Sound radiation from an unbaffled elastic plate strip of infinite length

The noise which is caused by vibration of steel plate girders of a motorway is studied. This type of noise is often referred to as elevated structure noise, and has its main component within the frequency range 500 Hz–1 kHz. Since a steel plate girder can be considered as a rectangular plate with an extreme aspect ratio, the sound field radiated by a baffled elastic plate strip of infinite length, which is the simplest model of a steel plate girder, was theoretically analysed in the preceding paper (Sakagami, K., Michishita, K., Morimoto, M., and Kitamura, Y. Sound radiation from a baffled elastic plate strip of infinite length with various concentrated exitation forces. Appl Acoust 55: 181–202). However, a real motorway construction is more appropriately modelled by an unbaffled strip. Thus the unbaffled condition has been analysed numerically in this paper. The method of equivalent sources is used. The main features of the radiated sound field are discussed in the light of numerical examples, and compared with those of a baffled strip. Two vibration patterns, i.e. an idealised piston vibration and flexural vibration excited by a moment are considered. Furthermore, in order to determine the far-field range of an infinitely long plate, the variation of the radiated sound field with the distance is discussed.