Abstract:

A sound source with the frequency below 70 Hz and transmitting level over 195 dB is needed to investigate global ocean climate change by acoustic monitoring like the ATOC project for long-range sound propagation of 5--10 Mm of gyre scale. Long-term operation of the source over 10 years is also required for this purpose. A low-frequency sound source of 70 Hz is planned using the same driving units of giant magnetostrictive type as the units used in the 200-Hz source for our tomography system [Nakamura et al., J. Acoust. Soc. Am. 96, 3256(A) (1994)]. Acceleration tests of long-term operation of the driving units were conducted to realize the equivalent operation to 10 years of the 70-Hz source in a shorter period. As the resonance frequency of the driving unit is about 1 kHz, the acceleration rate is 1000/70. The driving unit was operated 40 000 times for 1 min, which corresponds to a 30-min operation for the 70-Hz source every 4 h for 10 years. Variation of strain, impedance, and displacement-current sensitivity of five driving units were measured every 10 000 times operations. It was found there was no significant variation for these parameters during the experiment.