A 24.5 m long plane wave tube with 300 mm thick reinforced high density concrete sides and internal dimensions of 2 m x 2 m has been constructed to extend low frequency test capability of the Facilities. A large, optimised density, fibre glass, quarter-wave wedge provides an acoustical termination at one end of the tube and a sound source comprising 18 inch diameter loudspeaker vented enclosures, is placed at the other end. Plane wave response of the Tube extends from 15 Hz to 80 Hz and fully complies with provisional test requirements of ISO 3745 for free-field performance. Other tests indicate that the sound field can be varied from 10 Hz to 100 Hz while maintaining sound pressure level constant to within 0.5 dB of a preset nominal value (for low frequency calibration of microphones).

Applications include:

• Subjective testing associated with effects of infrasonic sound, low frequency testing of audio equipment.

• microphones and loudspeakers and determination of acoustical absorption coefficients of devices and material at very low frequencies (eg optimising performance for fitting to, or lining, air conditioning ducts).

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