Extraordinary transmission without resonance: Probing enhancements in acoustic transmission and energy density in hole arrays acting as a metasurface

Extraordinary transmission in wave physics, such as in optics or acoustics, is related to the theory of metamaterials in which resonances enhance wave transmission through apertures. Near-perfect acoustic transmission can, however, exist for subwavelength apertures in a thin wall without resonance, strongly contrasting to the situation in optics. We demonstrate this by experiments on airborne acoustic transmission through metal plates perforated with a variable number of circular holes at constant filling fraction $\ensuremath{\alpha}\ensuremath{\sim}0.12$ in a waveguide. Dissipative theory, including interactions between holes, shows that, at near-unity transmittance, the holes act as a low-inertance metasurface, which is verified by simulations. We also present a simple equation for the lossless, thin-wall case. Calculated enhancements in power transmission and acoustic energy density, which agree with the maximum measured values $\ensuremath{\sim}7$ and 27 for constant $\ensuremath{\alpha}$, are close to the lossless values 1/$\ensuremath{\alpha}$ and $1/(2{\ensuremath{\alpha}}²)$, respectively, and are also close to simulations, obtained at 1 kHz for one hundred 3.5 mm diameter holes in a 1 mm thick wall. Conditions for the maximum transmission enhancement in the general case are also obtained, demonstrating the counterintuitive result that the enhancement is optimal at $\ensuremath{\sim}50%$ power transmission.