Figure 5

Sound emission enhancement measurement of the double-walled metamaterial cavity.

(a) Photograph and (b) layout of the experimental set-up for measuring the emitted sound pressure outside the metamaterial cavity. To simulate a point source within the cavity, an acrylic cylinder (d = 15 mm) was inserted inside the double-walled metamaterial cavity to serve as a waveguide for the sound source located below the prototype. (c) The measured sound intensity with (P_m) and without (P_o) the double-walled meatamaterial cavity measured for θ = 90°. Background noise was found to be −10 dB, which yields a high signal-to-noise ratio for the measured values. (d) The acoustic gain calculated by the difference between P_m and P_o for θ = 90°. For the measured values with an air gap g = 20 mm, excellent agreement was found with the theoretical values with an air gap g = 13 mm. This difference is attributed to the reduced effective volume of the cavity due to the presence of the acrylic cylinder.