Vol.2 No.4 2010

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Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−261−Synthesiology - English edition Vol.2 No.4 (2010) 2 Elemental technologies and research scenarioElemental technologies are necessary to solve the two technical problems mentioned in the last chapter and to achieve a research goal of realizing the reliable environmental noise measurement.The first elemental technology is the uncertainty decrease in sensitivity calibration of laboratory standard microphones. Calibration uncertainty of laboratory standard microphones realized by NMIJ/AIST thus far was around 0.1 dB at the smallest. According to JIS, however, calibration uncertainty of a high-grade sound calibrator used to check sound level meters[14] (a portable sound source which generates the given sound pressure to calibrate the instruments) was less than 0.1 dB. The laboratory standard microphone with an uncertainty of 0.1 dB was not suitable as a reference to evaluate the performance of the sound calibrator. Therefore, an advanced calibration system of laboratory standard microphones became essential. Signal-to-noise ratio of the calibration system was improved by introducing the digital signal processing technique etc. and the uncertainty was decreased to 0.04 dB.The second elemental technology is the development of the method necessary to evaluate the uncertainty caused by the imperfection (existence of indirect sound) of the sound field which is used to calibrate acoustic measuring instruments. The developed method made it possible to evaluate the uncertainty quantitatively by visualizing the influence of the indirect sound and to remove unnecessary indirect sound by using the digital signal processing technique.Lastly, the traceability system of acoustic measuring instruments was required to return these research results to society. It is not until the traceability system is established that the advanced calibration technique of laboratory standard microphones and acoustic measuring instruments gives more reliable measurement results to end-users. NMIJ/AIST established the traceability system, providing acoustic standards as the highest level of accuracy, and calibration service providers could calibrate end-users’ acoustic measuring instruments using these acoustic standards.The traceability system is required to confirm the measurement capability of the constituent organizations in each level, namely NMIJ/AIST and calibration service providers. NMIJ/AIST internationally participated in several round robin tests among the national metrology institutes and Fig. 4 Schematic of primary calibration system of laboratory standard microphones by the coupler reciprocity method.Coupler reciprocity method uses two laboratory standard microphones, one as a transmitter and the other as a receiver. Sound wave is generated from the diaphragm of the transmitter by applying the input voltage, arrives at the diaphragm of the receiver through a cavity of the coupler and the output voltage is detected at the receiver.Sensitivity product of the two microphones are obtained from the cavity volume of the coupler and the voltage ratio between the input terminal of the transmitter and the output terminal of the receiver, by using a principle of an electro-acoustic transducer that the sensitivity of the transmitter is equal to that of the receiver. Introduction of one more laboratory standard microphone enables the sensitivity of each microphone to be determined by measuring the voltage ratios for the three combinations of the transmitter and the receiver.This method requires cancellation of influence caused by the output impedance of the receiver and the gain of the amplifier to measure the open-circuit output voltage of the receiver precisely. Thus, the calibration system has two signal paths, namely the acoustical path and the attenuator path. Ratio of the receiver’s output voltages between the two paths cancels this influence.Acoustical pathCouplerTransmitter microphoneReceiver microphonePre-amplifierMain amplifierBand-pass filterSignal outputAttenuator pathAttenuatorch Ach BFFT analyzer

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