Experimental characterization of a shape optimized acoustic lens: Application to compact speakerphone design.

This work presents the shape optimization and subsequent experimental validation of an acoustic lens with application to a compact loudspeaker, such as found in commercial speakerphones. The shape optimization framework is based on a combined lumped parameter and boundary element method model using free form deformation geometry parameterization. To test the optimized design, the loudspeaker lens is three-dimensionally printed and experimentally characterized under anechoic conditions on a finite baffle with respect to its off-axis frequency response. The overall tendencies of the frequency responses agree well between measurement and simulations within the optimization frequency range and at low frequencies. The optimization process is applied to a model including acoustic lumped parameter approximations. The shortcomings of the assumptions made in the model are revealed by laser Doppler vibrometer measurements of the loudspeaker driver and modelling of the mechanical vibrations of the lens.

Hybrid analytical-numerical optimization design methodology of acoustic metamaterials for sound insulation.

Acoustic metamaterials are becoming promising solutions for many industry applications, but the gap between theory and practice is still difficult to close. This research proposes an optimization methodology of acoustic metamaterial designs for sound insulation that aims to start bridging this gap. The proposed methodology takes advantage of a hybrid analytical-numerical approach for computing the sound transmission loss of the designs efficiently. As a result, the implementation of optimization techniques on numerical model designs becomes practically possible. This is exemplified with two test cases: (i) optimization of the sound transmission loss of a single gypsum board panel and (ii) optimization of the noise reduction of outdoor HVAC units. Two resonator designs, one used previously for sound radiation in flat panel speakers and the other for enhancing the sound transmission loss at the mass-air-mass resonance of double panels, are here optimized for the two test cases. This shows how an existing resonator can be adapted for new purposes, thus making the design of acoustic metamaterials efficient. The optimized metamaterials outperform the original designs as well as traditional approaches to sound insulation.

An analytical model for broadband sound transmission loss of a finite single leaf wall using a metamaterial.

Acoustic metamaterials (AM) have emerged as an academic discipline within the last decade. When used for sound insulation, metamaterials can show high transmission loss at low frequencies, despite having low mass per unit area. This paper investigates the possibility of using AMs to increase the sound insulation of finite single leaf walls (SLWs), focusing on the coincidence effect problem. Formulas are derived using a variational technique for the forced sound transmission of finite SLWs with a coupled array of single degree of freedom resonators. An analytical model is presented for this simple case, and the effects of the band gap in sound transmission and radiation are analyzed. Moreover, the influence of each parameter is studied, especially the presence of losses, giving way to an optimized way of designing this type of structure using constrained parameter optimization. Numerical validations are performed and discussed. Finally, some conclusions are drawn regarding the effectiveness of the proposed model, including possible applications.

An axisymmetric boundary element formulation of sound wave propagation in fluids including viscous and thermal losses.

The formulation presented in this paper is based on the boundary element method (BEM) and implements Kirchhoff's decomposition into viscous, thermal, and acoustic components, which can be treated independently everywhere in the domain except on the boundaries. The acoustic variables with losses are solved using extended boundary conditions that assume (i) negligible temperature fluctuations at the boundary and (ii) normal and tangential matching of the boundary's particle velocity. The proposed model does not require constructing a special mesh for the viscous and thermal boundary layers as is the case with the existing finite element method (FEM) implementations with losses. The suitability of this approach is demonstrated using an axisymmetrical BEM and two test cases where the numerical results are compared with analytical solutions.

A resonance shift prediction based on the Boltzmann-Ehrenfest principle for cylindrical cavities with a rigid sphere.

An investigation on the resonance frequency shift for a plane-wave mode in a cylindrical cavity produced by a rigid sphere is reported in this paper. This change of the resonance frequency has been previously considered as a cause of oscillational instabilities in single-mode acoustic levitation devices. It is shown that the use of the Boltzmann-Ehrenfest principle of adiabatic invariance allows the derivation of an expression for the resonance frequency shift in a simpler and more direct way than a method based on a Green's function reported in literature. The position of the sphere can be any point along the axis of the cavity. Obtained predictions of the resonance frequency shift with the deduced equation agree quite well with numerical simulations based on the boundary element method. The results are also confirmed by experiments. The equation derived from the Boltzmann-Ehrenfest principle appears to be more general, and for large spheres, it gives a better approximation than the equation previously reported.