In order to study the effect of damping layer applied to the ship hull structure on the radiated noise reduction, an experimental study on the vibration and acoustic intensity characteristic of rectangular steel plate with a damping layer was carried out.
The Summary of the results is :
(1) Radiated noise reduction due to vibration is lower for the plate contacting with water than for that in air.
(2) A damping layer is less effective near the point of excitation for high frequency range. But except of that, vibration reduction effect increases with increasing area of a damping area.
(3) As the measuring position is distinct from the point of excitation, its acoustic intensity level decreases more and more.

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Divers, who engage in search and/or rescue in a sunken or capsized ship, usually have no communication apparatus because of the complexity of the hull construction. For such divers, the most simple and effective way to provide some information is to transmit an acoustic signal directly by an underwater loudspeaker. Though the outer wall of the ship functions as a sound insulation wall, there is no appropriate evaluation method for the insulation effect of a wall to random sound pressure wave such as voice signal or noise.
In this paper, a statistical evaluation problem concerning sound insulation effect of a wall in underwater is considered based on the idea of statistics such as L_α noise level, defined as the (100-α) percentile point of the noise level probability distribution. Concretely, a single wall is employed as the most basic model of the hull and a new statistical treatment is first proposed for the transmitted sound pressure wave in the case where a Gaussian type random sound pressure wave with an arbitrary power spectral density is passed through the wall, and next, an explicit expression for the sound insulation effect, defined as the difference between the L_α of the incident sound intensity and that of transmitted sound intensity, is derived as a function of the system parameters.
Finally, the validity of the theoretical evaluation method is confirmed by an analog simulation and a water tank experiment using several kinds of single walls.

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