In order to apply the boundary element method to the rig, the surface is idealised into a simple closed surface and approximated by around 550 planar triangular elements. Generally, the positions of the elements are chosen so that their vertices are at the accelerometer points. The boundary element mesh of the CSRASYM is shown in figure 6. On each boundary element the surface vibration is specified by interpolating the values at the vertices of the components of the velocity normal to the element, derived from the accelerometer readings. At vertices where there was no accelerometer reading, the velocity is prescribed zero values. The particular BEM used was based on the integral equation of Kussmaul (1969). Further detail on the method employed is given in Kirkup (1989) and Kirkup and Henwood (1992). Figure 7 presents a comparison of the measured and computed sound power transfer function (with respect to cylinder pressure) over the range of 400Hz to 2400Hz.
In figure 7, the graphs of the sound power spectrum have peaks at 800Hz and 1120Hz. The peaks occur at these frequencies because the rig has structural resonances at 795Hz and 1121 Hz. Figure 8 illustrates the surface sound intensity with respect to cylinder pressure at 800Hz and 1120Hz. The shading scheme in figure 8 indicates the average intensity on each boundary element.