Ultrabroadband sound control with deep-subwavelength plasmacoustic metalayers

Controlling audible sound requires inherently broadband and subwavelength acoustic solutions, which are to date, crucially missing. This includes current noise absorption methods, such as porous materials or acoustic resonators, which are typically inefficient below 1 kHz, or fundamentally narrowband. Here, we solve this vexing issue by introducing the concept of plasmacoustic metalayers. We demonstrate that the dynamics of small layers of air plasma can be controlled to interact with sound in an ultrabroadband way and over deep-subwavelength distances. Exploiting the unique physics of plasmacoustic metalayers, we experimentally demonstrate perfect sound absorption and tunable acoustic reflection over more than two frequency decades, from several Hz to the kHz range, with transparent plasma layers of thicknesses down to $\lambda/1000$. Such unprecedented bandwidth and compactness opens new doors in a variety of applications, including noise control, audio-engineering, room acoustics, imaging and metamaterial design.