VRIJE UNIVERSITEIT BRUSSEL Acoustics - the Dept. of ...

4.2. REALIZATION OF ACOUSTIC ABSORPTION 67with perforated panels, one can adjust the friction term d as desired, e.g.by applying rock wool or glass wool in the air layer behind it. Moreover,there are also products available on the market that approach the resonatorof Helmholtz in shape : very thick embedded, sawed or milled wood fiberpanels. Indeed, the air mass in the holes, cuts or milled grooves can vibrate,while energy is dissipated in the porous fiber material. They absorb mainlyin the acoustic middle frequency range.N.B.: The air behind the big plate, as well as the air in the Helmholtzresonator undergoes compression and expansion as a whole, i.e. it is assumedthat no wave phenomena occur (λ > characteristic size).Figure 4.5: Schematic representation of a Helmholtz resonator.4.2.3 Porous acoustic absorbing materialsConsider a porous material which consists of fibers (diameter 2 to 20 µm)that are random oriented, and bonded to each other at their contact pointsusing a resin. Frequently used fiber materials for acoustic absorption areglass or mineral materials. Notwithstanding the specific mass of the glassor mineral fiber (approximately 2400 kg/m 3 ) the mass of the bulk porousmaterial is only between 30 and 200 kg/m 3 (average of about 100 kg/m 3 ).The porosity is thus very large : the ratio of pore volume to total volumeamounts to about 95 to 98%. The pores are all in connection to each other,