladder surface. These pulsations are transmitted through the tissues of the fish to the outside with little loss, since
the fish is virtually transparent to water-borne sound. In acoustic and electronic terms, therefore, the swim bladder is an impedance-matching device analogous to the large surface cone of a high fidelity loudspeaker.
7 . HYDRODYNAMIC AND SWIMMING SOUNDS
The movement of any object through the water will create displacement. Such displacements and compression waves may be rhythmic subsonic vibrations when produced by the fins and body of a swimming fish.
Such phenomena have been classified as hydrodynamic sounds by
Shishkova ( 1958a,b) and Moulton ( 1960b). Moulton also considered
153
of radius 3 cm at atmospheric pressure would be about 100 Hz. If we
treat the swim bladders of fishes as if they were spherical air bubbles,
the bladder dimensions in most individuals of squirrelfish, red hind, etc.,
would give resonant frequencies in the 50-100-H~ range. Even such
rough calculations indicate that the fundamental frequencies of most fish
sounds are in the same order of magnitude as the resonant frequencies of
the swim bladders. The resonance peak of fish swim bladders was found
to be flat, with a maximum rise of about 3-4 dB (Tavolga, 1964b). Conceivably, this would enable the fish to gain some efficiency from
resonance, but this gain would be quite small.
Harris (1964) investigated the properties of mathematical and physical models of vibrating swim bladders. He concluded that the pulsating
air bubble, such as a swim bladder, would have the elastic and resonant
qualities for a highly efficient underwater sound producer in the low frequency range that fishes usually produce. The toadfish, for example, develops sound pressures of up to 35 dB pb at distances within 5 meters,
and many species produce sounds powerful enough to be heard distinctly
above the sea surface. An underwater loudspeaker was constructed along
the physical principles of a toadfish swim bladder and was found to be
quite efficient in the 100-2000-Hz range (Tavolga and Wodinsky, 1963).
The mechanics of swim bladder sound production can be summarized
as follows: The fundamental frequency of the sound emitted is a direct
translation of the contraction frequency of the sonic muscles. The contractions of the sonic muscles produce volume and pressure changes in
the swim bladder, and, therefore, the entire surface of the bladder pulsates. The fact that the natural frequency of the swim bladder as an air
bubble is in the same order of magnitude as the vibration frequency of
the sonic muscles undoubtedly aids the efficiency of the system, and the
mechanical energy of the moving muscle is transmitted by way of air
pressure changes to the entire bladder surface. These pulsations are transmitted through the tissues of the fish to the outside with little loss, since
the fish is virtually transparent to water-borne sound. In acoustic and electronic terms, therefore, the swim bladder is an impedance-matching device analogous to the large surface cone of a high fidelity loudspeaker.
7 . HYDRODYNAMIC AND SWIMMING SOUNDS
The movement of any object through the water will create displacement. Such displacements and compression waves may be rhythmic subsonic vibrations when produced by the fins and body of a swimming fish.
Such phenomena have been classified as hydrodynamic sounds by
Shishkova ( 1958a,b) and Moulton ( 1960b). Moulton also considered