Novel silencers and absorbers for wind tunnels and acoustic test cells

Low frequency noise measurements in reverberation and free-field rooms require special attention as the sound field in this frequency range may be dominated by only a few modes (eigenfrequencies). This paper discusses how reverberation rooms may be adequately damped to smooth the spatial sound field and to increase measurement reproducibility and repeatability without violating standards. Absorber modules which are only 250 mm thick are described and their operating principles are discussed. The modules are used as linings for test cells and wind tunnels to provide free-field conditions down to 50 Hz for 1/3-octave band measurements. Their unprecedented acoustic effectiveness is demonstrated by fundamental investigations in well-conditioned test rooms and final verification in a number of rigorously silenced and anechoic facilities completed in the research and development centers of 5 different automobile manufacturers in Europe and the United States. Since the novel acoustic materials and modules also provide several advantages concerning spaciousness, cleanability and durability their practicability is demonstrated in a few typical examples of approximately 50 completed measuring environments.