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Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−268−Synthesiology - English edition Vol.2 No.4 (2010) AuthorRyuzo HoriuchiCompleted the master’s course in electrical engineering at the Faculty of Science and Engineering, Waseda University in 1989. Joined Electrotechnical Laboratory in 1990. Senior researcher, Acoustics and Ultrasonics Section, Acoustics and Vibration Metrology Division, NMIJ/AIST from 2001. Engages in research for precise acoustic measurement, especially for the development of acoustic standards. Studied at the National Physical Laboratory (NPL), UK for the development of infrasound standards from 2000 to 2001. Doctor of engineering.Discussion with Reviewers1 Primary calibration method other than coupler reciprocity methodQuestion (Akira Ono, AIST) In this paper, the coupler reciprocity method was used as a primary calibration method. Please introduce other methods if any. What kind of methods is adopted for primary calibration by the other national metrology institutes which participated in the international comparison? If some institutes used methods other than the coupler reciprocity method, please explain the reason.Answer (Ryuzo Horiuchi) The free-field sensitivity of a laboratory standard microphone is used as a reference in the secondary calibration of acoustic measuring instruments and it is normally determined by multiplying a correction term to the pressure sensitivity which was obtained by using the coupler reciprocity method. Direct primary calibration of the free-field sensitivity by using the free-field reciprocity method is not practically used because the pressure sensitivity can be more precisely and easily determined than the free-field sensitivity.In the free-field reciprocity method, two laboratory standard microphones are faced with each other in the anechoic chamber instead of the coupler’s cavity and the voltage ratio is measured in the same way as the coupler reciprocity method. However, this method takes a long time for the measurement of voltage ratio and requires strict measures to decrease the cross-talk because signal-to-noise ratio deteriorates at the lower frequency range. Influence of indirect sound should be also minimized because the calibration is conducted in the anechoic chamber. These reasons prevent the method from being used as a routine calibration service. Thus the other national metrology institutes use the coupler reciprocity method as a primary calibration method.There is the “laser-pistonphone” which is available as a primary calibration technique of the pressure sensitivity only for the lower frequency range. In this method, a piston attached to a shaker is used as a transmitter and generates sound pressure within the coupler. The vibration amplitude of the piston is optically measured and translated into the sound pressure. At the same time, output voltage of the laboratory standard microphone exposed to the sound pressure is measured and the pressure sensitivity is determined. NMIJ/AIST is developing a laser-pistonphone as a primary calibration system at infrasound range.2 Methods used by other national metrology institutes for evaluating influence of indirect sound Question (Akira Ono) I think the method developed by NMIJ/AIST for evaluating influence of indirect sound in the secondary calibration is an excellent research result. Do other national metrology institutes adopt a similar technique or not?Answer (Ryuzo Horiuchi) Other national metrology institutes use methods different from NMIJ/AIST’s to evaluate and minimize the influence of indirect sound in the secondary calibration of acoustic measuring instruments. TDS (Time Delay Spectrometry) eliminates indirect sound by using a narrow bandwidth filter which works considering the arriving time of indirect sound. MLS (Maximum Length Sequence) uses special random signals to obtain a pulse response rapidly. These methods have common characteristics of separating indirect sound from direct sound on the time domain. However, no methods have been internationally standardized yet. The FFT analyzer-based virtual pulse method developed by NMIJ/AIST will be adopted as one of the methods in the international standard under discussion.3 International level of acoustic standards and Japanese way to go in the futureQuestion (Akira Ono) Figure 8 seems to show little difference between the results of main institutes which participated in the international comparison organized by CCAUV and the results of institutes in Asia and Pacific area, from the point of view of uncertainty or deviation [23][24][25][26][27][28][29][30][31][32][33][34][35]T. Fujimori and H. Miura: New acoustic building of Electrotechnical Laboratory, Tsukuba Research Center, Proc. of technical committee on engineering acoustics, EA-80-19 (1980) (in Japanese).IEC 61094-8, Measurement microphones Part8: Methods for free-field calibration of working standard microphones by comparison Working Draft 6 (2009).T. Fujimori, R. Horiuchi and S. Sato: Measurement of acoustic center position of LS2P laboratory standard microphones, Journal of Acoustical Society of Japan, 58(9), 579-585 (2002) (in Japanese).R. Horiuchi and T. Fujimori: Traceability on acoustic measurement under JCSS, Journal of INCE/J, 30(5), 381-383 (2006) (in Japanese).K. Seta: Activity for global mutual recognition arrangement of measurement standards, AIST Today, 1(6), 26 (2001) (in Japanese).ISO/IEC 17025 (JIS Q 17025), General requirements for the competence of testing and calibration laboratories (2005).R. Barham: Report on key comparison CCAUV.A-K1, Metrologia, 40-09002 (2003).V. C. Henriquez and K. Rasmussen: Final report on the key comparison CCAUV.A-K3, Metrologia, 43-09001 (2006). R. Horiuchi, H. Takahashi, T. Fujimori and S. Sato: Final report on key comparison APMP.AUV.A-K1, Metrologia, 44-09001 (2007).JCT 21500-05, Japan calibration service system, Specific application document for the application of ISO/IEC 17025 to sound pressure level (2009) (in Japanese).H. Takahashi, T. Fujimori and R. Horiuchi: Development of acoustic standards for airborne ultrasound, Journal of Acoustical Society of Japan, 65(1), 34-39 (2009) (in Japanese).R. Horiuchi, T. Fujimori and S. Sato: Research and development for infrasound standards, Journal of Acoustical Society of Japan, 62(4), 338-344 (2006) (in Japanese).IEC TS 61094-7, Measurement microphones Part7: Values for the difference between free-field and pressure sensitivity levels of laboratory standard microphones (2006).

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