IEC 61094-5:2001 pdf download

IEC 61094-5:2001 pdf download.Measurement microphones – Part 5: Methods for pressure calibration of working standard microphones by comparison.
5.1.2 General principles using simultaneous excitation
In order for the two microphones to be simultaneously exposed to essentially the same Sound pressure it is usually necessary for the two diaphragms to be separated by less than a tenth of the wavelength at the highest frequency of interest, For frequencies up to 20 kHz, this can be achieved by mounting the two microphones face to face separated by less than 1 mm in either a coupler or a jig. Couplers usually contain an integral sound source; jig mounted microphones are usually exposed to an externally produced sound field In order to reduce the effect of systematic differences in Sound pressure between the two microphone positions, for example caused by some asymmetries, the following procedure shall be used: after the ratio of the microphone pressure sensitivities is first determined, the microphones shall be interchanged. and the measurement repeated. The sensitivity is then calculated from the mean of the two ratios. Examples of practical arrangements and precautions to be taken are given in annex A.
5.1.3 General principles using sequential excitation
In order for the two microphones to be sequentially exposed to essentially the same sound pressure, either the exchange of microphones shall not change the Sound pressure significantly or any significant change shall be detected and corrected. This can be achieved by incorporating a Sound source, a monilor microphone, and the test/reference microphone in a coupler. In any design of coupler, the monitor microphone shall accurately sense changes in the Sound pressure at the test/reference microphone position. Examples of practical arrangements are given In annex B.
5.2 Measuring the output voltages of the microphones
The output of a test or reference microphone may be determined as the open-circuit voltage by use of the insert voltage technique (see 5.3 of IEC 61094.2) or by using a measuring system consisting of a high impedance microphone preamplifier and a voltmeter (see annex C).
The method used to measure the output of the test microphone shall be stated on any calibration certificate.
6 Factors influencing the pressure sensitivity
6.1 General
The pressure sensitivity of a measurement microphone may depend on environmental conditions. Further, the definition of the pressure sensitivity Implies that certain requirements be fulfilled by the measurements. It is essential during a calibration that these conditions are controlled sufficiently well so that the resulting uncertainty components are small.
6.2 Microphone pr.ssur. .quallzlng leakage
The cavity behind the diaphragm is normally fitted with a narrow pressure-equalizing tube to permit the static pressure to be the same on both sides of the diaphragm. Consequently, at very low frequencies, this tube also partially equalizes the Sound pressure. If, during the calibration, the sound which Is coherent with that on the diaphragm is Incident on the pressure- equalizing tube, then this could change the apparent sensitivity at low frequencies and the result would not be the true pressure sensitivity.
In a jig, where Sound is Incident on the pressure equalizing tube. It is necessary to determine the size of this change by comparing calibrations made in the jig with calibrations made in a coupler that does not expose the pressure equalizing tube to the sound field,
In a coupler an ring can be used to seal the gap between the coupler and the microphone. If this is done, care should be taken to ensure that the 0 ring does not exert undue force on the microphone and cause a change In sensitlvlty
6.3 Polarising voltage
If the test microphone requires an external polarising voltage, then the polarising voltage actually used during the calibration shall be reported.
If the reference microphone requires an external polarising voltage, then any difference between that applied when it was calibrated and that applied when it is used as the reference m,ctophone shall be allowed for In the uncertainty calculations (see annex D).
6.4 Ground shield reference configuration
When the open-circuit voltage is measured, the ground-shield configurations given in IEC 61094-1 or IEC 61094-4 shall be used.
If the instruction manual specifies a maximum mechanical force to be applied to the central electrical contact of the microphone, this limit shall not be exceeded.
6.5 Pressure distribution over the diaphragms
The definition of the pressure sensitivity assumes that the sound pressure over the diaphragm is applied uniformly. Pressure non-uniformity over the surface of the diaphragm can be minimised by using a radially symmetric Sound source but some non-uniformity at high frequencies would remain even with a perfect source. The output voltage of a microphone presented with a non-uniform pressure distribution over the surface of the diaphragm will differ from the output voltage of the microphone when presented witl a uniform pressure distribution having the same mean value, because usually the microphone is more sensitive to a sound pressure at the centre of the diaphragm.
When comparing microphones of the same diameter, the sound pressure distribution during a calibration should be uniform over the surface of the diaphragms. However, it is difficult to control this condition in an actual calibration set-up. In order to validate calibrations performed in any particular jig or coup’er. it is recommended they be compared with calibrations performed in a variety of other jigs and couplers. For microphones with circular housings, the effect of the combined asymmetries in the Sound field and microphone may become evident when one microphone is rotated about its axis of symmetry.