Sound absorption

· Sound absorption usually has little effect on noise control but has an important effect on sound quality.

· Sound absorption can be useful in the control of noise that spreads by reflections from the ceiling of offices and factories or by multiple reflections along corridors and ducts.

Types of absorber

The materials used for the purpose of absorbing sound can be classified into 3 main types:

a. Porous absorber for high frequencies.

b. Panel absorber for lower frequencies.

c. Cavity absorber for specific lower frequencies.

a. Porous absorber

Porous sound absorber

· Porous absorber consists of cellular materials such as fiberglass and mineral wools.

· The air in the cells provides a resistance to sound waves.

· Porous material used for sound absorption includes acoustic tiles, acoustic blankets and special coating such as acoustic plaster.

· The absorption of porous materials is most effective at frequencies above 1 kHz.

b. Panel absorber

A panel absorbers The fibreboard panels are suspended from the ceiling with

the sizes varying to give a broadband absorption field.

· A panel absorber is created when you place a sheet of plywood or fibreboard, with insulation glued to the back of it, over an air cavity.

· Panel or membrane absorbers are constructed from fixed sheet of continuous materials with a space behind them; the space may be air or may contain porous absorbent.

· The panels may be made of plywood or existing elements such as suspended ceilings.

· A panel absorber is most effective for low frequencies in the range 40 to 400Hz.

· The two factors determining the frequency of absorption are:

1. The mass or density of the panel.

ii. The depth of the air cavity, i.e. depth of the sealed timber frame

c. Cavity absorber / Helmholtz resonators

· Cavity absorbers or Helmholtz resonators are enclosure of air with one narrow opening. The bandwidth is often quite narrow but really very highly efficient.

· The opening act as an absorber when air in the opening is forced to vibrate and the viscous drag of air removes energy from the sound waves.

· In practice, the cavity may cover with absorbent other than air.

· The panel build in with Helmholtz Resonator will absorb low and low mid frequencies.

· This panel must be placed at the back centre of the room and is very efficient in small rooms.

Дата добавления: 2015-11-14