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The boundary element solution procedure for exterior periodic acoustic problems fails at frequencies associated with the eigenfrequencies of the corresponding interior problems. A new technique is developed to overcome this problem in the indirect boundary element method by expanding the integral equations through the application of multi-valued impedance boundary conditions. The effectiveness of this newly developed UNequal Impedance technique for Qualitative Evaluation of acoustic response (UNIQUE) is demonstrated by applying this procedure for the solution a series of exterior acoustic problems.

A technique is developed which utilizes numerical models and in situ measurements to characterize acoustic fields and identify noise sources. The numerical model is based on boundary element numerical procedures and is used to characterize the acoustic cavity. Passive boundary conditions in the form of impedance boundary conditions are used for the numerical model. The sound field is found by measuring several pressures in the acoustic field and using the data to overdetermine the numerical problem and solve for the complete sound field. The model can then be used to identify acoustic energy paths in the field. The model can also be used to evaluate design alternatives. The method has been tested analytical and verified. In addition, the sensitivity of the method to error in the impedance data or measurement data was tested and found to be acceptable.