自由壁半楕円音響管内の音波伝搬
Sound Propagation in Soft-Walled Cylindrical Waveguides of Semi-Elliptical Cross-Section

Akio Hasegawa, Otohiko Nomoto, Toshiaki Kikuchi
1974 Acoustical Science and Technology  
In the Free −walled acoustic waveguides with semi − circu ! ar 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 avaiIable range for us 岨 1 皿 easurement , theoretical analysis was performed on the acoustic characteristics of the free − walled waveguides with semi − elliptical cross − sectiQn with the aim of expanding this frequency interval . The normal modes
more » ... . The normal modes of semi − ellipse cut along minor and major axes correspond to φ負 and di2 , respectively ( cf . Fig.10 ) , and the 2nd norma1 皿 odes for the semi − ellipse of both types are com 皿 on and are given by φ 窪 1 。 Experiments were performed on the semi − elliptical waveguide made of foam − polystyrol ( cf. Fig.12 ) and on the approximately sem 玉 ・ elliptical waveguide made of freely suspended vinyl sheet . The free−wall condition had been su 伍 clently satis 丘ed 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 measure − ment for the semi − ellipse with eccentricity O. 9. For the semi − ellipse cut along minor axis , φS ユ increases and φ 窒 , decreases as compared with the semi − circular waveguides of equa ! 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 φe ユ exceeds that of φ g2 , thus it results in the decrease of the frequellcy interval between cut − offs ( see also Fig.21 ) . By the way , the range of frequency 皿 easurement can be extended as shown in Fig. 24 by working out so as to supPress the second φ 31 mode by adjusting the position of a hydrophone on the axis of symmetry ( nodal line of φ 81 ) . In this 丘 gure , the ordinate φ ; cos −1 c / cp , where c is the free − medium vel ity of sound , and cp 三 s axial phase velocity ,
doi:10.20697/jasj.30.4_219 fatcat:yyrvzhi5ujhqhpzbtbn32oohka