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Sound Field Control for a Low-Frequency Test Facility
The two largest problems in controlling the reproduction of low-frequency sound for psychoacoustic experiments is the effect of the room due to standing waves and the relatively large sound pressure levels needed. Anechoic rooms are limited downward in frequency and distortion may be a problem even at moderate levels, while pressure-field playback can give higher sound pressures but is limited upwards in frequency. A new solution that addresses both problems has been implemented in the laboratory of Acoustics, Aalborg University. The solution uses one wall with 20 loudspeakers to generate a plane wave that is actively absorbed when it reaches the 20 loudspeakers on the opposing wall. This gives a homogeneous sound field in the majority of the room with a flat frequency response in the frequency range 2-300 Hz. The lowest frequencies are limited to sound pressure levels in the order of 95 dB. If larger levels are needed, a hybrid mode can be used to utilize the pressure-field conditions at frequencies up to approx. 30 Hz while the higher frequencies are controlled by plane-wave generation. This approach allows for playback of levels at the lowest frequencies in the order of 125 dB while still maintaining a homogeneous sound field for the entire frequency range 2- 300 Hz.
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