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Sound Isolation and Giant Linear Nonreciprocity in a Compact Acoustic Circulator

Science  31 Jan 2014:
Vol. 343, Issue 6170, pp. 516-519
DOI: 10.1126/science.1246957

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Acoustically Isolated

The control of sound transmission is desirable in a number of circumstances from noise suppression to imaging technologies. Fleury et al. (p. 516; see the cover; see the Perspective by Cummer) studied a subwavelength acoustic meta-atom consisting of a resonant ring cavity biased by an internally circulating fluid. The direction of rotational flow of the fluid (air) changed the resonant properties of the ring cavity, allowing the propagation of sound waves within the cavity to be controlled. With several ports connected to the cavity, sound could be directed to a certain port while isolating transmission in another.

Abstract

Acoustic isolation and nonreciprocal sound transmission are highly desirable in many practical scenarios. They may be realized with nonlinear or magneto-acoustic effects, but only at the price of high power levels and impractically large volumes. In contrast, nonreciprocal electromagnetic propagation is commonly achieved based on the Zeeman effect, or modal splitting in ferromagnetic atoms induced by a magnetic bias. Here, we introduce the acoustic analog of this phenomenon in a subwavelength meta-atom consisting of a resonant ring cavity biased by a circulating fluid. The resulting angular momentum bias splits the ring’s azimuthal resonant modes, producing giant acoustic nonreciprocity in a compact device. We applied this concept to build a linear, magnetic-free circulator for airborne sound waves, observing up to 40-decibel nonreciprocal isolation at audible frequencies.

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