The noise radiating from a structural surface area is determined by dimensions of the radiating area and its vibration speed, The radiation coefficient can be defined as a value of the noise power as divided by the product of the square of vibration speed and the radiating area.
For a simple scale structure, the radiation coefficient is known, which is not applicable as it is to actual structure, because such structure is vibrated in so complicated radiating surface. Rugged radiating surfaces should be considered to include some ineffective radiating areas.
In this report, a measuring method based upon experimenta l data on hydraulic machineries and its related piping is described, which is effective for determining experimental values of effective radiating area. Thus known radiation coefficients became available. The above data will be useful for the theoretical calculation of predict of noise to be made

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In the field of A. C. (Air Cushion) technology, the dimensionless power coefficient, Cp, becomes rapidly high with small h/t, where h is the ground height and t the jet thickness, and theoretical predictions of Cp based on four familia theories are rather different.
Therefore, in order to find out the optimal Cp it is necessary to examine the characteristics of the hovering power through detailed experiments, for the purpose of obtaining a ratio, h/t in which Cp becomes minimal. Experiments were conducted with a two-dimensional model in the range of h/t3 and the results obtained are as follows:
(a) The optimal value of h, t was 1.1-1.5, so that h/t<1, which is used in practical A. C., must be modified.
(b) Reduction in jet setting angle from 90°to 45°, resulted in an improvement in Cp from about 0.7 to 0.5,
(c) Because of Cp<1, hovering power is smaller than the power of the ideal plenum chamber type A. C., (Pt=Pc), so that, in this range of Cp, the peripheral jet type A. C. is superior.
(d) Optimal power performance was given at a cushion pressure coefficient of 0.8, and a mass flow coefficient of 0.65.

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