Special test facilities at NAL
- 2 high intensity noise rooms
- 4 anechoic rooms
- 2 reverberation rooms
- 1 large quiet room
- 1 large horizontal plane wave tube
These acoustical test facilities were specifically designed to provide testing capability that covers the full range of human hearing and extends into infrasonics. They provide a superior environment for acoustical testing of materials, small machines and acoustical absorbers, and for research into the effects of noise on people.
Eight of the ten facilities are contained in a 'sound shell', which is a 58 m x 50 m hall with 280 mm thick concrete walls. The hall itself is vibrationally isolated from the main National Acoustic Laboratories building. The two high-intensity noise rooms are located outside the hall for acoustical isolation purposes.
The 'sound shell' isolation has been achieved through provision of separate foundations and narrow gaps between the two buildings which are caulked by elastic materials at appropriate places, thus preventing entry of rain or objects which may jam and form a bridge for transmission of vibrational energy. Other services are supplied through flexible couplings to ensure that the two buildings are kept vibrationally isolated from each other. A special acoustically silenced air conditioning plant, which serves only the Facilities, has been installed to provide a controlled temperature and humidity air supply to all the facility test rooms.
Free-Field Performance Capability
Frequency response range of the horizontal plane wave tube and four anechoic rooms has been independently tested as required by construction contract specifications and complies with inverse square law characteristics measured in accordance with precision methods of ISO 3745. The Large Horizontal Plane Wave Tube conforms from 15Hz to 80Hz, Large Anechoic Room for frequencies above 50Hz, two medium Anechoic Rooms above 90Hz and a Small Anechoic Room above 190Hz. The Small Anechoic Room is also radio frequency shielded for acoustically testing RF sensitive devices. Upper limit ISO 3745 testing was restricted to 12kHz for the anechoic rooms in accordance with contract specifications; however, the untested useful upper limit, which has not been tested to date, is well above this figure and exceeds 20,000Hz.
Acoustical background noise inside any of the anechoic rooms is not able to be determined with available instrumentation. Present measurement capability is limited to -15dB Sound Pressure Level (SPL) because of the need to have a 10dB measurement margin above the instrumentation noise floor (expressed as equivalent acoustical input) of -25dB SPL.
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