We explore the slit-width dependence of the resonant transmission of sound in air through both a slit array formed of aluminum slats and a single open-ended slit cavity in an aluminum plate. Our experimental results accord well with Lord Rayleigh's theory concerning how thin viscous and thermal boundary layers at a slit's walls affect the acoustic wave across the whole slit cavity. By measuring accurately the frequencies of the Fabry-Perot-like cavity resonances, we find a significant 5% reduction in the effective speed of sound through the slits when an individual viscous boundary layer occupies only 5% of the total slit width. Importantly, this effect is true for any airborne slit cavity, with the reduction being achieved despite the slit width being on a far larger scale than an individual boundary layer's thickness. This work demonstrates that the recent prevalent loss-free treatment of narrow slit cavities within acoustic metamaterials is unrealistic.
The "acoustic double fishnet" is a structure with holes running from its front to back faces, yet at a characteristic frequency it transmits very little sound. The transmittance of this structure, which is comprised of a pair of closely spaced, periodically perforated plates, is determined experimentally and analytically. The surprising acoustic properties are due to hybridization between a two-dimensional resonance within the gap between the plates, and pipe modes within the holes. At the center of the stop band the input impedance is imaginary, interpreted as a negative product of effective bulk modulus and density.
The acoustic transmission of a closely spaced pair of patterned and perforated rigid plates is explored in air. The structure resembles an acoustic double fishnet design, with each plate modified such that the gap between them acts as an array of Helmholtz resonators. This allows the center frequency of the stop band to be reduced by a factor greater than 2 from the value obtained for the conventional acoustic double fishnet design. Experimental results accord well with the predictions of a finite element model.
The acoustic transmittance of two closely spaced solid plates, each perforated with a square array of cylindrical holes, exhibits a band of near-perfect acoustic attenuation originating from hybridization between a resonance in the gap separating the plates and pipe resonances in the holes. Displacement of one plate relative to the other, such that the holes are no longer aligned, or an increase in the plate separation leads to an increased center frequency of the stop band. This ability to easily tune the frequency of the stop band may prove advantageous.
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