Vol.2 No.4 2010

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Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−259−Synthesiology - English edition Vol.2 No.4 (2010) The Japanese Measurement Law has regulated “specific measuring instruments” essential for business dealings or certification. To realize the reliable measurement, the law provides items to be tested for structure and measurement accuracy, and the corresponding criteria to be reached. The sound level meter is designated as one of the specific measuring instruments and has been tested. At present, about fifty thousand sound level meters have passed the testing and they are used for official noise measurement.In the testing procedure of the sound level meter, the sound pressure applied to the microphone, namely the sound pressure at the tip of the sound level meter is precisely measured to evaluate the indication of the sound level. A laboratory standard microphone is used to determine this sound pressure. As shown in Fig. 2, it is a special measurement microphone superior to the others in stability[7,8]. In other words, sensitivity of the laboratory standard microphone as a sensor of the sound pressure, namely the ratio of the output voltage to the input sound pressure, is adopted as the national standard in acoustic measurement (the acoustic standard). The laboratory standard microphone used as a reference to test the sound level meter is named the reference standard for the sound level. NMIJ/AIST has calibrated a lot of reference standards for the sound level for nearly half a century including the time of the former Agency of Industrial Science and Technology /Electrotechnical Laboratory[9,10].The Japanese testing system has allowed only sound level meters which passed the testing to be used for business dealings and certification, and this will be continued into the future. However, the testing does not cover the concept of uncertainty which has been internationally expressed in recent years. Uncertainty of calibration results is not expressed in the testing reports of the reference standards for the sound level. In the testing procedure of the sound level meter as a specific measuring instrument, it is judged to fulfill the criterion only by comparing its indication of the sound level with the value determined by the reference standard for the sound level and determining that it is within the normal tolerance.Uncertainty is now introduced into IEC and ISO international standards and JIS, which prescribe specification and calibration methods of the measuring instruments. From the scientific point of view, any measurement result essentially has some extent of uncertainty. Uncertainty of the measurement results should be properly stated to ensure objective reliance. Moreover, World Trade Organization /Technical Barriers to Trade (WTO/TBT) arrangement was made among the countries including Japan in 1995. Each country took similar responsibility to ensure the conformity to the international standards of measuring instruments in its country. Each country was required to develop the domestic traceability systemTerm 2 of measuring instruments to the national standard and to verify technical equivalence of national standards among the countries.Similarly in the acoustic area, uncertainty evaluation of acoustic measuring instruments became essential to verify their conformity to the corresponding standards[11]. Establishment of a measurement traceability system (Fig. 3) was indispensable to verify that end-users’ acoustic measuring instruments fulfill the standards.In addition to the environmental noise measurement, measurement of the noise from household appliances (refrigerator, washing machine, vacuum cleaner, etc.) and information equipment (copying machine, personal computer, printer, etc.) has recently become important. Measurement microphones are used to evaluate the noise from these kinds of equipment[12,13].Manufacturers of household appliances and information equipment cannot obtain reliable data on the acoustic Fig. 1 Measurement microphone (left) and sound level meter (right).Measurement microphone is a sensor of sound pressure and generates the output voltage proportional to the sound pressure applied to the diaphragm (circular surface in the left figure). Type WS1 and WS2 microphones are available for the audible frequency range (20 Hz to 20 kHz). They are different in size and in suitable frequency range. The larger (left) is type WS1.Sound level meter senses sound pressure by the measurement microphone at the tip (left in the right figure) and calculates sound pressure level or sound level. Sound level meter consists of microphone, amplifier, frequency weighting circuit, calculation circuit and indicator.

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