Journal of Sound and Vibration (1986) 104(1), 127-136 MODE COUPLING AND ENERGY PARTITION OF SOUND IN A SYSTEM OF PLATE JUNCTIONS B. M. GIBBs Department of Building Engineering, University of Liverpool, Liverpool L69 3BX, England (Received 26 November 1984, and in revisedform 7 February 1985) A description is given of a method of analysis of sound waves generated at a series of T-junctions of thin plates as a result of a wave incident on one of the plates. Mode coupling and energy partition are calculated for longitudinal, transverse shear and bending waves obliquely incident and it is seen that the transmission coefficient is strongly dependent on angle. Thus normal incidence characteristics are not a good indication of wave transmission in the configurations considered. The description is extended to include the ettect of internal losses and the case is included where non-coupled edges parallel to the junctions are simply supported. The approach exploits the virtues of Algol 68 programming language and can be easily modified to describe any plate system where the junctions are parallel. 1. INTRODUCTION In structure-borne sound propagation there is, in addition to dissipative losses, attenuation as a result of reflections at discontinuities such as at junctions of structural elements. The mechanism of bending wave transmission at such junctions is generally well understood and has led to analyses of complex systems of coupled plates such as in the case of large buildings [1] and ship structures [2]. The role of mode coupling in the transfer of vibrational power in such studies is less well understood. In a system of connected thin plates the bending vibration is accompanied by in-plane vibration fields composed of compressional and transverse shear waves. The assumption is often made that the bending mode is the main component of energy transfer and previous analyses in which this vibration only is considered have yielded fair agreement with measurement [3]. It has also been shown by the use of Statistical Energy Analysis (S.E.A.) that consideration of other modes of vibration yields calculated vibration levels which differ by less than 2 dB from those obtained when using simpler theory in which bending vibration only is considered [4]. The case studied was that of adjacent rooms but the same has been observed for large building structures where the source and receiver positions are several rooms or structural elements removed [1]. The net effect may therefore seem little altered by consideration of bending vibration only but there is still a requirement for study of mode coupling in detail. The sound transmission through a structure may be reduced by correct location of dissipative damping for example. The damping which may have been positioned in order to reduce bending vibration in a structural element will be less effective if the important mode of vibration in that element is in-plane. It has also been shown that once sound is converted into in-plane vibration it can travel appreciable distances, through several intervening junctions, with little attenuation before converting to bending vibration [5]. Most analyses of this problem have so far involved the use of S.E.A. and 127 0022-460x/86/010137+ 10 $03.00/0 9 1986Academic Press Inc. (London) Limited