Acoustic metamaterials comprised of dead-end pores and black hole effect for low frequency sound absorption in linear and nonlinear regimes

Brooke, DC 2021, Acoustic metamaterials comprised of dead-end pores and black hole effect for low frequency sound absorption in linear and nonlinear regimes , PhD thesis, University of Salford.

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The aim of this work is to design and test acoustic metamaterial absorbers particularly for mitigation of low frequency sound of high intensity. The absorbers designed are formed of a series of plates with a central perforation, separated by air cavities. Two types of structures are investigated: the first has a central perforation with a constant radius (pancake structure), while in the second the pore radius gradually decreases along the thickness of the absorber (profile structures). In the structures of the first type, the wave speed reduction is abrupt, while in the second a gradual impedance matching with air is achieved. The structures developed are tested in a range of various experimental set-ups. This includes performing measurements in a conventional impedance tube (linear regime), in a specially modified impedance tube that allows pressures (RMS) up to 250 Pa using sine wave excitation (weakly nonlinear regime for continuous sound) and in a shock tube (nonlinear regime for pulsed sound of amplitude of up to 100 kPa). The models developed allow the predictions of the metamaterial structure performance at low and high sound pressure levels. In order to test the models, the absorbers of various dimensions are built, tested and the results of the measurements are compared with the model predictions. The analytical model for the pancake absorber is used to derive simple formulae for the frequency and the peak value of the absorption coefficient at the lowest frequency resonance in the linear regime, depending on the geometrical parameters. This model is complemented by a Transfer Matrix Model (TMM) and Finite Element Model (FEM) for both pancake and profile structures. The latter accounts for the influence of the structural resonances and tortuosity effect of the plates on the absorber performance. To investigate the nonlinear regime, flow resistivity measurements are performed on the samples to directly measure Forchheimer’s nonlinearity parameter. Flow resistivity measurements at low flow rates show that the periodic set of cavities does not modify resistivity significantly when compared to a simple perforated cylinder with same thickness. As flow rate increases, the flow resistivity grows linearly according to Forchheimer’s law and has a significant dependence on the absorber thickness. A nonlinear numerical model is developed accounting for the growth of flow resistivity with particle velocity amplitude in the central perforation and compared with the measurements at high amplitudes of the continuous incident sound. It is confirmed by measurements, that the peak absorption coefficient values for both types of absorbers decrease as the sound amplitude grows (irrespective of dimensions of pore radius and value of open surface area ratio). Where the peak values of absorption coefficient for the pancake absorbers are shown to be significantly reduced compared to the profile structures as amplitude strength grows to nonlinear regime. The resonance frequencies, however, remain close to their measured values independent of amplitude strength and is advantageous for both structures. Measurements in a shock tube are performed in both rigid backing and transmission set-ups, in time domain. Fast Fourier Transform (FFT) is later performed to investigate the signals. It is demonstrated that the profile absorber design is advantageous for the absorption of high amplitude pulsed sound.

Item Type: Thesis (PhD)
Contributors: Umnova, O (Supervisor) and Leclaire, P (Supervisor)
Schools: Schools > School of Computing, Science and Engineering > Salford Innovation Research Centre
Funders: Defence Science and Technology Laboratory (DSTL)
Depositing User: Daniel Brooke
Date Deposited: 05 Oct 2021 14:22
Last Modified: 05 Nov 2021 02:31

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