Effect of power line interference on microphone calibration measurements made at or near harmonics of the power line frequency

Abstract

The electrical measurements required during the primary calibrations of laboratory standard microphones by the reciprocity method can be influenced by power line interference. Because of this influence, the protocols of international inter-laboratory key comparisons of microphone calibrations usually have not included measurements at power line frequencies. Such interference has been observed in microphone output voltage measurements made with a microphone pressure reciprocity calibration system under development at NIST. This system was configured for a particular type of standard microphone in such a way that measurements of relatively small signal levels, which are more susceptible to the effect of power line interference, were required. This effect was investigated by acquiring microphone output voltage measurement data with the power line frequency adjusted to move the frequency of the interference relative to the center frequency of the measurement system passband. These data showed that the effect of power line interference for this system configuration can be more than one percent at test frequencies harmonically related to the power line frequency. These data also showed that adjusting the power line frequency to separate the interference and test frequencies by as little as 1.0 Hz can reduce the effect of the interference by at least an order of magnitude. Adjustment of the power line frequency could enable accurate measurements at test frequencies that otherwise might be avoided.

New Measurement Service for Determining Pressure Sensitivity of Type LS2aP

“…The NIST results in CCAUV.A-K3 agreed well with the key comparison reference values. Subsequent improvements to this test bed system are: control of the ambient static (barometric) pressure used for calibrations so that calibrations can be performed at the reference pressure specified in documentary standards [ 2 – 7 ] and key comparisons, automated measurement and recording of the static pressure and acoustic coupler temperature at each frequency of measurement, the use of the voltmeter in its most accurate alternating voltage measurement mode, the use of new programmable highpass and lowpass filters to provide bandpass filtering with greater dynamic range and lower distortion than the previously used filter set, an improved technique for measuring the front cavity depths of individual Type LS2aP microphones [ 9 ], and adjustment of the power line frequency as needed to reduce interference effects at calibration frequencies harmonically related to 60 Hz [ 10 ]. …”

“…adjustment of the power line frequency as needed to reduce interference effects at calibration frequencies harmonically related to 60 Hz [ 10 ].…”