Mimicking the cochlea with an active acoustic metamaterial

Abstract

The human ear is a fascinating sensor, capable of detecting pressures over ten octaves of frequency and twelve orders of magnitudes. Here, following a biophysical model, we demonstrate experimentally that the physics of a living cochlea can be emulated by an active one-dimensional acoustic metamaterial. The latter solely consists on a set of subwavelength active acoustic resonators, coupled to a main propagating waveguide. By introducing a gradient in the resonators' properties, we establish an experimental set-up which mimics the dynamical responses of both the dead and the living cochleae: the cochlear tonotopy as well as the low-amplitude sound amplifier are reproduced.

Cochlea-inspired design of an acoustic rainbow sensor with a smoothly varying frequency response

“…The pressure and volume velocity at the input of each element can be calculated once p end and q end are found, by iteratively applying Eq. (10) starting from the last element. The pressure in the resonators can then be calculated with expressions given in the Supplementary material.…”

“…Also, although in many current applications advanced digital systems currently appear to provide the most effective frequency analysers, it is still worthwhile to investigate alternative acoustic or mechanical 9 technologies for future or niche applications. In the acoustic regime, a nonlinear acoustic rainbow sensor with side branches of quarter-wave resonators was recently presented 10 , which aims to mimic the response characteristics of both the passive cochlea, at high input levels, and the active cochlea, at low input levels. In this case, the passive system had a smoothly varying frequency response but the active system did not, which the authors attribute to reflected waves at low levels and postulate that these may also occur in the cochlea.…”

“…4a, b predict that the designed system can be used as an acoustic rainbow sensor, where different frequency components are filtered at different positions, since each element acts as a bandpass filter with a smooth frequency response centred at a different frequency. Therefore, the different frequency components can be captured by introducing small microphones in the cavities of the Helmholtz resonators, as was done in a system with quarter-wave resonators in 10 .…”

Mimicking the active cochlea with a fluid-coupled array of subwavelength Hopf resonators

“…Rupin et al [21] have recently produced promising experimental results, demonstrating that a graded array of cylindrical resonators can mimic both the cochlea's frequency separation and non-linear amplification properties. Their set-up was scaled up for practicality and employed an array of cylindrical quarter-wavelength resonators with a microphone-andspeaker amplification system.…”

“…In this work, we will introduce a Hopf-type nonlinearity directly to the wave-propagation problem by supposing that the resonators are equipped with an appropriate forcing mechanism (as was realized by Rupin et al [21]). Based on an eigenmode decomposition, we will explore the Hopf-type behaviour of this system and show that the crucial cochlea-like properties of (1.1) are retained by the coupled subwavelength structure.…”