  Metal reflects electromagnetic waves because of the large conductivity that
is responsible for dissipation. During which the waves undergo a 180$^\circ$
phase change that is independent of the frequency. There is no counterpart
material for acoustic waves. Here we show that by using an array of acoustic
resonators with a designed high-density dissipative component, an "acoustic
metal" can be realised that strongly couples with sound over a wide frequency
range not otherwise attainable by conventional means. In particular, we show
the acoustic Faraday cage effect that when used as a ring covering an air duct,
99% of the noise can be blocked without impeding the airflow. We further
delineate the underlying volume requirement for an acoustic metal based on the
constraint of the causality principle. Our findings complement the missing
properties of acoustic materials and pave the way to the strong wave-material
couplings that are critical for the applications as high-performance audio
devices.
