General The three fundamental wave types that can propagate in plates are bending@ shear and longitudinal waves. A wave type that is incident on a junction can be transmitted and reflected as all three types as illustrated in Sketch 2.1. This wave propagation in structures is sometimes referred to as structure-borne sound (Derivation 3). This Item permits estimation of the ratio of transmitted to incident wave power@ that is@ the transmission efficiencies of an arbitrary junction of between two and six coupled thin isotropic plates. The plates can either be coupled directly as in Sketch 2.1@ or via a beam element as shown in Sketch 2.2. There are two main applications of this Item. Firstly@ it allows the evaluation@ of the attenuation expressed as the transmission loss@ Rijpr@ of vibrational power through different plate junctions@ and the determination of the percentage of the incident wave power that is being transmitted as bending@ shear and longitudinal wave power. Secondly@ the Data Item allows the calculation of diffuse field transmission efficiencies in 1/3-octave bands@ octave bands or at a user defined constant bandwidth. These can be used to calculate coupling loss factors for SEA (Statistical Energy Analysis) models. Transmission efficiencies of coupled plates are functions of the angle of incidence@ ?? If the movement of the junction is not restricted@ as is assumed in this Item@ then bending waves and at least one of the other wave types (longitudinal or shear in the plates or torsion or longitudinal in the beam element) will normally be present@ and then the transmission efficiencies will also be functions of frequency. The transmission efficiencies for coupled plates have also some application for coupled curved plates and for coupled beams. Above the ring frequency@ a curved plate behaves with good accuracy as a flat plate. Transmission efficiencies for flat plates can therefore be used to describe a similar coupling between curved plates for frequencies above their ring frequencies. For coupled beams@ the transmission efficiencies are identical to those of a similar plate system at normal incidence (??0).