Experimental study of sound propagation in a flexible duct

Propagation of sound in a flexible duct is investigated both theoretically and experimentally. Strong np coupling of sound and flexural waves on the d uct wall is found when the wall-to-air mass ratio is of the order of unity. The axial phase speed of sound approaches the in vacuo speed of flexural w aves (subsonic in this case) at low frequencies. However, a speed higher th an the isentropic sound speed in free space (340 m/s) is found beyond a cri tical frequency which is a function of the mass ratio. Experiments using a duct with a finite section of tensioned membrane are compared with the prop agating modes pertaining to the infinite membrane model. Satisfactory quant itative agreement is obtained and the measured phase speed ranges from 8.3 to 1348 m/s. In the moderate frequency range, the theory predicts high spat ial damping rate for the subsonic waves, which is consistent with the exper imental observation that subsonic waves become increasingly undetectable as the frequency increases. Substantial sound reflection is observed at the i nterface between the rigid and the flexible segments of the duct without cr oss-section discontinuity, which, together with the high spatial damping, c ould form a basis for passive control of low-frequency duct noise. (C) 2000 Acoustical Society of America. [S0001-4966(00)03308-7].