Acoustic resonant surface: from nearly-total reflection to nearly-total absorption of sound
Invited paper
University of Salford
Tuesday 2 june, 2015, 09:00 - 09:20
0.7 Lisbon (47)
Abstract:
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.