Randall P. Wagner
David J. Evans
Administrative and Logistics:
Please contact the technical staff before shipping instruments or standards to the address listed below.
Pressure calibrations (Service ID Nos. 25010C and 25020C) are performed on Type-L one-inch microphones* satisfying the requirements of American National Standard S1.12-1967 (R1977), Specifications for Laboratory Standard Microphones and its impending revision. The microphones submitted for pressure calibration must be suitable for use with the calibrating couplers shown in Figures 6 and 10 of the applicable American National Standard S1.10-1966 (R1986).
* Type-L one-inch is a commercial designation for these microphones.
Further information is contained in the references for acoustic measurements.
Since American National Standards Institute publications S1.10-1966 (R1986) and S1.12-1967 (R1977) were issued, certain types of half-inch diameter precision microphones have attained widespread use. Therefore, NIST has developed procedures (Service ID Numbers 25030C and 25040C) for determining the pressure response levels of half-inch microphones by comparison with NIST-owned Type-L standard microphones, which in turn are calibrated periodically by the reciprocity technique. The technique used, precautions to be observed, and uncertainties of measurement are essentially the same as those given above for one-inch microphones. Significantly different aspects of the procedures for half-inch microphones, such as ground shield configuration, are described in the test report. Since several half-inch laboratory standard microphones have been available for only a relatively short time, their long-term stability has yet to be determined.
The free-field response levels of certain Type L microphones (e.g., Western Electric Type 640AA condenser microphones) can be computed from pressure response levels reported by NIST. However, certain precautions must be taken, and there is some degradation in accuracy.
Therefore, for the most demanding free-field measurement requirements, NIST offers a calibration service (Service ID No. 25050C) for determining the free-field response levels for half-inch microphones. The calibrations are made over the frequency range of 2.5 kHz to 20 kHz at normal incidence. Response levels (sensitivity levels) are reported in terms of open-circuit voltage per unit sound pressure (in the absence of the microphone) of a plane progressive wave whose direction of propagation is normal to the plane of the diaphragm. The calibrations are performed in a well-characterized anechoic chamber. A typical expanded uncertainty in this calibration is approximately 0.21 dB or less at each frequency within the range of 2.5 kHz to 6.3 kHz and 0.15 dB or less at each frequency within the range of 6.3 kHz to 20 kHz. Calibrations can be performed with or without protective grids on the microphone. For the most precise free-field measurements, the customer should contact the NIST staff person cited at the beginning of this section for recommendations prior to submitting the microphone to NIST for calibration.
Acoustical measurement services are available by special arrangement. These services include extended frequency ranges of calibration, additional data points, measurements at very low sound pressure levels, and calibration of certain pistonphones and acoustic calibrators. NIST has a large general-purpose anechoic chamber available for special calibrations requiring such a facility. The frequency-dependent and position-dependent acoustical performance of this chamber, including extremely low background noise, was carefully controlled during design and construction and is documented in archival journal publications.
Earphones are tested on the NIST 9-A Coupler from 125 Hz to 8000 Hz. Measurements of audiometer/earphone response and linearity can be made at very low sound pressure levels.
Calibration of Pressure and Gradient Microphones, V. Nedzelnitsky, Encyclopedia of Acoustics, Ed. in Chief M. J. Crocker, John Wiley & Sons, Inc., New York, 1869-1879 (1997).
Primary Method for Free-Field Calibration, V. Nedzelnitsky , AIP Handbook of Condenser Microphones, Eds., G. S. K. Wong and T. F. W. Embleton, Am. Inst. of Physics Press, Woodbury, NY, 103-109 (1995).
Laboratory Microphone Calibration Methods at the National Institute of Standards and Technology, U.S.A., V. Nedzelnitsky, AIP Handbook of Condenser Microphones, Eds., G. S. K. Wong and T. F. W. Embleton, Am. Inst. of Physics Press, Woodbury, NY, 145-161 (1995).
Method for Calibration of Microphones, Amer. Natl. Stand. Inst. S1.10-1966 (R1986), New York, NY.
Specifications for Laboratory Standard Microphones, Amer. Natl. Stand. Inst. S1.12-1967 (R1977), New York, NY.
Free-Field Reciprocity Calibration of Microphones, E. D. Burnett and V. Nedzelnitsky, J. Res. Natl. Bur. Stand. (U.S.) 92 (2), 129 (Mar.-April 1987).
Traceability of Acoustical Instrument Calibration to the National Bureau of Standards, V. Nedzelnitsky, Proc. INTER-NOISE 80, II, Dec. 8-10, 1980, Miami, FL, G. C. Maling, Jr., Ed., Poughkeepsie, NY: Noise Control Foundation, 1043 (1980).
Calibration of Laboratory Condenser Microphones, V. Nedzelnitsky, E. D. Burnett, and W. B. Penzes, Proc. 10th Transducer Workshop, Transducer Committee, Telemetry Group, Range Commanders Council, Colorado Springs, CO (June 1979).
Acoustical Properties of the National Bureau of Standards Anechoic Chamber, W. Koidan and G. R. Hruska, J. Acoust. Soc. Am. 64 (2), 501 (Aug. 1978).