Vol.2 No.4 2010

33/68

Research paper : How the reliable environmental noise measurement is ensured (R. Horiuchi)−263−Synthesiology - English edition Vol.2 No.4 (2010) 4 Evaluation of sound field4.1 Indirect soundSecondary calibration of acoustic measuring instruments is made in comparison with a reference standard microphone, usually within an anechoic chamber. A lot of sound absorbing wedges protrude from the inside wall of the anechoic chamber including its ceiling and bottom to minimize indirect sound. A floor necessary to carry measuring instruments into the anechoic chamber has a special structure of a wire meshed floorTerm 5 to decrease sound reflection[23]. However, actual sound field within the anechoic chamber is still influenced by indirect sound, due to incompleteness of sound absorbing wedges and the wire meshed floor. For the NMIJ/AIST’s facility shown in Fig. 5, the degree of indirect sound is approximately 1 to 2 % of the direct sound. Furthermore, structures necessary to fix the reference microphone (laboratory standard microphone with pre-determined free-field sensitivityTerm 6) or acoustic measuring instruments to be calibrated also influence sound reflection. Deviation of the sound field from an ideal situation due to indirect sound cannot be theoretically estimated and thus the corresponding uncertainty must be experimentally evaluated.4.2 Secondary calibration of acoustic measuring instrumentsDescription in the following sections of chapter 4 is focused on calibration of measurement microphones. However, a similar approach can be applied to sound level meters.Two secondary calibration methods are applicable to acoustic measuring instruments, namely sequential method and simultaneous method[24]. Both methods have the common procedures as follows. The reference microphone and the test microphone (the measurement microphone to be calibrated) are placed ahead of a loudspeaker. Ratio of the output voltages between the two microphones, namely the ratio of sensitivities is measured. Sensitivity of the test microphone is determined as the product of this ratio by the sensitivity of the reference microphone. Two methods are different in the placement of the microphones.In the sequential method, the reference microphone is replaced by the test microphone and the output voltage of the microphone is sequentially measured. This method assumes that equal sound pressure is applied to both microphones during the measurement. However, actual sound pressure fluctuates because the characteristics of the loudspeaker changes with the generation of heat and this phenomenon results in calibration uncertainty.In the simultaneous method, both microphones are placed at close positions and are exposed to the sound field simultaneously. Fluctuation of sound pressure caused by the loudspeaker’s instability does not cause a problem because the output voltage ratio of the microphones becomes stable due to the cancellation effect. Measurement time can be decreased by half compared with the sequential method. As described later, however, uncertainty related to the sound field increases because the two microphones are placed at different positions within the sound field; partly because sound pressure has a spatial distribution in the sound field and partly because the existence of one microphone disturbs the sound field to which the other microphone is exposed.A drawback of the sequential method can be solved by placing a third microphone in front of the loudspeaker to monitor the fluctuation of sound pressure and by correcting the change. In this research, the sequential method was adopted because it can evaluate the uncertainty more Fig. 6 Schematic of secondary calibration system of measurement microphones by the sequential method.A lot of sound absorbing wedges protrude from the inside wall of the anechoic chamber to minimize indirect sound. However, it is quite difficult to realize an ideal free-field even in the high-performance anechoic chamber. As will be explained later, reflection from the object closest to the test or reference microphone has dominant influence. In this measurement system, reflection is mainly caused by the upper end of vertical rod which supports the microphone (area with oblique lines in the figure).Fig. 7 Frequency characteristics of ratio of free-field sensitivities between reference and test microphones.Influence of indirect sound is visualized as vertical fluctuation in the blue curve. The pink curve has smooth frequency characteristics because this influence was minimized by using digital signal processing technique as will be explained later.Sound sourceMonitor microphone tocorrect fluctuation ofsound sourceMicrophonesupporting rodWire meshed floorSound absorbingwedgesTest or refference microphone 051015200.00.51.01.5Frequency (kHz)Ratio of free-field sensitivities (dB)

※このページを正しく表示するにはFlashPlayer9以上が必要です