Broadband metamaterial for nonresonant matching of acoustic waves

Abstract

Unity transmittance at an interface between bulk media is quite common for polarized electromagnetic waves incident at the Brewster angle, but it is rarely observed for sound waves at any angle of incidence. In the following, we theoretically and experimentally demonstrate an acoustic metamaterial possessing a Brewster-like angle that is completely transparent to sound waves over an ultra-broadband frequency range with >100% bandwidth. The metamaterial, consisting of a hard metal with subwavelength apertures, provides a surface impedance matching mechanism that can be arbitrarily tailored to specific media. The nonresonant nature of the impedance matching effectively decouples the front and back surfaces of the metamaterial allowing one to independently tailor the acoustic impedance at each interface. On the contrary, traditional methods for acoustic impedance matching, for example in medical imaging, rely on resonant tunneling through a thin antireflection layer, which is inherently narrowband and angle specific.

Enhanced acoustic transmission through a slanted grating

“…The successful demonstration of EOT therefore raised the question of whether the same subwavelength effect can be achieved in acoustics. D'Aguanno et al [2] answered the question in the affirmative, demonstrating theoretically and experimentally an acoustic grating that is completely transparent to sound waves. The Brewster-like effect was explained via surface impedance matching between the exterior air and the effectively rigid grating.…”

“…The grating displays effective long-wavelength properties easily estimated for rigid grating elements, which allows tuning the grating porosity to achieve the desired intromission angle [2]. Maurel et al [1] also provide a clear explanation of the phenomenon as impedance matching but in the context of acoustics of fluids with anisotropic inertia.…”

“…The mechanism behind EAT is, as noted by D'Aguanno et al [2], impedance matching. The grating displays effective long-wavelength properties easily estimated for rigid grating elements, which allows tuning the grating porosity to achieve the desired intromission angle [2].…”

“…D'Aguanno et al [2] showed that such a "single layer grating" (SLG) with rigid filling fraction f exhibits total transmission for an acoustic plane wave incident at intromission angle θ i satisfying cos θ i = 1 − f . The angle is defined relative to the slab normal.…”

Geometrical isotropy in perforated plates with subwavelength holes decorated with Archimedean patterns

“…The investigations on acoustic transmission through subwavelength apertures restricted to plates perforated with periodic arrays of apertures showed that the geometrical parameters of the hole array are important in the ultrasonic transmission through perforated plates [10][11][12]. Recently, many phenomena regarding the extraordinary acoustic transmission through subwavelength hole arrays have been demonstrated [13][14][15][16][17] The acoustic transmission through a plate perforated with a periodic array (or semiregular tilings) of subwavelength apertures [18] showed strong resonant transmissions that were attributed to the coherent diffraction by the long-range order of the quasiperiodic structure. It was also studied on a plate with quasiperiodic surface corrugations [19].…”