自由壁半楕円音響管内の音波伝搬

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In the Free-walled acoustic waveguides with semi-circular cross-section, the ratio of cut-off frequencies of lowest normal mode and next normal mode is given by 5. 14 : 3. 83=1. 34. The frequency interval between the cut-offs being the available range for usual measurement, theoretical analysis was performed on the acoustic characteristics of the free-walled waveguides with semi-elliptical cross-section with the aim of expanding this frequency interval. The normal modes of semi-ellipse cut along minor and major axes correspond to φ^e_11 and φ^o_11 respectively (cf. Fig. 10), and the 2nd normal modes for the semi-ellipse of both types are common and are given by φ^o_21. Experiments were performed on the semi-elliptical waveguide made of foam-polystyrol (cf. Fig. 12) and on the approximately semi-elliptical waveguide made of freely suspended vinyl sheet. The free-wall condition had been sufficiently satisfied by the foam-polystyrol as was evidenced from the coincidence of theoretical and experimental results-phase velocity -for the semi-circular waveguide (cf. Fig. 14). Fig. 15 is an example of the measurement for the semi-ellipse with eccentricity 0. 9. For the semi-ellipse cut along minor axis, φ^o_21 increases and φ^e_11 decreases as compared with the semi-circular waveguides of equal sectional area (Fig. 19), so that the frequency interval between cut-offs widens (see also Fig. 21), while for the semi-ellipse cut along major axis, the increase of φ^o_11 exceeds that of φ^o_12, thus it results in the decrease of the frequency interval between cut-offs (see also Fig. 21). By the way, the range of frequency measurement can be extended as shown in Fig. 24 by working out so as to suppress the second φ^o_21 mode by adjusting the position of a hydrophone on the axis of symmetry (nodal line of φ^o_21). In this figure, the ordinate Ф=cos^-1 c/c_p, where c is the free-medium velocity of sound, and c_p is axial phase velocity.
社団法人日本音響学会の論文
1974-04-01

自由壁半楕円音響管内の音波伝搬

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