On the Sound Transmission through a Truncated Conical Shell and its Coupling to a Cylindrical Shell

摘要

The present paper deals with the vibroacoustic behaviour of a truncated isotropic conical shell either rigidly backed or coupled to a stiffened cylindrical shell for the purpose of understanding airborne sound transmission through aircraft cockpit. To understand sound transmission mechanisms through a truncated cone and quantify the influence of its coupling to a cylinder on them, experimentation has been carried out on three cases : (a) a rigidly backed truncated cone, (b) an assembly of a truncated cone and a stiffened cylinder, (c) a stiffened cylinder. The lower and upper ring frequencies of the cone are fR2=1800Hz and fR1=4500Hz and its critical frequency is fc=12000Hz. fR2 and fRl correspond to the ring frequencies of cylindrical shells with radius equal to the larger and the smaller radii of the cone. The Noise Reduction (NR) of the truncated cone for cases a and b, and the NR of the cylinder for case c, have been measured under diffuse sound field conditions in the 100Hz-10kHz range. The two major results are the superposition of the NR curves of the truncated cone for cases a and b above 0.56~*fR2 and the similitude of the transmission behaviour of the truncated cone to that of a cylinder.