Sound absorption and noise control

How absorptive materials absorb sound

Sound absorbers are porous, soft or flexible. Absorption works by three mechanisms:

1. Porous absorption: Sound enters the connected pores of materials such as mineral wool, glass wool, acoustic tile, carpets and curtains. The air in the pores moves, and friction with the fibres changes the sound energy into a little heat.
2. Panel (membrane) absorption: Thin panels of plywood or hardboard over an air space vibrate when sound strikes them, and absorb mostly low frequencies by internal damping.
3. Cavity (Helmholtz) resonators: Perforated boards with an air space behind; air in the holes resonates and loses energy, mostly in a narrow band.

The absorption coefficient $\alpha$ is the fraction of incident sound absorbed (0 for a perfect reflector, 1 for an open window).

General considerations for noise control in buildings

1. Site selection and planning: Set back from highways, railways and airports. Put noisy zones away from quiet rooms. Use trees, fences and mounds as buffers.
2. Building layout: Group noisy and quiet spaces separately; a service core between.
3. Insulation: Heavy walls, floors, double glazing, sealed doors.
4. Absorption inside rooms using acoustic ceilings and carpets.
5. Control at the source: Silent machines, vibration mounts and enclosures for generators and pumps.
6. Services: Isolate ducts, pipes; lined ducts, silencers.
7. Legal regulation: Noise limits and horn restrictions.