Randall P. Wagner

New Measurement Service for Determining Pressure Sensitivity of Type LS2aP Microphones by the Reciprocity Method

A new National Institute of Standards and Technology (NIST) measurement service has been developed for determining the pressure sensitivities of American National Standards Institute and International Electrotechnical Commission type LS2aP laboratory standard microphones over the frequency range 31.5 Hz to 20 000 Hz. At most frequencies common to the new service and the old service, the values of the expanded uncertainties of the new service are one-half the corresponding values of the old service, or better. The new service uses an improved version of the system employed by NIST in the Consultative Committee for Acoustics, Ultrasound, and Vibration (CCAUV) key comparison CCAUV.A-K3. Measurements are performed using a long and a short air-filled plane-wave coupler. For each frequency in the range 31.5 Hz to 2000 Hz, the reported sensitivity level is the average of data from both couplers. For each frequency above 2000 Hz, the reported sensitivity level is determined with data from the short coupler only. For proof test data in the frequency range 31.5 Hz to 2000 Hz, the average absolute differences between data from the long and the short couplers are much smaller than the expanded uncertainties.

Determination of acoustic center correction values for type LS2aP microphones at normal incidence

Frequency-dependent acoustic center correction values are required to obtain accurate microphone calibrations in the free-field by the reciprocity technique. These values were determined for IEC type LS2aP microphones at normal incidence by utilizing the theoretical inverse relationship between the sound pressure amplitude at the acoustic center of a receiver and the distance between acoustic centers of source and receiver. A dynamic signal analyzer was used to measure the gain factor between the amplified output voltage of the receiver and the source input voltage at 500-Hz intervals in the extended frequency range 2–50 kHz. This procedure allowed all the data for a microphone pair to be gathered within several hours for microphone diaphragm separations from 101–311 mm at 10-mm intervals. At each frequency, the reciprocal of this gain factor as a function of microphone diaphragm separation was fit to a straight line after correction for atmospheric effects, including attenuation of sound caused by atmosp...

Non-contact Methods for Measuring Front Cavity Depths of Laboratory Standard Microphones Using a Depth-Measuring Microscope

To achieve an acceptable degree of accuracy at high frequencies in some standardized methods for primary calibration of laboratory standard (LS) microphones, the front cavity depth lfc of each microphone must be known. This dimension must be measured using non-contact methods to prevent damage to the microphone diaphragm. The basic capabilities of an optical depth-measuring microscope were demonstrated by the agreement of its measurements within 0.7 μm of the known values of reference gage blocks. Using this microscope, two basic methods were applied to measure lfc. One (D) uses direct measurements at the microphone front surface annulus and conventional data reduction techniques. The other (GB) uses measurements at the surface of a gage block placed on the annulus, and plane-fitting data reduction techniques intended to reduce the effects of the slightly imperfect geometries of the microphones. The GB method was developed to provide a smoother surface of measurement than the relatively rough surface of the annulus, and to simulate the contact that occurs between the annulus and the smooth, plane surface of an acoustic coupler during microphone calibration. Using these methods, full data sets were obtained at 33 measurement positions (D), or 25 positions (GB). In addition, D and GB subsampling methods were applied by using subsamples of either the D or the GB full data sets. All these methods were applied to six LS microphones, three each of two different types. The GB subsampling methods are preferred for several reasons. The measurement results for lfc obtained by these methods agree well with those obtained by the GB method using the full data set. The expanded uncertainties of results from the GB subsampling methods are not very different from the expanded uncertainty of results from the GB method using the full data set, and are smaller than the expanded uncertainties of results from the D subsampling methods. Measurements of lfc using the GB subsampling method with only nine measurement positions exhibit expanded uncertainties (with coverage factor k = 2) within 4 μm, and can improve the uncertainty of microphone calibrations by an order of magnitude over the result from use of generic standardized microphone type nominal lfc values and tolerance limits.

Effect of Power Line Interference on Microphone Calibration Measurements Made at or Near Harmonics of the Power Line Frequency.

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.

Pressure reciprocity calibrations of laboratory standard microphones at the National Institute of Standards and Technology

The reciprocity technique has long served as a method for pressure calibration of microphones. It is a primary method, which determines microphone sensitivities from first principles and does not require a previously calibrated acoustic transfer standard. For calibrations of laboratory standard microphones, this method is standardized and utilized at national measurement institutes worldwide. Standard microphones calibrated by reciprocity are in turn used to calibrate additional microphones and sound calibrators, which apply known sound pressures to calibrate acoustical measuring devices and systems. Reciprocity calibrations done at the National Institute of Standards and Technology (NIST), which is the national measurement institute for the U.S., provide its customers with accurate results traceable to the International System of Units (SI). These customers and organizations that utilize their services perform large numbers of secondary and further calibrations and measurements concerned with hearing con...

