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Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−260−Synthesiology - English edition Vol.2 No.4 (2010) characteristics of their own products and ensure the quality of the products internationally until their acoustic measuring instruments are verified to be in conformity with the international standards or the equivalent JIS. The establishment of a traceability system for acoustic measuring instruments was an essential technology to develop the products envisioned on a road map of future technical development by the industrial world, namely products with high quality which are environmentally friendly and safe. Such a change of needs in the industrial world resulted in the establishment of a new calibration service system of acoustic standards, which is different from the traditional testing system.For the reliable environmental noise measurement, end-users should not only use the traceable measuring instruments but evaluate the influence specific to the measurement site such as environmental conditions (temperature, static pressure and wind) and indirect sound from the surroundings. Uncertainty related to a sound field (a space in which the sound wave is transmitted) containing indirect sound has not been technically evaluated up to this point. Influence of indirect sound depends on relative positions between acoustic measuring instruments and sound reflecting objects such as the ground or buildings. In the past, measured data were averaged by changing the position of the measuring instrument. Uncertainty caused by indirect sound could be decreased by this method, but the obtained data were not reliable enough because the remaining uncertainty could not be quantitatively evaluated.The calibration of acoustic measuring instruments had the same technical problem. Acoustic measuring instruments are usually calibrated within an anechoic chamber which is designed to minimize the influence of indirect sound. However, even a high-performance anechoic chamber cannot realize a space completely free from indirect sound. Indirect sound was the main cause of uncertainty in the calibration of acoustic measuring instruments. JIS of sound level meters[6] requires uncertainty decrease by placing the sound source and the sound level meter at several positions and by averaging the measured data. However, it is just one of the procedures to judge if the specification of the sound level meter fulfills the criterion, and the uncertainty still remained not properly evaluated.Considering such a situation, NMIJ/AIST developed the technique necessary to evaluate the uncertainty of the sound field caused by indirect sound and thus solved the technical problem.Furthermore, there was one more technical problem in the calibration of acoustic measuring instruments. Acoustic standards realized so far did not have measurement uncertainty small enough to evaluate the conformity of some acoustic measuring instruments to the corresponding standard. Development of advanced (high-precision) acoustic standards was essential to decrease the uncertainty.In the following chapters, research results are described which were done to solve these problems and to realize the reliability of environmental noise measurement.Fig. 2 Laboratory standard microphone.Laboratory standard microphone is one of the measurement microphones but its sensitivity must be sufficiently stable compared with the others because it is required to be reliable as an acoustic standard. It has a special structure around the diaphragm (gold part and indicated by an arrow in the figure) to protect the diaphragm when it is fitted to the coupler for the sensitivity calibration as will be explained later. Left is type LS1P and right is LS2aP microphone, respectively.Fig. 3 Schematic of traceability system in acoustic measurement.Laboratory standard microphones possessed by a high-ranking calibration service provider are calibrated by using a primary calibrated system NMIJ developed. Lower-ranking provider’s microphones are calibrated in comparison with higher-ranking provider’s microphones. End-users’ acoustic measuring instruments are calibrated in comparison with higher or lower ranking provider’s microphones.Sound calibratorsSound level metersMeasurement microphonesAcoustic measuring instrumentsPrimary calibration system of pressure sensitivities on laboratory standard microphonesLaboratory standard microphonesLaboratory standard microphonesEnd-usersCalibration service providersNational Metrology Institute of JapanCalibration service providers

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