Giant Acoustic Concentration by Extraordinary Transmission in Zero-Mass Metamaterials

We demonstrate 97%, 89%, and 76% transmission of sound amplitude in air through walls perforated with subwavelength holes of areal coverage fractions 0.10, 0.03, and 0.01, respectively, producing 94-, 950-, and 5700-fold intensity enhancements therein. This remarkable level of extraordinary acoustic transmission is achieved with thin tensioned circular membranes, making the mass of the air in the holes effectively vanish. Imaging the pressure field confirms incident-angle independent transmission, thus realizing a bona fide invisible wall. Applications include high-resolution acoustic sensing.

Figure 1

(a) Setup. A probe microphone near the tube walls probes the pressure. (b) Transmission intensity coefficient $|T{|}^2$. (c) Reflection intensity coefficient $|R{|}^2$.

Figure 2

(a) Setup (top view). (b) Images (interpolated from 2D arrays of points on square lattices—32 points along the duct axis) of the instantaneous pressure distribution at 1.2 kHz for no wall, a perforated wall with bare holes, and a wall with otherwise identical membrane-covered holes. (c) Images for the case of a perforated wall for acoustic incidence at $\theta =30°$. (d) Equivalent images for $\theta =60°$.

Figure 3

(a) Pressure distributions from a point source with no wall in the middle. (b) Corresponding image when a perforated wall with membranes is installed. Data were collected in the same way as for Fig. 2, here on a square lattice of points of separation 40 mm.