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Noise and airflows in five types of valves were measured and the power level of the noise radiating from a valve outlet was experimentally related to the mechanical power of the air stream through the valve. The mechanical power W_m of an isentropic flow through a valve can be calculated by equation (1), where G is the weight flow of air, C_p the specific heat with constant air pressure, T_<1T> the total temperature in inlet pipe, P_<2S> the atomospheric pressure, P_<1T> the total pressure at valve inlet and K the ratio of specific heats. In order to determine W_m, the upstream temperature and pressure T and P, the differential pressure across the orifice plate ΔP and the total pressure P_<1T> were measured over a wide range of weight flow and of pressure differential across the valve as is shown in Fig. 1. Acostic power W_a was determined as follows. Octave band sound pressure levels were measured in an anechoic room as six points on the hemispherical surface of radius 1. 8m with the origin at the valve outlet. The acoustic power W_a was calculeted by equation (2), where L_p is the mean sound pressure level averaged over six points and r(1. 8m) is the radius of the hemisphere. In Fig. 4 and 5 acoustic power W_a as a function of mechanical power W_m and the conversion efficiency η=W_a/W_m from mechanical power to sound are shown with the total pressure as a parameter. In Fig. 6〜8 some properties of η are summerized. η varies in the order of 10^<-5>〜10^<-4> for ball valve and 10^<-6> for diaphragm valve and is constant for higher total pressure but at low pressures the values are different according to the type of the valve. It was only for ball valve that the peak value of η nearly coincided with the critical pressure. Sound power level increases rapidly up to a constant at about 2. 5kg/cm^2a and doubling the weight flow of air at constant pressure may increase the power level by as much as 5 dB (Fig. 9, 10). As is shown in Fig. 11 and 12, the shape of the power spectrum does not depend on the weight flow of air but on the total pressure at valve inlet, especially for high frequencies. The sound power level of the valve noise can be calculated from equation (3), under the assumption that the efficiency η is expressed by equation (4), where n and k are constant. Values of n and k of the equation were determined for each type of valves from the experimental values of η as is shown is Fig. 4 and 5. By substituting the values of n, k(Table 1) and equation (5) into equation (3), empirical formulae for each type of valves were obtained.