Basic construction of a micro-perforated absorber: Perforated plate, air layer, rigid back panel.
Left side: In a normal Helmholtz resonator.
Right side: In a micro-perforated absorber.
Micro-perforated absorbers consist of a perforated plate with a multitude of tiny holes arranged in front of an enclosed air volume. When excited by a sound wave the mass of air in the holes oscillates in front of the elastic air volume. Thus we are dealing with an acoustic mass-spring system operating on the principle of the Helmholtz resonator.
Micro-perforated absorbers are effective over a relatively broad band of frequencies. The perforated portion of their surface only adds up to a few percent, and no additional frictional resistance is required. Absorption is entirely due to the viscous friction of the air in the holes. It originates in the respective acoustical boundary layer, that is in the area where the velocity profile changes. The holes of micro-perforated absorbers are so small that this boundary layer extends over the entire cross-section of the holes. The thermal conductivity of the plate material is important, because a significant part of the frictional heat generated in the air is carried off by the plate. Micro-perforated absorbers can be made from different materials, including transparent acrylic glass.