BS EN 61094-3-2016 pdf free download.Electroacoustics — Measurement microphones Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique
6 Factors influencing the free-field sensitivity
6.1 General The free-field sensitivity of a laboratory standard microphone depends on polarizing voltage, as it has an electrostatic transductions mechanism, and the environmental conditions. The basic mode of operation of a polarized electrostatic microphone assumes that the electrical charge on the microphone is kept constant at all frequencies. This condition cannot be maintained at very low frequencies and the product of the microphone capacitance and the polarizing resistance determines the time constant for the flow of charge to and from the microphone. While the open-circuit sensitivity of the microphone, as obtained using the insert voltage technique, will be determined correctly, the absolute output from an associated preamplifier to the microphone will decrease at low frequencies in accordance with this time constant.
The construction principles of laboratory standard microphones imply that the static pressure behind and in front of the diaphragm shall remain the same. To comply with this a pressure equalizing tube is used to connect the back cavity of the microphone to the external medium. The effect of this tube is that the free-field sensitivity will approach zero at very low frequencies (below a few hertz). The technique described in this standard is not suitable for determining the free-field sensitivity in this frequency range. Furthermore, the definition of the free-field 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 Polarizing voltage The sensitivity of a laboratory standard microphone is approximately proportional to the polarizing voltage and thus the polarizing voltage actually used during the calibration shall be reported. To comply with IEC 61 094-1 , a polarizing voltage of 200,0 V is recommended.
6.3 Shield configuration The open-circuit voltage, and therefore the free-field sensitivity, depends on the shield configuration. Consequently, IEC 61 094-1 specifies a reference mechanical configuration for the shield for use in determining the open-circuit voltage. While the reference mechanical configuration is essential, the shield can either be grounded (grounded-shield configuration), or the output voltage from the microphone can be applied to the shield (driven-shield configuration). It shall be stated whether the driven-shield or grounded-shield configuration was used in the measurements.
The same shield configuration shall apply to both transmitter and receiver microphones during the calibration
. If any non-standard configuration is used, the results of a calibration shall be referred to the reference mechanical configuration. If the manufacturer specifies a maximum mechanical force to be applied to the central electrical contact of the microphone, this limit shall not be exceeded.
NOTE 1 When the shield is driven, the loading impedance as seen from the microphone is maximized, and it can be described more accurately than in the case of using the grounded shield configuration. In the ideal case, in which the microphone is a perfectly linear and passive device and the shield is either grounded, or driven from a zero source impedance, there is no difference between the open-circuit sensitivity with grounded or driven shield.
NOTE 2 In the driven-shield configuration, applying the output voltage from the microphone to the shield means that any difference between the signal applied on the shield and on the centre-pin of the microphone is negligible. NOTE 3 If a microphone is connected to a preamplifier by means of an adapter there is the possibility that the open-circuit voltage of the microphone is not determined properly by the insert voltage technique at high frequencies. The deviations depend on the load impedance as seen from the microphone.
6.4 Acoustic conditions The free-field sensitivity of a microphone depends on the geometrical configuration of the housing containing the preamplifier. For this reason, the microphone and the shield configuration shall be attached to a cylinder whose diameter is equal to the nominal diameter of the microphone, see Table 1 and Table 2 in IEC 61 094-1 :2000. The length of the cylinder shall be long compared to the diameter of the microphone.
A minimum length of twenty times the diameter of the microphone with a gradually tapered transition to the supporting structure is recommended. This arrangement shall also apply to the transmitter microphone. The definition of the free-field sensitivity of a microphone refers to the sound pressure in an undisturbed plane progressive wave. In the far field of a sound source located under free-field conditions, spherical waves are encountered which, at a sufficient distance from the source, are approximately plane waves in a limited region. Thus, the distance between the receiver microphone and the transmitter microphone shall be great enough to ensure approximately plane waves in a suitable region around the receiver microphone (see 7.3). Conversely, the influence of reflections from the interior surfaces of an anechoic chamber usually increases as the distance between the two microphones is increased. Also the scattering factor S(f,θ) depends on the character of the sound field and can only be unambiguously defined for a true plane progressive wave. Therefore, the metrological conditions should be carefully chosen and it may be preferable to carry out calibrations at more than one distance to assess the calibration uncertainty attributable to dependence on these conditions.
6.5 Position of the acoustic centre of a microphone The position of the acoustic centre of a microphone can be determined from measurements of the sound pressure produced by the microphone when used as a sound source in a free field, as a function of distance r from an arbitrarily chosen reference point of the microphone. In a limited region of the far field, the sound pressure, corrected for the effect of sound attenuation, will follow the 1 /r-law, r being referred now to the acoustic centre of the microphone. Thus, when plotting the inverse value of the measured sound pressure as a function of the distance from an arbitrarily chosen reference point of the microphone (most conveniently the centre of the diaphragm), a straight line can be fitted (e.g. by the methods of least squares) through the plotted values. The intersection of this straight line and the abscissa axis determines the position of the acoustic centre relative to the reference point.BS EN 61094-3 pdf download.