Science 343, 516–519 (2014)

A diode allows flow in one direction but not the other. Electrical circuits frequently use this concept, but it could also find numerous applications in acoustics — reducing noise in ultrasound imaging, for example. Romain Fleury and his colleagues have now designed and fabricated just such a compact acoustic diode that operates at audible frequencies.

Their device is a sealed rectangular toroid with a major radius of approximately 70 mm. This dimension is much smaller than the wavelength of the audible sound waves that enter the resonator through a waveguide. If the frequency of the incoming wave is equal to the resonant mode of the cavity, the wave can travel both clockwise and anticlockwise around the device before leaving at one of two exit waveguides, which are positioned 120° around from the input.

Fleury and the team used electric fans to circulate air around the device, splitting the resonant frequency of the clockwise and anticlockwise modes. Thus, the input sound wave can only travel one way around the torus — it is heard at one exit but not the other.