音波の振幅測定による変位の測定法

プローブ形状の影響

抄録

The acoustic resonance frequency of an air column in a tube or duct is altered by the presence of an object facing to one of its openings. Alternatively, acoustic pressure inside responding to constant excitation should change according to the distance, thus can be a measure of the displacement of that object. Investigations reported by the author on this subject are further extended. For both two-dimensional and axially symmetric configurations, effects of the sensor geometry, specifically of the flunge-width at the open end of the sensor tube/duct on the sound pressure-displacement characteristics is investigated experimentally. Simple calculation scheme developed previously gives fairly good agreement with the experiment, although not complete in details. Good linearity of the displacement-acoustic pressure relation is obtained for rather restricted range of frequency. The width of that frequency range is found to be only weakly dependent on the flunge width, while calculation predicts rather clear dependence. The useful range of the displacement is clearly dependent on the flunge width. The wider flunge gives the wider measurement range. The acoustic pressure is more sensitive to small change in the excitation frequency for narrower flunge. Such sensitivity is not desirable since it leads to erroneous response by un-intended change in, or error in setting of, the excitation frequency. It is concluded that as far as situation allows it, use of wider flunge geometry is preferable.