Acoustic resonant surface: from nearly-total reflection to nearly-total absorption of sound

Invited paper

Logan Schwan

University of Salford

Tuesday 2 june, 2015, 09:00 - 09:20

0.7 Lisbon (47)

While surfaces with periodic profiles (such as diffusers) are used widely to control acoustic waves at higher frequencies, their application at low frequencies is hindered by the sizes required for an effective performance. Here, the concept of a tailored surface with small resonant roughness elements arranged on the plane is reported. The resonance frequency of the elements is low so that a scale separation exists between their size and the long wavelength of sound at resonance. Due to this multiple interactions between the resonators are confined within a boundary layer. Two-scale asymptotic homogenisation is applied to describe the behaviour of the resonant array in terms of an effective admittance. Its frequency dependence, inherited from the basic elements, is responsible for tunable surface conditions and the unconventional reflections. It is shown that close to resonance frequency the pressure field on the surface can be nearly cancelled leading to a phase shift in reflected wave and a memory effect. The array can also behave as an absorbing layer. Based on homogenisation, an array is designed using 2D resonators (slotted cylinders) arranged on a hard surface. It is then analysed with Multiple Scattering Theory (MST) for comparison with the predictions of the analytical homogenisation model. The basic hypothesis of a sharp scale separation is challenged by the MST and the influence of surface roughness is discussed. Finally, an experimental prototype is designed with 3D resonators. An array of 961 spherical resonators arranged on a fibreboard is built and tested. The system is excited with a series of transient and harmonic signals from a point source. Results of the measurements confirm the efficiency of the structure, as predicted by the analytical model.

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