Next Generation Acoustic Measurements of Microphones

NIST calibration customers are increasingly sending newer types of microphones to NIST for calibration. These microphones can meet new and more demanding customer needs because the microphone properties enable them to be calibrated to smaller uncertainties over a wider frequency range than those of current NIST SP250 measurement services. For example, improved uncertainties in next-generation microphone pressure calibration services would enable NIST customers to realize the potential of new and evolving instruments such as very stable acoustical calibrators. Because customers calibrate these devices by comparison with the NIST-calibrated microphones as references, the uncertainty in the reference microphone calibration is an important component of the uncertainty in the customer calibration of these acoustical calibrators. These acoustical calibrators are used to provide traceability for many thousands of other acoustical measuring instruments used in critical measurements for product design and conformance, worker and military personnel safety, and health care. These acoustical measuring instruments include measuring microphone systems, sound level meters, personal sound exposure meters (dosimeters), sound power measurement systems, and audiometric equipment.

Customer outreach activities have confirmed the need for improved uncertainties in reference microphone calibrations to calibrate new acoustical calibrators. According to representatives of instrument manufacturers, calibration laboratories, and a consultant with extensive knowledge of aircraft and other noise regulations and measurements, these improved uncertainties are required to meet:

• Competitive and contractual pressures to produce and to use instruments with state-of-the-art uncertainties. This is a matter of business survival.
• Requirements for reliable measures of noise emission as close to a regulated or mandated limit as possible, without exceeding that limit.
• Legal metrology needs and the desire to avoid costly litigation and damages. Use of the best instruments and methods with the lowest uncertainties to obtain legally unassailable results is often considered essential by contractual parties, regulators, the regulated, and independent testing authorities.

Provision of next-generation microphone pressure calibration services by NIST will enable customers to meet competitive and contractual pressures to provide state-of-the-art measurements, to satisfy legal metrology needs and avoid costly litigation and damages, and to achieve reliable measurements of noise emission close to regulated or mandated limits. 

This project will develop and validate the measurement procedures, instrumentation, and quality system documentation to allow NIST to provide next-generation microphone pressure calibration services that meet the needs of our customers. The project will build upon prior success in developing non-contact procedures to measure microphone front cavity depths, and in selecting and controlling stable electronic bandpass filter sets to achieve suitably high signal-to-noise ratios in the electrical measurements. Project activities will include the design, testing, and integration of the measurement system components in the evolving testbed (including the bandpass filter set, signal generator, signal conditioner, custom-built system controller, voltmeter, temperature probe, barometer, etc.), development of the measurement procedures for next-generation acoustic pressure-field calibrations of microphones, and evaluation of the initial system by measuring the performance of different small couplers in partially overlapping frequency ranges. The project will complete all technical developments needed to bring the next-generation measurement system online for customer calibrations, including refinement of the system and/or measurement procedures based on the testing results; establishment of the measurement uncertainty; and documentation of the system, measurement procedures, and quality system requirements.