AN ACTIVE ANECHOIC TERMINATION FOR LOW-FREQUENCIES WITH MEAN FLOW

This paper deals with the behavior of an active anechoic termination f or low frequencies (<150 Hz) in the presence of mean flow. The auxilia ry source is an oscillating valve positioned within the flow which can be considered as an acoustic source in series with a primary source. This system was previously used to eliminate flow periodic fluctuation s in exhaust from reciprocating engines. The modeling (briefly recalle d here) is based on the propagation of plane waves in flow, the princi ple of the superposition of linear acoustics, and electroacoustic anal ogies. It is first shown that the reflection coefficient can be minimi zed for a uniform duct in the case of a fixed valve positioned in inci dence. It is then demonstrated that the reflection coefficient can be controlled by oscillating the valve to provide an active anechoic term ination. This active method allows the passive influence of the valve to be reduced in an acoustic circuit through limitation of head-loss. The theoretical study was validated by an experimental application con firming the viability of this low-frequency termination. A 5% lower re flection coefficient was obtained from 40 to 110 Hz, with a Mach numbe r of around 0.05.