6 Concluding Discussion
As verification of the subroutine ABSEMGEN, the results
in figure 3 compare well with the results in [20], wherein
the radiation ratio curve for a cube shielded by another cube is given.
Figure 3 shows the occurance of near-resonances and near-antiresonances
in the acoustic system.
At the low frequencies, where the shield is much smaller than the
acoustic wavelength, the shield does not appear to have a significant effect on
the global properties of the acoustic field. The sound pressure plots
in figures 4, 5 and 6 show the
influence of the shield on the sound pressure field
near the acoustic resonant frequencies. Figures 4 and 6 show the shield has
the effect of increasing the farfield sound pressure at the acoustic
resonances and figure 5 illustrates the opposite effect at the anti-resonant
frequency.
Figure 7 shows the effect of the shield near its
first structural resonant frequency.
The coupling between the acoustic field and the structural vibration
acts to reduce the effect of the shield on the sound pressure field.
The boundary and shell element method is thus a useful generalisation
of the traditional BEM: greatly enhancing the range of applications.
The subroutine ABSEMGEN, which implements the BSEM,
is described in greater detail in reference [20]. The
subroutine clearly has a wide range of applications - its application
to the problem of predicting the effect of a shield on the noise
from an engine is considered in reference [21].