Index of content:
Volume 125, Issue 1, January 2009
- ARCHITECTURAL ACOUSTICS 
125(2009); http://dx.doi.org/10.1121/1.3026328View Description Hide Description
The performance of hybrid passive-active absorbers at normal incidence has been previously demonstrated. However, their feasibility in realistic environments remains to be confirmed. This paper proposes a hybrid passive-active liner absorbing at oblique incidence. The passive absorber is a microperforated panel backed by a 5 cm deep air layer and a rigid ending. An active multichannel cell, composed of five single-input single-output controllers, releases the sound pressure at the entrance of the air cavity in a reduced area of the microperforated panel absorber. Hybrid passive/active absorption of the prototype is measured in an anechoic room up to a maximum incidence angle of . For instance, at such a system is able to provide an average absorption of 94% between 266 and 1500 Hz. In addition, the performance that would provide such a hybrid absorber in a reverberant room has been quantified theoretically by an index describing the active-to-passive reverberation time ratio in terms of the active-to-passive absorption area and absorption coefficient ratios. The proposed prototype, implementing 7% of the active proportion area, would afford a reverberation time reduction between 77.5% and 12% from 100 to 500 Hz.
A broadband fast multipole accelerated boundary element method for the three dimensional Helmholtz equation125(2009); http://dx.doi.org/10.1121/1.3021297View Description Hide Description
The development of a fast multipole method (FMM) accelerated iterative solution of the boundary element method (BEM) for the Helmholtz equations in three dimensions is described. The FMM for the Helmholtz equation is significantly different for problems with low and high (where is the wavenumber and the domain size), and for large problems the method must be switched between levels of the hierarchy. The BEM requires several approximate computations (numerical quadrature, approximations of the boundary shapes using elements), and these errors must be balanced against approximations introduced by the FMM and the convergence criterion for iterative solution. These different errors must all be chosen in a way that, on the one hand, excess work is not done and, on the other, that the error achieved by the overall computation is acceptable. Details of translation operators for low and high , choice of representations, and BEM quadrature schemes, all consistent with these approximations, are described. A novel preconditioner using a low accuracy FMM accelerated solver as a right preconditioner is also described. Results of the developed solvers for large boundary value problems with are presented and shown to perform close to theoretical expectations.