Ralph T. Muehleisen
David C. Swanson
Graduate Program in Acoust., Penn State Univ., P. O. Box 30, State College, PA 16804
A theoretical investigation into the effects of mean flow (of Mach number M) on the plane wave radiation resistance of a source mounted in the side of a duct is made. The radiation resistance is an important parameter in the design of active noise control systems. It is found when the source is in a nonturbulent region the overall radiation resistance is increased by a factor of 1+6M[sup 2]. If the source is in a region of turbulence the radiation resistance is only increased by a factor of 1+M[sup 2]. Without turbulence the downstream radiated power is decreased by a factor of 1/(1+M)[sup 3] while the upstream radiated power is increased by the factor 1/(1-M)[sup 3]. With turbulence the factors are reduced to 1/(1(plus or minus)M). The difference in the turbulent and nonturbulent expressions is because of a velocity discontinuity at the edges of the piston that is modified in the presence of turbulence. Because the downstream radiated power is reduced, an active noise control system that works primarily by out-of-phase cancellation will be less efficient. However, since the upstream radiated power is increased, a system that works primarily by mutual coupling of the sources may be enhanced.