NMI is responsible for maintaining and disseminating Australia's standards for acoustics (sound power level, sound pressure level and sound intensity level), vibration (acceleration and velocity) and ultrasound (ultrasound power).
NMI undertakes research and development activities as part of its charter to continually improve the Australian standards. This includes development of novel measurement techniques to assist in the measurement of air and structure borne sound and vibration, and development and implementation of equipment to provide the highest possible accuracy in a calibration facility. The dissemination of measurement standards and supply of research assistance to Australian industry for the measurement of sound and vibration are also undertaken.
NMI participates in numerous international technical committees to give Australian representation in such bodies as ISO and IEC, and supply training and support to the measurement institutes of a number of other countries.
Some of our recent research and development activities are described below.
Successful calibration of noise measuring equipment to international standards requires an anechoic space. NMI has a medium-sized anechoic room with a cut-off frequency of 160 Hz. It is constructed of graded density absorber that has been in use for this purpose successfully for over 20 years. A recent requirement for ISO 17025 accreditation has led NMI to revisit and justify the claim that the space is anechoic to the required standards. To carry out such a survey is both expensive in time and equipment.
As the chamber is used in a well defined and limited manner, NMI adopted a modified approach that has justified the claim of anechoicity for the method of use. The results have demonstrated that it is possible to carry out such a survey in a timely and simplified manner, that the space meets all expectations for anechoicity and the room can be used for free-field calibrations of microphones and sound level meters between 31.5 Hz and 20 kHz with satisfactory uncertainties.
BH Meldrum and A Thorley (2002) A Simplified Method for the Determination of the Quality of an Anechoic Space at the CSIRO National Measurement Laboratory. Proceedings of the Australian Acoustical Society Conference, Adelaide
A set of four, novel bells comprising a major triad and octave has been designed and cast in bronze by an Adelaide bronze artist. The bells have a particularly pleasing strike tone as well as beautiful decoration. To identify the origins of the bell tones, and to assist in tuning, the bells have been modelled using finite element analysis. Various aspects of the acoustics of the bells were studied, including the synergies between shape symmetries and modal symmetries and their effects on the tuning and timbre.
This investigation concluded that the decoration on the bell has a crucial acoustic role in addition to its artistic and symbolic ones. Depending on the symmetries of the decoration with respect to the modes, the tuning can be extraordinarily sensitive. In the case of these bells the tuning sensitivity is such that even the very difficult to control surface patination plays a crucial role.
S Thwaites and H Printer (2002) Acoustics of the Arpeggio Series 1 Set of Bells. Proceedings of the Australian Acoustical Society Conference, Adelaide, pp 176–185
NMI has recently commissioned a new Bruel and Kjaer 9699/5998 two-port reciprocity calibration system for the calibration of 1 inch and ½ inch microphones. This new apparatus complements the existing three-port coupler which is the Australian standard for microphone reciprocity, the performance of which has been verified during participation in several international intercomparisons.
The interferometer used for the primary acceleration standard is being upgraded. This upgrade will permit the use of the method of 'sine approximation' (method 3 in ISO 16063:11) as well as the traditional 'fringe counting' and 'minimum point' (methods 1 and 2 in ISO 16063:11) for determination of absolute displacement, velocity and acceleration as well as absolute phase for the calibration of reference accelerometers.