ACT WRM perforated panels have circular or linear holes, also called multiple Helmholtz resonators, in the cavity of which porous material has been placed.
In such a configuration, the incident wave is unable to make the base panel move; only the vibration of air in the holes is relevant. The air inside the porous material and in the cavity acts as a spring secured at one end by light air masses which vibrate in the holes.
The system has a resonance frequency which is controllable by changing the thickness of the air gap, the size and shape of the holes and the depth of the hole neck.