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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク measurement microphone calibration ： ウィキペディア英語版 measurement microphone calibration In order to take a scientific measurement with a microphone, its precise sensitivity must be known (in volts per pascal). Since this may change over the lifetime of the device, it is necessary to regularly calibrate measurement microphones. This service is offered by some microphone manufacturers and by independent testing labs. Microphone calibration by certified labs is ultimately traceable to primary standards a (National) Measurement Institute such as the National Physical Laboratory in the UK, PTB in Germany and NIST in the USA, where the reciprocity calibration (see below) is the internationally recognised means of realising the primary standard. Laboratory standard microphones calibrated using this method are used in-turn to calibrate other microphones using comparison calibration techniques (‘secondary calibration’), referencing the output of the ‘test’ microphone against that of the reference laboratory standard microphone. A microphone’s sensitivity varies with frequency (as well as with other factors such as environmental conditions) and is therefore normally recorded as several sensitivity values, each for a specific frequency band (see frequency spectrum). A microphone’s sensitivity can also depend on the nature of the sound field it is exposed to. For this reason, microphones are often calibrated in more than one sound field, for example a pressure field and a free field. Depending on their application, measurement microphones must be tested periodically (every year or several months, typically), and after any potentially damaging event, such as being dropped or exposed to sound levels beyond the device’s operational range. == Reciprocity calibration == Reciprocity calibration is currently the favoured primary standard for calibration of measurement microphones. The technique exploits the reciprocal nature of certain transduction mechanisms such as the electrostatic transducer principle used in condenser measurement microphones. In order to carry out a reciprocity calibration, three uncalibrated microphones $i$, $j$ and $k$ are used. Microphones $i$ and $j$ are placed facing each other with a well known acoustical coupler between their diaphragms, allowing the acoustic transfer impedance $Z\_$ to be easily modelled. One of the microphones is then driven by a current $I\_i$ to act as the source of sound and the other responds to the pressure generated in the coupler, producing an output voltage $U\_j$ resulting in the electrical transfer impedance $Z\_$. Provided that the microphones are reciprocal in behaviour, which means the open circuit sensitivity in V/Pa as a receiver is the same as the sensitivity in m³/s/A as a transmitter, it can be shown that the product of the transmission factors $M\_i$, $M\_j$, and the acoustical transfer impedance equals the electrical transfer impedance. :$Z\_\; =\; \backslash frac\; =\; M\_i\; \backslash ;\; Z\_\; \backslash ;\; M\_j$ Having determined the product of the transmission factors for one pair of microphones, the process is repeated with the other two possible pair-wise combinations $ik$ and $jk$. The set of three measurements then allows the individual microphone transmission factor to be deduced by solving three simultaneous equations. :$M\_i\; =\; \backslash sqrt\}\backslash frac\}$ The electrical transfer impedance is determined during the calibration procedure by measuring the current and voltage and the acoustic transfer impedance depends on the acoustical coupler. :$Z\_\; =\; \backslash frac\; =\; \backslash frac$ Commonly used acoustical couplers are free field, diffuse field and compression chamber. For free field conditions between the two microphones the sound pressure in the far field can be calculated and it follows :$Z\_\; =\; \backslash frac\; e^r\}\; e^)\}$ where $r$ is the distance between the microphones. For diffuse field conditions follows :$Z\_\; =\; \backslash frac$ where $A$ is the equivalent absorption area and $d\_c$ is the critical distance for reverberation. For compression camber conditions follows :$Z\_\; =\; \backslash frac$ where $V\_0$ is the air volume in the chamber. The technique provides a measurement of the sensitivity of a microphone without the need for comparison with another previously calibrated microphone, and is instead traceable to reference electrical quantities such as volts and ohms, as well as length, mass and time. Although a given calibrated microphone will often have been calibrated by other (secondary) methods, all can be traced (through a process of dissemination) back to a microphone calibrated using the reciprocity method at a National Measurement Institute. Reciprocity calibration is a specialist process, and because it forms the basis of the primary standard for sound pressure, many national measurement institutes have invested significant research efforts to refine the method and develop calibration facilities. A system is also commercially available from Brüel & Kjær. For airborne acoustics, the reciprocity technique is currently the most precise method available for microphone calibration (i.e. has the smallest uncertainty of measurement). Free field reciprocity calibration (to give the free-field response, as opposed to the pressure response of the microphone) follows the same principles and roughly the same method as pressure reciprocity calibration, but in practice is much more difficult to implement. As such it is more usual to perform reciprocity calibration in an acoustical coupler, and then apply a correction if the microphone is to be used in free-field conditions; such corrections are standardised for laboratory standard microphones (IEC/TS 61094-7) and are generally available from the manufacturers of most of the common microphone types. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「measurement microphone calibration」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.