The Transmission of Forced Waves at a Junction

A finite size plate excited by airborne sound has both forced and freely propagating bending waves. The forced waves are generated by the airborne sound excitation and the freely propagating waves are generated when the forced waves are reflected at a discontinuity such as the edges of the plate, studs or stringers. Unless the plate is very heavily damped, the transverse vibration of the plate is dominated by the freely propagating waves. However below the critical frequency of the plate, the direct airborne sound transmission of the plate is dominated by the forced waves because their radiation efficiency is very much greater than that of the freely propagating waves. The prediction of airborne flanking sound transmission requires the prediction of the transmission of the bending waves at the junction of plates. This paper shows that this transmission is different for forced and freely propagating bending waves below the critical frequency of the plate from which the bending waves are incident upon the junction. It is shown that it is not possible to separately calculate the intensity of a forced incidence wave and its freely propagating reflected wave because the cross terms do not vanish when the incident wave is forced.