Vol.2 No.4 2010

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Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−264−Synthesiology - English edition Vol.2 No.4 (2010) precisely by introducing the monitor microphone. Figure 6 shows a schematic of secondary calibration system of measurement microphones by the sequential method.4.3 Visualization of influence by indirect soundIndirect sound reaches the microphone through a path different from direct sound and they interfere with each other. Thus on the frequency domain, sound pressure at the microphone position has local minimums and maximums alternately. Indirect sound adds small waves to the frequency characteristics of the microphone output voltage which would be inherently smooth without indirect sound. Influence of indirect sound changes with relative positions of the loudspeaker, microphone and sound reflecting objects. Reflection from the object closest to the microphone has dominant influence.Influence of indirect sound also depends on the shape of microphone housing and even on the slight difference of positions between the reference and test microphones. Therefore, measured sensitivity ratio between the microphones has frequency characteristics as shown in the blue curve of Fig. 7. The amplitude of the waves is equal to the uncertainty caused by indirect sound and frequency of the waves corresponds to the distance between the sound reflecting object and the microphone. Frequency dependence of sensitivity ratio enabled the specification of the most influential object and taking the measures necessary to decrease indirect sound.In some cases, influence of indirect sound was decreased by averaging the sensitivity ratio in the vicinity of the measurement frequency. This method is not appropriate from the point of view of calibration results and uncertainty.4.4 Reduction of influence by indirect sound − application of sound absorbing material −The author tried to decrease the influence of indirect sound simply by covering the sound reflecting object with sound absorbing material. This method was effective if the reference and test microphones belonged to the same type but it was not sufficient for the different types.4.5 Reduction of influence by indirect sound − application of digital signal processing technique −Uncertainty caused by indirect sound can be decreased if the indirect sound is separated and removed from the direct sound on the time domain, since the indirect sound reaches the microphone later than the direct sound. However, simple application of pulse waveform with short duration as an input signal cannot give sufficient signal-to-noise ratio because the energy of the waveform is essentially distributed to the frequencies other than the measurement frequency. In this research, NMIJ/AIST developed a virtual pulse method to solve this problem[25]. This method makes use of computer simulations to determine time response which would be obtained by the application of pulse waveform. Sufficient signal-to-noise ratio can be ensured because a continuous waveform is used to measure the data necessary for simulation. The virtual pulse method could not be realized until digital signal processing by the FFT analyzer was introduced into the measurement.In the method, time response is calculated on condition that a virtual pulse signal is applied to the system with transfer function as shown in the blue curve of Fig. 7. Only direct sound is taken from the calculated pulse response waveform by applying the time window function and it is transformed into the frequency domain. As a result, smooth frequency characteristics not influenced by indirect sound can be achieved as shown in the pink curve of Fig. 7.After indirect sound was removed by digital signal processing technique, still remaining small uncertainty related to the signal processing, namely uncertainty caused by slight difference of parameters used in the signal processing were evaluated. These were such differences as the frequency bandwidth of the pulse waveform and duration and center position of the time window function to remove indirect sound.5 Establishment of traceability system on acoustic measuring instruments by JCSSAs described in chapter 1, a new calibration service system of acoustic standards, different from the traditional measurement management system based on the testing of sound level meters, was required. Basis of the new system was JCSS (Japan Calibration Service System) in the Japanese Measurement Law[26]. In the system, firstly NMIJ/AIST evaluates calibration uncertainty of laboratory standard microphones as national standards and calibrates laboratory standard microphones of calibration service providers in comparison with one of the national standards. Then calibration service providers calibrate end-users’ acoustic measuring instruments in comparison with one of their laboratory standard microphones and ensure traceability to the national standards. Special attention was paid to the following points to establish the system.・ The system allows calibration service providers to develop individual measurement management procedures. In addition to the receiving supply of national standards directly from NMIJ/AIST, they can also get acoustic measuring instruments traceable to the national standards from other high-ranking calibration service providers. Furthermore, measurement management is possible by using acoustic measuring instruments other than laboratory standard microphones as working standards for daily calibration use.

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