Acoustic transmission through compound subwavelength slit arrays

Abstract

The angular dependence of the transmission of sound in air through four types of 2D slit-arrays formed of aluminium slats is explored, both experimentally and numerically. For a simple, subwavelength periodic slit-array, it is well known that Fabry-Perot-like wave-guide resonances, supported by the slit-cavities, hybridising with bound acoustic surface waves, result in 'Enhanced Acoustic Transmission' at frequencies determined by the length, width and separation of each slit-cavity. We demonstrate that altering the spacing or width of some of the slits to form a compound array (i.e. an array having a basis comprised of more than one slit) results in sharp dips in the transmission spectra, that may have a strong angular dependence. These features correspond to 'phase resonances', which have been studied extensively in the electromagnetic case. This geometry allows for additional near-field configurations compared to the simple array, whereby the field in adjacent cavities can be out-of-phase. Several types of compound slit-array are investigated; one such structure is optimised to minimise the effect of boundary-layer loss mechanisms present in each slit cavity, thereby achieving a deep, sharp transmission minimum in a broad maximum.PACS numbers: 43.20.+g, 43.20.Mv, 43.20.Ks, 43.20.Fn The experimental discovery of Extraordinary Optical Transmission (EOT) through subwavelength hole arrays[1] opened a whole new area of research into how structured resonant layers can affect the propagation of light. This research has been extended to the acoustic case, where similar behaviour is observed, sometimes termed Enhanced Acoustic Transmission (EAT) (Not extraordinary, since longitudinal sound waves have no cutoff when propagating through gaps/holes in rigid bodies with sound hard walls) [2][3][4][5][6][7]. The observed phenomena for both electromagnetic and acoustic cases in such structures is due to complex interplay between surface-wave modes and wave-guide modes, the exact nature being dependent on many structural parameters [6][7][8]. Other types of transmission anomaly have been discovered in the electromagnetic case that stem from EOT. One such anomaly is the 'Phase resonance', which appears as a sharp dip in the transmission of transverse magnetic polarised light through so called 'compound grating' structures, gratings with structure factor comprised of multiple elements [9][10][11][12][13][14]. In the case of a 2D metal slit-array this can be achieved by having unequally sized slits, or multiple slits in each period. In a singularly periodic grating structure, symmetry requires that the fields in all slit-cavities are identical when excited by a normally incident planar wave. Compound gratings introduce new degrees of freedom to the near field configurations, and at specific frequencies fields in adjacent cavities may be both out-of-phase with one another and strongly enhanced [10], leading to 'phase-resonant' features in their electromagnetic response. Being simply a lattice/symmetry phenomena, there is an ex...

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