Pressure reciprocity calibration of a MEMS microphone

This article reports the first use of the pressure reciprocity technique to calibrate a micro-electromechanical system (MEMS) microphone. This standardized primary calibration method is conventionally used to calibrate laboratory standard microphones. Results for the pressure reciprocity calibration of a MEMS microphone and two laboratory standard microphones are presented for the frequency range 100-10 000 Hz. Because the amplifier in the MEMS microphone package prevents reciprocal operation, this microphone was used only as a receiver of sound. A description of the procedure is presented along with checks of the measurement results and data regarding the uncertainties of these results.

Long-Term Stability of One-Inch Condenser Microphones Calibrated at the National Institute of Standards and Technology

The devices calibrated most frequently by the acoustical measurement services at the National Institute of Standards and Technology (NIST) over the 50-year period from 1963 to 20121 were one-inch condenser microphones of three specific standard types: LS1Pn, LS1Po, and WS1P. Due to its long history of providing calibrations of such microphones to customers, NIST is in a unique position to analyze data concerning the long-term stability of these devices. This long history has enabled NIST to acquire and aggregate a substantial amount of repeat calibration data for a large number of microphones that belong to various other standards and calibration laboratories. In addition to determining microphone sensitivities at the time of calibration, it is important to have confidence that the microphones do not typically undergo significant drift as compared to the calibration uncertainty during the periods between calibrations. For each of the three microphone types, an average drift rate and approximate 95 % confidence interval were computed by two different statistical methods, and the results from the two methods were found to differ insignificantly in each case. These results apply to typical microphones of these types that are used in a suitable environment and handled with care. The average drift rate for Type LS1Pn microphones was −0.004 dB/year to 0.003 dB/year. The average drift rate for Type LS1Po microphones was −0.016 dB/year to 0.008 dB/year. The average drift rate for Type WS1P microphones was −0.004 dB/year to 0.018 dB/year. For each of these microphone types, the average drift rate is not significantly different from zero. This result is consistent with the performance expected of condenser microphones designed for use as transfer standards. In addition, the values that bound the confidence intervals are well within the limits specified for long-term stability in international standards. Even though these results show very good long-term stability historically for these microphone types, it is expected that periodic calibrations will always be done to track the calibration history of individual microphones and check for anomalies indicative of shifts in sensitivity.

NIST System for Measuring the Directivity Index of Hearing Aids under Simulated Real-Ear Conditions.

The directivity index is a parameter that is commonly used to characterize the performance of directional hearing aids, and is determined from the measured directional response. Since this response is different for a hearing aid worn on a person as compared to when it is in a free field, directivity index measurements of hearing aids are usually done under simulated real-ear conditions. Details are provided regarding the NIST system for measuring the hearing aid directivity index under these conditions and how this system is used to implement a standardized procedure for performing such measurements. This procedure involves a sampling method that utilizes sound source locations distributed in a semi-aligned zone array on an imaginary spherical surface surrounding a standardized acoustical test manikin. The capabilities of the system were demonstrated over the frequency range of one-third-octave bands with center frequencies from 200 Hz to 8000 Hz through NIST participation in an interlaboratory comparison. This comparison was conducted between eight different laboratories of members of Working Group S3/WG48, Hearing Aids, established by Accredited Standards Committee S3, Bioacoustics, which is administered by the Acoustical Society of America and accredited by the American National Standards Institute. Directivity measurements were made for a total of six programmed memories in two different hearing aids and for the unaided manikin with the manikin right pinna accompanying the aids. Omnidirectional, cardioid, and bidirectional response patterns were measured. Results are presented comparing the NIST data with the reference values calculated from the data reported by all participating laboratories.

Developments at the National Institute of Standards and Technology in pressure calibration of laboratory standard microphones.

Current research effort aims at improving the apparatus and methods for determining the pressure sensitivities of IEC types LS1Pn and LS2aP laboratory standard microphones. Among the improvements that are being systematically incorporated in an evolving test bed is the capability to operate at adjustable power line frequencies other than the usual 60 Hz. Suitable choices of line frequency relative to frequencies of calibration and adjustable bandpass filter characteristics can be used to improve the signal‐to‐noise ratios of measurements performed near the usual line frequency and its first few harmonics. This can enable the use of relatively large volume couplers for which uncertainties in microphone front cavity volume and equivalent volume, capillary tube effects, and heat conduction corrections have a lesser influence than they have for small‐volume couplers. Another improvement aims to control and to stabilize the ambient static pressure during microphone calibrations, to reduce or eliminate the effe...

Methods for hearing aid directivity index measurements under simulated real‐ear conditions

One technique traditionally used to measure the directivity index of hearing aids involves acquiring data with sound source locations at multiple azimuth angles restricted to the horizontal plane. For measurement of the directivity index of a hearing aid under simulated real‐ear conditions, this technique provides limited accuracy due to the inherent asymmetry of the directional response of a manikin‐mounted aid. To address this limitation, Working Group 48 of the American National Standards Institute (ANSI) Accredited Standards Committee S3 on Bioacoustics worked on the development of standardized procedures with sound source locations out of the horizontal plane. One procedure given consideration specifies sound source locations on the surface of an icosahedron. The revision of ANSI Standard S3.35 eventually developed by this working group specifies 48 sound source locations on a spherical surface centered about the point on the manikin bisecting the line joining the centers of the ear canals. Details r...