Whales found in the north Pacific are known to migrate over several thousand kilometers, from the Alaskan coast where they heartily feed during the summer to low latitude waters where they breed during the winter. Therefore, it is assumed that whales are using the “deep sound channel” for their long-distance communication. The main objective of this study is to clarify the behaviors of baleen whales from the standpoint of acoustical oceanography. Hence, authors investigated the possibility of long distance communication in various species of baleen whales, by simulating the long-distance propagation of their sound transmission, by applying the mode theory to actual sound speed profiles and by simulating their transmission frequencies. As a result, the possibility of long distance communication among blue whales using the deep sound channel was indicated. It was also indicated that communication among fin whales and blue whales can be made possible by coming close to shore slopes such as the Island of Hawaii.

In marine research, measures against self-noise of an observatory ship are important. Generally, the self-noise is measured after the completion of ships. It is difficult to predict this noise level beforehand. Then, an attempt is made to determine the noise emitted from various elements of a structure. The finite difference time domain method is applied to obtain sound fields, including that of a plate in water. The time behavior of the sound wave emitted from a sound source placed near the upper part of a plate is investigated. As a result, the reflected and re-radiated waves from the plate including the head wave resulting from the longitudinal and traverse waves in the plate are able to be visualized. In the case of the plate with a branch plate, the suppression of the wave which propagates at the inside of the plate with the length of the branch plate is shown.

The mysterious mass stranding of whales often arises in specific areas in the world. Moreover, vibrational noise, suspected to originate from large-scale construction in coastal areas, is problem. These problems could be considered to be related to sound propagation near coastal areas with a special geometrical structure and an ambient noise environment. We set up a propagation model in a coastal area, and simulated low-frequency sound propagation near the coast using the parabolic equation method (PE method) and the normal mode method. It has been shown that low-frequency sound cannot propagate from the ocean to the coast owing to the cutoff effect of shallow water that occurs when there is a water layer with a basement half space. However, when the water layer has a sediment layer, mode coupling occurs between the water layer and the sediment layer and propagation attenuation rapidly decreases. Sound can propagate to the coast in this case.

The association between the pulse shape of a probe pulse and phase modulation in the proposed method of phase information transmission by utilizing a phase conjugate is studied by simulation. As a result, it is found that phase modulation is possible irrespective of both pulse waveform and carrier signal type, and that the phase of the reconstructed pulse at the sound source varies according to the amount of phase modulation added at the array. Some carrier signals conserve such a particular position as an inflection point, which can be utilized as a reference point of the phase.

We studied the degradation of the source pulse form in long-range propagation of low-frequency sound in the sound fixing and ranging (SOFAR) channel in the ocean. We made the numerical simulation of the source pulse propagation with changing the parameters of the simulation; carrier frequency, source pulse width, and propagation range. In long-range propagation such as 1000 km, 2000 km and 3000 km, there are a limited number of combination of the frequency and the pulse width for minimizing the consequent pulse degradation. These results are applicable to the design of the actual ocean propagation experiment itself and also of sound source equipment for the experiments.