BS EN 61094-3 pdf free download

BS EN 61094-3 pdf free download. Electroacoustics — Measurement microphones Part 3: Primary method for free-field calibration of laboratory standard microphones by the reciprocity technique.
5 Principles of free-field calibration by reciprocity 5.1 General principles 5.1 .1 General A reciprocity calibration of microphones may be carried out by means of three microphones, two of which shall be reciprocal, or by means of an auxiliary sound source and two microphones, one of which shall be reciprocal. NOTE 1 If one of the microphones is not reciprocal it can only be used as a sound receiver. NOTE 2 Laboratory standard microphones are reciprocal when used within their linear operating range. 5.1 .2 General principles using three microphones Let two of the microphones be coupled acoustically under free-field conditions. Using one of them as a sound source and the other as a sound receiver, the electrical transfer impedance is measured. When the acoustic transfer impedance of the system is known, the product of the free-field sensitivities of the two coupled microphones can be determined. Using pair-wise combinations of three microphones, three such mutually independent sensitivity products are available, from which an expression for the free-field sensitivity of each of the three microphones can be derived. 5.1 .3 General principles using two microphones and an auxiliary sound source First, let the two microphones be coupled acoustically under free-field conditions, and the product of the free-field sensitivities of the two microphones be determined as described in 5.1 .2. Next, let the two microphones be sequentially presented to the same sound pressure, set up by the auxiliary sound source under identical free-field conditions. The ratio of the two output voltages will then equal the ratio of the free-field sensitivities of the two microphones. Thus, from the product and the ratio of the free-field sensitivities of the two microphones, an expression for the free-field sensitivity of each of the two microphones can be derived. NOTE In order to obtain the ratio of free-field sensitivities, a direct comparison method can be used, and the auxiliary sound source can be another type of transducer or a third microphone having mechanical or acoustical characteristics which differ from those of the microphones being calibrated. 5.3 Insert voltage technique The insert voltage technique is used to determine the open-circuit voltage of a microphone when it is electrically loaded. Let a microphone having a certain open-circuit voltage and internal electrical impedance be connected to an external electrical load impedance. To measure the open-circuit voltage, an impedance, small compared to the load impedance, is connected in series with the microphone and a calibrating voltage applied across it. Let a sound pressure and a calibrating voltage of the same frequency be applied alternately. When the calibrating voltage is adjusted until it gives the same voltage drop across the load impedance as results from the sound pressure on the microphone, the open-circuit voltage will be equal in magnitude to the calibrating voltage. 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.BS EN 61094-3 pdf download.