Experimental demonstration of a three-dimensional bioinspired tonotopic metasensor

Spiral structures are widely recurrent in nature to serve different purposes, including the spatial mapping of acoustic frequencies in the mammalian cochlea-a feature referred to as tonotopy. Motivated by this fundamental characteristic, we explore the elastodynamics of a three-dimensional seashell-like structure with frequency-selective capabilities and, in addition, a polarization-dependent response, a feature rarely found in nature. We experimentally demonstrate how these properties can be exploited to discriminate between out-of-plane and in-plane waves, while producing a discrete spectrum that displays tonotopic behaviour. The polarization capabilities are a consequence of the realization of a tonotopic response in the spiral plane and perpendicular to it. Results can be of interest for the design of low-power, low-latency smart sensors for structural health monitoring and non-destructive testing, where discrimination between frequency and polarization is usually accomplished through digital signal processing.