Vol.2 No.4 2010

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Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−266−Synthesiology - English edition Vol.2 No.4 (2010) stability of traveling standards (laboratory standard microphones to be calibrated), analyzed calibration results of the participants, and developed the method to link the results of this international comparison to those of corresponding international comparisons by CCAUV[31]. Figure 8 shows results of the international comparison on pressure sensitivity of LS1P microphones. Figure 8 revealed that each participant’s result agrees to the others within the uncertainty of the participant. Thus, mutual equivalence of acoustic standards among the participants was verified based on this data.6.2 Validation of measurement capability of calibration service providersNMIJ/AIST supported publishing a guide[32] for applying technical requirements given in ISO/IEC 17025 to the field of precise acoustic measurement. Calibration service providers established their quality systems by using this guide. Round robin tests were conducted to verify their measurement capability and NMIJ/AIST provided reference values as criteria. NMIJ/AIST also supported NITE from the technical viewpoint in the assessment of calibration service providers for accreditation. At the end of August, 2009, six calibration service providers were accredited as JCSS providers and their measurement capability was verified accordingly.7 Research resultsIn this research, NMIJ/AIST made technical development indispensable to ensure the reliable environmental noise measurement and thus to sustain safety in our daily life. Firstly, primary calibration system of pressure sensitivities on laboratory standard microphones was advanced, resulting in a new calibration service of acoustic standards. Uncertainty caused by electrical characteristics of the calibration system was minimized to the limit at the present time. This revealed that instability of the microphone sensitivity is a dominant factor of uncertainty and that different types of microphones have different degrees of stability. Secondly, NMIJ/AIST developed the method essential to evaluate the uncertainty related to the sound field. This evaluation has been an unsolved problem in secondary calibration of end-users’ acoustic measuring instruments, conducted in comparison with laboratory standard microphones. Introduction of digital signal processing technique succeeded in decreasing the influence of indirect sound.Lastly, the above technical development resulted in the new calibration service system of acoustic standards, ensuring traceability based on the Japanese Measurement Law. Measurement capability of calibration service providers was verified by several ways: supply of acoustic standards, discussion of the guide used to take technical requirements into account, provision of reference values in the round robin tests, and technical support in the assessment.Besides calibration service providers, NMIJ/AIST was also assessed to get accreditation and took part in several international comparisons to verify the equivalence of national standards among the main institutes. NMIJ/AIST acted as a pilot laboratory in the first international comparison conducted in Asia and Pacific area.These results greatly contribute to not only secure the performance of end-users’ acoustic measuring instruments but also to improve the reliability of measurement by end-users. The method to evaluate indirect sound enabled the operator to find the cause and degree of indirect sound clearly. Thus, influence of indirect sound could be easily decreased by taking suitable measures and the effect of the measures could be quantitatively evaluated. As a result of the development, reliability of the measurement results became almost independent of the operator’s skill and even an inexperienced operator could easily get reliable data.8 ConclusionNMIJ/AIST developed a calibration service system of acoustic standards based on the Japanese Measurement Law, with acoustic standards at the highest level of accuracy in the traceability system and started a new calibration service to meet the demands of the times.The future theme is to expand the calibration frequency of acoustic standards outside of the audible frequency range. Equipment generating airborne ultrasound over 20 kHz is increasing around our living circumstances. However, sound pressure level cannot be quantitatively evaluated because acoustic standards are not yet established in the airborne ultrasound range. To discuss the problem of human safety under the exposure to strong airborne ultrasound, development of acoustic standards in high frequency range is essential[33].On the other hand, complaints against infrasound less than 20 Hz are increasing. Although common measurement procedures have been suggested for infrasound, reliability of the measurement results cannot be ensured without acoustic standards. Acoustic standards in low frequency range should also be essentially developed[34]. NMIJ/AIST is conducting research and development of acoustic standards and measurement technology necessary to expand the available frequency range.AcknowledgementsI am deeply grateful to Mr. Takeshi Fujimori, former staff of NMIJ/AIST and Dr. Sojun Sato, Professor of Akita Prefectural University and ex-staff of NMIJ/AIST as well as many persons concerned for much valuable advice.

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