Slip Clutch

(3) A standard capacitor is used to measure the driving current of the reciprocal transducer.9

(4) The necessary calculations are performed by a simple dial-type analog computer coupled to the switch (see Figure 3). The voltages that result from the acoustical transfers are duplicated with an electrical network that includes a logarithmic potentiometer. The angular positions of the potentiometer shaft are proportional to the logarithms of the voltages to be measured, and these shaft positions are transferred to the answer dial in such a way that the necessary multiplications and divisions are accomplished in a manner analogous to the use of a slide rule.

(5) The closed coupler reciprocity technique normally yields the pressure response of a microphone, but the American Standards Association Specification for General Purpose Sound-Level Meters SI.4-1961 requires a microphone which has a flat random-incidence (diffuse-field) response. In this calibrator, the deviation of the coupler from a simple acoustical element with increasing frequency is empirically matched to the correction between the random-incidence response and the1 pressure response. The calibration is then effectively in terms of a diffuse-sound field, which is the environment to which a sound-level-meter microphone is most commonly exposed. Figure 4 shows the degree to

•A. K. Nielson, "A Simplified Technique for the Pressure Calibration of Condenser Microphones by the Reciprocity Method," Acoustica, Vol 2 #3, 112-118 (1952).

which the random-incidence correction matches the deviation of the coupler impedance with frequency.

Procedure

To calibrate a microphone one inserts the microphone into the cavity, clamps it in place, and connects it to the instrument. Then one sets a reference to the barometric pressure and proceeds to make four dial settings. At the completion of the fourth setting, the random-incidence (diffuse-field) microphone sensitivity in db re lv/yubar is read directly from a dial on the instrument panel to an accuracy which varies from 0.2 db at low frequencies to 0.7 db at 7 kc.

The Calibration, Absolute and Traceable

The Type 1599-A Microphone Reciprocity Calibrator is a primary and accurate calibrator of General Radio Company's pzt microphones below 1 kc, while above 1 kc (where the dimensions of the cavity become comparable with the wavelengt hs of the sound) it is direct reading in the random-incidence (diffuse field) response of the microphones. The calibration of a Type 1560-P3 PZT Microphone by use of this instrument depends on the measurement of length (volume of the cavity) and electrical impedance (capacitance and resistance), and is thus "traceable" to nbs calibrations of those units. An independent cross-check is also possible by comparison with an nbs calibrated 6 40-A A microphone, as shown later.

Figure 4. Average deviation between calibration by the Type 1559-A Microphone Reciprocity Calibrator and calibration at random incidence.

Figure 4. Average deviation between calibration by the Type 1559-A Microphone Reciprocity Calibrator and calibration at random incidence.

A Standard Source

The Type 1559-A Microphone Reciprocity Calibrator can also be used as a precision acoustic source for setting the reference level of a sound measurement system. When the instrument is to be used as a precision source, the sensitivity of a microphone must first be determined by the procedure outlined above. This sensitivity is used as a reference to set the driving current for the reversible transducer. For a given input voltage a known sound-pressure level will then be produced in the cavity. A sound-measurement system connected to the output of the microphone can then be set to indicate this known level.

Comparison with Standard Microphone

This technique for producing a known sound level in the cavity can also be used to compare the sensitivitv of a General Radio Type 1560-P3 PZT Microphone to that of a Western Electric 640-AA Laboratory Standard Microphone or to calibrate a sound-measuring system that uses such a microphone. After the Type 1559-A Microphone Reciprocity Calibrator is set to produce a known sound field as above, the Type 1560-P3 PZT Microphone is replaced by the G40-AA microphone with the adaptor sleeve furnished with the calibrator. The sound level in the cavity is then measured by the G40-AA microphone and its associated measurement system, and the system calibration can be set according to the known level. Or, if the sensitivity of the 640-AA is known and the electrical response of the associated system is known, one can then compare the sound level measured with the G40-AA and that produced in the cavity. When this is done, the primary calibration of a Type 1560-P3 PZT Microphone made with a Type 1559-A Microphone Reciprocity Calibrator has been compared to the National Bureau of Standards calibration of a 640-AA microphone, and the agreement is better than 0.2 db for frequencies below 1 kc.

Generator and Detector

The accessories required are a generator and a detector. The Type 1311-A Audio Oscillator, which supplies 10 fixed frequencies between 50 and 5000 cps, is recommended. For continuously adjustable frequencies, either the Type 1210-C Unit R-C Oscillator or the Type 1304-B Beat-Frequency Audio Generator can be used. For recording applications or for fixed installations, the latter is preferable. The detector can be either the Type 1551-C Sound-Level Meter or the Type 1558-A Octave-Band Noise Analyzer, usually the instrument whose microphone is to be calibrated.

Applications

Use of the Type 1559-A Microphone Reciprocity Calibrator effectively eliminates instrument error from sound-level measurements; the observer can then concentrate his attention on the acoustical factors, such as environment and microphone placement. Hitherto, one could readily calibrate the electrical part of a sound-measuring system, but the calibration of the microphone was restricted to a check on the microphone's sensitivity at a single frequency. A complete calibration of the microphone could be made only at a qualified laboratory.

The need for proveçl accuracy, including calibration of the microphone, in the field of sound-level measurements, is illustrated by a noise specification such as MIL-E-22842 (Ships), which is referenced to M1L-STD-740 (Ships). The latter specification requires that the microphone used in 1 he sound-measuring system be calibrated every six months. With both the supplier and the purchaser performing noise measurements according to the specifications, accuracy of calibration will be an important factor in the rapid and economical elimination of any discrepancies between the respective measurements.

The manufacture and purchase of equipment to a particular noise specification is not restricted to military applications. Some examples are: (1) The air-moving indust ry which has specifications on the noise levels that may be generated by fans, blowers, etc.; (2) communities with ordinances restricting the maximum noise level produced by trucks and other vehicles; and (3) airports that have limits on the noise level produced by aircraft using their facilities. Industrial hygienists require proof of the calibration accuracy of their sound-level measuring equipment. In general, requirements for sound-measuring equipment are becoming more stringent because users demand correlation of measurements taken at different locations and consistency of measurements taken at different times.

Simple to operate, direct reading, and accurate, this calibrator provides a reliable and consistent means of standardizing sound measurements.

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