The ability to determine the direction of propagation of an acoustic wave is crucial to sound source detection, localization, and identification. Current techniques based on sensing acoustic pressure using membranes at multiple spatial locations tend to have directionality and output sensitivity that depend strongly on the sound frequency, which compromises performance, especially in miniaturized applications. Unlike pressure, which is a scalar, the fluctuating flow of the acoustic medium (i.e., the acoustic particle velocity) is a vector, which inherently contains the direction information of sound propagation. Here, we experimentally demonstrate an alternative, and more direct, method of directional sound detection by sensing acoustic flow using nanofibers, which are driven by viscous forces applied by the fluctuating medium. The measured output signal of this prototype sensor exhibits bidirectional response and flat frequency response across a wide range of frequencies from 100 Hz to 10000 Hz. While pressure-sensing microphones now enjoy an annual market in the billions of units sold, a broadband sound flow sensor with almost full fidelity will provide considerable benefits.
