1. The only references found about the noise source characteristics were related to acoustic similarity laws.[9-121 The basic concept of the acoustic similarity law is that the radiated sound can be expressed as a nondimensional product of the Reynolds number raised to an exponent 13,the Mach number raised to an exponent 01,a spectral distribution function, F(ST), and an acoustic frequency response function, G(,Ye). Here the F(ST) function is related to the aerodynamic sound with Re and Ma. By this similarity law, Chanaud concluded that in general the dipole relationship applies to the centrifugal impeller sound.[9] Neise[ 121 shows that the primary causes of fan noise are the forces of the blades. In order to obtain acoustic signal from the unsteady force fluctuation of the blades, we use Lowson's equation[ 13). Lowson derived the formula of predicting the acoustic field generated by the moving point force from the wave equation. By applying this equation to each blade element, we can predict the acoustic pressure. But Lowson's equation is applied only in the free field. The effects of the solid boundaries can't be considered. So the BEM(Boundary Element Method) is introduced to calculate the solid boundary to take into account the scattering effect of the aerodynamic source.
2. 2-1. Acoustic Calculation
3. Ageneral formula for the sound field of a point force in arbitrary motion is derived by Lowson in 1965 as below-[ 131
4. Equation (1) refers to the effect of an isolated point force. In order to apply it to an impeller blade, the distributed loading over the impeller blade is divided by small point elements, as is common in propeller noise theory. The force on the blade is represented by a point force, which act on that element. By applying the equation (1) to each blade element, we can predict the acoustic pressure in the free field. But, the effects of the scattering and diffraction due to the wedge can not be considered. Only the behavior of the noise source and its radiation to the free field can be estimated. 2-2. Flow calcu