航行中の船舶から放射される海中雑音の干渉効果 Interference of Underwater Noise Emitted by Cruising Vessel
The underwater noise emitted by a cruising vessel has several transmission rays namely direct ray and reflected rays from the sea bottom and sea surface. Interference occurs when the sound waves are received at a hydrophone simultaneously. Actual interference of underwater noise was observed in sea areas of 24m and 70m depth. Observed underwater noise emitted by an approaching vessel at 9 knots was analyzed every 10 sec at a distance of 0.5 mile from the closest point of approach, and a spectrogram was calculated from the spectrum data. The theoretical interference of underwater noise in shallow water was obtained using an equation that considered the range of transmission path from the cruising vessel to the hydrophone as the reflection number. This report compares the theoretical and actual interference.<br>The characteristics of interference were obtained. Numbers of transmission rays by the theoretical interference (μ=0.5) were more than six rays at 0.5 mile away, but at 0.05 mile from tue closest point of approach, the number decreased to two rays. This was becouse the reflected transmission range is much longer than the direct one, so the reflected sound pressure level is lower than the direct sound pressure level at short range. If the reflection number at the sea bottom increased, the reflected sound pressure level decreased in inverse proportion to the reflection number (at the sea bottom) power of μ. Therefore, the spectrum pattern will change with the amplitude reflection coefficient at the sea bottom (μ). A theoretical interference striped pattern appeared for the shallow water rather than the deep water. Actual interference striped pattern appeared for the aes area of 24m depth at all hydrophone depths and the sea area of 70m depth at 5, 10, 60m hydrophone depths. This conforms with the theoretical pattern for the sea area of 24m depth of all hydrophone depths.
日本水産学会誌 64(1), 48-55, 1998-01-15