Electrico

SOUND ABSORPTION

Sound absorbing materials are used to reduce the reverberant (reflected) sound (noise) inside a room.

Porous sound absorber:

Polystyrene is not a porous sound absorberbecause it is not a porous material. The air pores (voids) in a polystyrene material are not interconnected. Therefore, one cannot blow air through the material. The interconnectivity of the pores in amaterial is one of the essential properties of that material that makes it an efficient porous sound absorbing material. That is why glass fibre, mineral wool, polyester fibre, polyurethane foam and,curtains, carpets, etc. absorb sound. The sound energy is dissipated in such materials by being converted to heat energy through frictional resistance within the interconnected pores.

The surfacecharacteristics of a porous material must also not be such as to prevent the sound waves from penetrating the material. An impermeable surface would not allow the sound to enter the material. Forexample, if four thick coats of PVA paint were applied to a polyurethane foam board, the sound absorption of the material would be drastically reduced. However, a very thin, impermeable layer of plasticor vinyl on, for example some acoustic ceiling panels, do not reduce the amount of sound absorption by much, except that high frequency sound absorption is decreased somewhat.

The mass density of aporous material also has an influence on the amount of sound absorbed by the material. For example, the mass density of ordinary ‘think pink’ fibreglass and polyester ‘isotherm’ roof insulation isonly about 10 – 12 kg/m3. To be effective sound absorbers, these materials should have a mass density of between 32 – 80 kg/m3.

Panel sound absorber:

If thin sheets (say 25 mm thick) ofpolystyrene were mounted on battens against a wall with an air space behind the polystyrene sheets, a relatively inefficient panel sound absorber is presented. A panel sound absorber is only useful to...