  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.
