151
For a monopole or dipole source in a free field Γ would always be positive. In
Fig. 11, for all three frequencies Γ is initially negative but becomes positive at a
distance of 3–5 m from the source. The ratio Γ approaches +1 at large distances as
would be expected.
Time averaged intensity provides a physics based approach to source localization with wide applicability with regard to source type, acoustic environment,
and time waveform. While it won’t work for all possible sources and environments
it may well work for all situations where localization from point measurements
is possible.
5 Conclusion and Future Directions
Numerous observations of acoustical behaviors in multiple fish species strongly
suggest that fish have the capacity for sound source localization. However, there are
no good examples of sound localization capacities in a single species that provide a
comprehensive theoretical explanation. There remain a number of important questions that should be addressed in future work on sound source localization by fishes,
several of which are briefly detailed below:
1) Which end organs are utilized by fish in sound source localization? All teleost
fishes possess three inner ear end organs (the saccule, utricle, and lagena) that
contain functionally similar hair cells with functional overlap in both auditory
and vestibular modalities, but their respective contributions to sound localization
remain largely unclear.
2) Is binaural integration essential for sound source localization in teleost fishes?
The importance of binaural input from the end organs in sound source localization has not been rigorously tested. Also, to what extent is sound source localization possible with a single ear? Can the 180° ambiguity be resolved with just a
single ear in a non-otophysan fish? Most models for pressure detection require
two ears.
3) Under what controlled conditions can sharks and other fish without a gas bladder resolve the 180° ambiguity and if so how? Sharks and other elasmobranch
fishes are able to locate sound sources from relatively far distances despite their
probable lack of pressure sensitivity and thus are apparently able to localize
sound on the basis of acoustic particle motion alone.
4) Are fish cognizant of sound source location when local particle motion vectors
do not point toward the sound source? A recent preliminary reanalysis of the
results from the midshipman dipole localization by Zeddies et al. (2012) suggests that fish could potential use a time-average intensity approach for locating
sound sources. This area of research warrants further study to determine how
fish may use this information as well as determine under what potential acoustic
environments the time averaged intensity does point away from the direction of
the sound source.
Directional Hearing and Sound Source Localization in Fishes
For a monopole or dipole source in a free field Γ would always be positive. In
Fig. 11, for all three frequencies Γ is initially negative but becomes positive at a
distance of 3–5 m from the source. The ratio Γ approaches +1 at large distances as
would be expected.
Time averaged intensity provides a physics based approach to source localization with wide applicability with regard to source type, acoustic environment,
and time waveform. While it won’t work for all possible sources and environments
it may well work for all situations where localization from point measurements
is possible.
5 Conclusion and Future Directions
Numerous observations of acoustical behaviors in multiple fish species strongly
suggest that fish have the capacity for sound source localization. However, there are
no good examples of sound localization capacities in a single species that provide a
comprehensive theoretical explanation. There remain a number of important questions that should be addressed in future work on sound source localization by fishes,
several of which are briefly detailed below:
1) Which end organs are utilized by fish in sound source localization? All teleost
fishes possess three inner ear end organs (the saccule, utricle, and lagena) that
contain functionally similar hair cells with functional overlap in both auditory
and vestibular modalities, but their respective contributions to sound localization
remain largely unclear.
2) Is binaural integration essential for sound source localization in teleost fishes?
The importance of binaural input from the end organs in sound source localization has not been rigorously tested. Also, to what extent is sound source localization possible with a single ear? Can the 180° ambiguity be resolved with just a
single ear in a non-otophysan fish? Most models for pressure detection require
two ears.
3) Under what controlled conditions can sharks and other fish without a gas bladder resolve the 180° ambiguity and if so how? Sharks and other elasmobranch
fishes are able to locate sound sources from relatively far distances despite their
probable lack of pressure sensitivity and thus are apparently able to localize
sound on the basis of acoustic particle motion alone.
4) Are fish cognizant of sound source location when local particle motion vectors
do not point toward the sound source? A recent preliminary reanalysis of the
results from the midshipman dipole localization by Zeddies et al. (2012) suggests that fish could potential use a time-average intensity approach for locating
sound sources. This area of research warrants further study to determine how
fish may use this information as well as determine under what potential acoustic
environments the time averaged intensity does point away from the direction of
the sound source.
Directional Hearing and Sound Source Localization in Fishes