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Acoustic Cloaking
Figure 5. The pyramidal acoustic carpet cloak of Zigoneanu et al. (2014). Reprinted with permission.
Design and Realization of TA Devices
We follow the progression of Figure 3, starting with carpet cloaking. Popa et al. (2011) demonstrated the first acoustic carpet cloak in air using a 2D inertial design. Anisotropic density was realized with thin, heavy (relative to air) plates perforated to allow the air to permeate the plates stacked with air gaps between them, giving a mass density ratio of about 5 to 1 in orthogonal directions. Scanned microphone measurements showed good cloaking for incident waves of center frequency 3 kHz with a 3 dB bandwidth of 1 kHz. The broadband nature of the device can be ascribed to the long wavelength, 10 cm at 3 kHz in air, compared with the lattice constant for the perforations (5 mm) and the plate spacing, yielding good effective medium properties.
Zigoneanu et al. (2014) fabricated a fully 3-dimensional om- nidirectional carpet cloak based on the same design prin- ciples. A pyramidal structure (Figure 5) rendered a region of space three wavelengths in diameter invisible to sound. Ex- periments were performed using a Gaussian pulse of 600μs half-amplitude duration modulated with a 3 kHz sinusoid. The measured response is shown in Figure 6.
Figure 6. (a) The setup for the 3D carpet cloak of Figure 5. (b) Mea- sured reflected pressure fields for three cases: ground only, object, object+cloak. Reprinted from (Zigoneanu et al., 2014) with permission. (Movie version https://www.youtube.com/watch?v=k13L8u2tACY)
Kan et al. (2013) presented an experimental demonstration of an acoustic cloak designed to hide an object in a cor- ner. The device used thin slabs in air separated to provide a mass anisotropy ratio greater than 6 at 1700 Hz operat- ing frequency. This design could, in principle, be adapted to manipulate the acoustic field near boundaries of arbitrary curved geometry.
Zhang et al. (2011) gave the first demonstration of cylindri- cal cloaking in water at ultrasonic frequencies. The design is unique in that it uses a 2-dimensional network of 1D chan- nels in the radial and circumferential directions, where each channel is an acoustic circuit of TA-defined lumped param- eters.
42 | Acoustics Today | Winter 2015