123
1.1 Underwater Sound and the Acoustic Cues Available
to Fish
Sound is a mechanical disturbance that propagates as a longitudinal wave through
a medium (e.g., air, water, or other material, see Beranek 1954). When a disturbance occurs, energy is radiated away from the source in the form of sound pressure and particle motion. In water close to the source, the pressure is high and the
particle motion is large due to incompressible fluid flow. Away from the source,
the pressure remains relatively large with small but finite particle motion. Sound
pressure is a scalar quantity that conveys the magnitude of pressure fluctuations
relative to the ambient pressure at a fixed point of measurement. Pressure is the
component of sound that we are most accustomed because it is the component that
most terrestrial ears sense and is what is measured using a microphone or hydrophone. Sound pressure sensors only convey the amplitude and time course of
pressure fluctuations and not information related to sound source directionality. In
contrast, particle motion is the actual displacement of the particles constituting
the medium and is a vector quantity having both magnitude and direction. Particle
motion sensors can thus convey information about source direction and angle. In
air, particle motion decays to small values very rapidly and we rarely are subject
to it, but due to the higher density and correspondingly longer wavelengths of
underwater sound, it remains at relatively high amplitudes at greater distances
from the underwater sound source (particle motion predominates within about a
wavelength from the source). For example, in water the wavelength of 100 Hz (a
man’s advertisement call) is approximately 15 m. Thus, particle motion may provide fish with salient acoustic cues at large distances from sound sources and offer
directional information to fish that they may be able to exploit for sound source
localization. Conversely, traditional “terrestrial” localization cues, i.e., the ITDs
and ILDs computed via the comparison of the pressure waveform at two different
pressure sensors (e.g., left and right eardrums), are thought to be negligible for
most fish. This, in part, is due to the speed of underwater sound which is nearly
five times faster in water than in air, reducing underwater ITDs by ~80 % (relative
to terrestrial ITDs). Thus, the maximum ITD experienced by a fish with an interaural distance of less than 3 cm should be 30 μs or less, which is at or near the
threshold ITD of the most sensitive terrestrial vertebrates (Grothe et al. 2010).
Furthermore, because fish are approximately the same density as water there is
little or no attenuation of sound as it travels from one ear through the head to the
other ear (i.e., ILDs are also ~0 dB). However, in fish the orthogonal orientation
of the left and right saccule with their complementary hair cell orientations could
Directional Hearing and Sound Source Localization in Fishes
1.1 Underwater Sound and the Acoustic Cues Available
to Fish
Sound is a mechanical disturbance that propagates as a longitudinal wave through
a medium (e.g., air, water, or other material, see Beranek 1954). When a disturbance occurs, energy is radiated away from the source in the form of sound pressure and particle motion. In water close to the source, the pressure is high and the
particle motion is large due to incompressible fluid flow. Away from the source,
the pressure remains relatively large with small but finite particle motion. Sound
pressure is a scalar quantity that conveys the magnitude of pressure fluctuations
relative to the ambient pressure at a fixed point of measurement. Pressure is the
component of sound that we are most accustomed because it is the component that
most terrestrial ears sense and is what is measured using a microphone or hydrophone. Sound pressure sensors only convey the amplitude and time course of
pressure fluctuations and not information related to sound source directionality. In
contrast, particle motion is the actual displacement of the particles constituting
the medium and is a vector quantity having both magnitude and direction. Particle
motion sensors can thus convey information about source direction and angle. In
air, particle motion decays to small values very rapidly and we rarely are subject
to it, but due to the higher density and correspondingly longer wavelengths of
underwater sound, it remains at relatively high amplitudes at greater distances
from the underwater sound source (particle motion predominates within about a
wavelength from the source). For example, in water the wavelength of 100 Hz (a
man’s advertisement call) is approximately 15 m. Thus, particle motion may provide fish with salient acoustic cues at large distances from sound sources and offer
directional information to fish that they may be able to exploit for sound source
localization. Conversely, traditional “terrestrial” localization cues, i.e., the ITDs
and ILDs computed via the comparison of the pressure waveform at two different
pressure sensors (e.g., left and right eardrums), are thought to be negligible for
most fish. This, in part, is due to the speed of underwater sound which is nearly
five times faster in water than in air, reducing underwater ITDs by ~80 % (relative
to terrestrial ITDs). Thus, the maximum ITD experienced by a fish with an interaural distance of less than 3 cm should be 30 μs or less, which is at or near the
threshold ITD of the most sensitive terrestrial vertebrates (Grothe et al. 2010).
Furthermore, because fish are approximately the same density as water there is
little or no attenuation of sound as it travels from one ear through the head to the
other ear (i.e., ILDs are also ~0 dB). However, in fish the orthogonal orientation
of the left and right saccule with their complementary hair cell orientations could
Directional Hearing and Sound Source Localization in Fishes