In this experiment, we used sound stimuli of different durations to demonstrate that the auditory evoked potential (AEP) of fishes was contaminated with microphonic potential. We demonstrated that the AEP contained a component that had double the frequency as that of the sound stimulus. The durations of the AEPs were in proportion with the durations of the sound stimuli. “Auditory brainstem response” has been considered to be the dominant component of the AEP of fishes. However, this study suggests that some of the AEP components of fishes are derived from peripheral organs such as the saccule, lagena and utricle in the inner ear.

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A novel fiber Bragg grating (FBG) hydrophone array has been proposed in order to realize multipoint underwater sound detection with thermal stabilization. The interrogation technique of the FBG hydrophone to be arrayed is based on an intensity-modulation scheme, in which a tunable laser is used for an optical source with its oscillation wavelength tuned to a slope of the reflection spectrum curve of the FBG. The partially reflected light from the FBG is modulated by an acoustic wave and offers a sensor output. For fabricating the FBG hydrophone array, an optical switch and a specially designed feedback circuit have been employed. The arrayed FBG hydrophones are connected in parallel with the output ports of the optical switch so that the time-division multiplexed (TDM) operation is performed by selecting the optical paths to the hydrophones. In the TDM operation, the feedback circuit enables us to control the oscillation wavelength automatically in accordance with the reflection spectrum for the selected FBG hydrophone. In addition, the automatic wavelength control allows the stabilization of the sensor output against temperature variations. In the experiment, two-point TDM sound detection with a thermally stabilized operation has been successfully demonstrated.

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The application of the sound wave which propagates through long range in the ocean is studied to monitor global warming. Recently, parabolic equation method (PE method) is often used as a sound propagation simulation technique. The PE method can calculate the sound field in high accuracy when the propagation range is short and the frequency is low. However, the error grows in a usual computational method when the propagation range becomes mega-range. Especially, the phase error in the time domain was not examined. The solution of PE method is a product of envelope function and Hankel function. The influence of Hankel function becomes big in the time domain because the received pulse consists of a lot of frequencies. This is a cause of the error of the pulse propagation time. In this paper, we theoretically outlined the difference between the phase error in the frequency domain and the phase error in the time domain. Moreover, the phase error in the time domain was simulated by selecting the sound speed in the envelope function and in the Hankel function. As a result, we show the importance of making the sound speed in the envelope function and in the Hankel function the same.

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