Abstract
A 15-m-long flexible waveguide was successfully used to create a traveling wave for frequencies below 300 Hz in water. A cylindrical waveguide that allows only plane-wave propagation was designed and fabricated from Plexiglas. This flexible material reduced the effective stiffness and thus the sonic speed and wavelength of the disturbance generated by a J9 underwater sound projector flanged to one end. In addition, the energy of the wave was dissipated as it traveled along the waveguide; consequently, little if any reflection occurred at its end. The fish were placed inside a PVC mesh cage and positioned in the waveguide approximately 3 m down from the J9. Then they were exposed to either a continuous wave or a pulsed wave similar to manmade sources. Afterwards the fish were held for a specified time period and then sacrificed. The auditory organs were treated with fixative and removed. Then they were shipped overnight to the University of Maryland where the tissue was immediately prepared for scanning electron microscopy to determine if sensory hair cells had been destroyed. This method has yielded dependable data to help assess the effects of low-frequency sound on hearing in fish. [Work supported by ONR.]
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