Vol.2 No.4 2010

39/68

Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−269−Synthesiology - English edition Vol.2 No.4 (2010) 4 International and societal role of NMIJ/AIST in the futureQuestion (Katsuhisa Kudo, Evaluation Division, AIST) Acoustic standards are essential in a lot of fields such as industrial, scientific and technological fields with wide-ranging end-users. Please tell us the international and societal role NMIJ/AIST should fulfill in the development of acoustic standards considering the technical trend.Answer (Ryuzo Horiuchi) As described in the main text, measurement of the noise from household appliances or information equipment has become important lately. Acoustic power level is mainly used to evaluate these noise sources instead of sound pressure level or sound level. Although traditional sound level intuitively gives noisy circumstances at the measurement points, acoustic power level can give total acoustic output generated by the noise source.To ensure the reliable measurement of the acoustic power level, absolute calibration technique on a “reference sound source” should be established. It is a sound source specially designed for the precise measurement of the acoustic power level and it constantly generates noise with a wide bandwidth. The acoustic power level of the test sound source can be calibrated in comparison with the reference sound source whose acoustic power level is pre-determined. To secure the reliable measurement results for end-users, NMIJ/AIST is planning to develop precise calibration technique of reference sound sources and establish practical standards of the acoustic power level.5 Problem requiring most time to solveQuestion (Katsuhisa Kudo) Please tell us the most difficult problem you have faced and the measures taken to solve the problem, ranging from the development of acoustic standards to the realization of the calibration service.Answer (Ryuzo Horiuchi) The most difficult technical problem in the development of acoustic standards was to discover the cause of instability in the pressure sensitivity of laboratory standard microphones. As described in the main text, the primary calibration system of laboratory standard microphones was advanced and uncertainty related to the electric circuit of the calibration system was minimized to the limit at present. After the improvement of the system, I got suspicious about the stability of the microphone sensitivity and studied the cause of the instability. In other words, the instability of the microphone sensitivity could not be observed until the system was advanced.Possible causes of instability were physical distortion of the microphone described in the main text, sensitivity dependence on environmental conditions such as temperature and static pressure, poor insulation of the microphone, application of bias voltage necessary for the workings of the microphone, physical force applied to the microphone by its connection to the pre-amplifier. However, causes except the physical distortion of the microphone could not explain the measurement results.The unstable phenomenon could not be observed by the simple repetition of the measurement. Usually the measured value has very small deviation but at a certain time it suddenly changes to an unexpected value. Quite a long time was spent to obtain a set of data under some measurement condition to confirm the stability. Besides the repetition of the measurement, measurement conditions were changed by trial and error. It took three or four years to come to the conclusion.from the reference value. It seems that, seen from another angle, Asia and Pacific institutes have smaller deviation from the average while some of the main institutes have larger deviation. Does this imply that calibration technique for acoustic standards became mature and it was transferred to the developing countries, resulting in technical equivalence among the countries? I would appreciate it if you could give me your view.Furthermore, what will be necessary in the future for Japan to surpass the other countries from the point of view of reliability in acoustic measurement? Do you have any suggestions to the middle class of the traceability system (calibration service providers), to the lower class (end-users who conduct measurement at the site), or to the manufacturers of acoustic measuring instruments?Answer (Ryuzo Horiuchi) As the reviewer pointed out, the results of the international comparison (Fig. 8) show that there is little difference between the developing institutes in Asia and the leading institutes which have developed acoustic standards. This is due to the following reasons peculiar to acoustic standards. Many of the institutes in the world use the same type of primary calibration systems for laboratory standard microphones produced by one manufacturer of acoustic instruments. Calibration results can be reproduced once the operator becomes proficient in the calibration procedure. Technical information necessary for uncertainty analysis can be got without much difficulty. Therefore, even the institute with limited experience in microphone calibration can realize acoustic standards equivalent to those developed by the leading institutes. It is natural that calibration results obtained by using the same type of primary calibration systems have little deviation in the international comparison.Just five institutes in the world (only NMIJ/AIST in Asia) have the advanced technology to develop primary calibration systems. The system developed by each leading institute has electrical or mechanical elements slightly different from other institutes. International comparison is the only way to validate their equivalence to the other institutes. It is concluded under the present technical situation that the international comparison shows good agreement among the institutes irrespective of calibration system specifications. Study of reasons for the remaining deviation would be necessary to improve the reliability of acoustic standards.On the other hand, NMIJ/AIST is requested to take the technical leadership such as in the development of a new acoustic standard to surpass the other institutes from the point of view of reliable acoustic measurement. As described in the main text, our present theme is to expand the frequency range of acoustic standards. NMIJ/AIST is developing technical basis to sustain our daily life without any health damages caused by airborne ultrasound or infrasound.At the same time, NMIJ/AIST should publish the research results by further study of measurement reliability from various viewpoints. In the international comparison, common calibration principles and traveling standards are adopted to validate the equivalence among the results of participants. Further consideration would be necessary to confirm the consistency among the results obtained by different calibration principles or by different types of acoustic standards. For example, influence of microphone types (LS1P or LS2aP) on the measurement results is not well evaluated yet.Finally, as a suggestion to the calibration service providers, manufacturers and end-users of acoustic measuring instruments, I would like to appeal to them of certain necessary points to ensure the reliability of their measurement results. In addition to ensuring the traceability to national standards, uncertainty analysis on various components inherent in the measurement methods should be considered. I think the evaluation on the influence of indirect sound is a good example.

※このページを正しく表示するにはFlashPlayer9以上が必要です