Localised sound optimisation in small enclosures

In this work an application was developed which uses the Sequential Quadratic Pro- gramming (SQP) optimisation algorithm to optimise boundary absorption in order to reduce the sound pressure level in a localised sampling region in a small enclosure over a low frequency range. Three different boundary absorber models were used; an unconstrained complex impedance, a porous absorber and a composite absorber. Absorbers were applied either as panels or using smaller area patches. Experiments were performed to test the methodology on a simplified computationally conservative rectangular enclosure, specially designed to be computationally efficient, and on a more realistic and computationally intensive train driver's cabin. The methodology was shown to be successful since the sound pressure level in the localised sampling region was reduced significantly on average in all cases tested. The sound pressure was not only reduced from the initial random designs used to initiate the optimi- sation but also from the rigid and uniform absorption backgrounds. Furthermore the global sound pressure level evaluated over the entire enclosure was also reduced, a desirable side effect of the local optimisation. Remarkably, patch applications of absorber were shown to be very successful at reducing the local sound pressure level despite their small area (i.e. approximately one sixth the size of a panel). Even using just one or two patches could have a significant effect. In general, the results validate the use of optimisation for acoustic design of small enclosures, and since re-alistic boundary absorption was designed and placed in a realistic train drivers cabin to reduce the real problem of lowering sound level for the train driver, optimisation has been demonstrated to be a practical approach for real world problems.