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Sound absorption is the prevention of reflection of sound or alternatively, a reduction in the sound energy reflected by the surfaces of a room.

ound absorption:

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Sound insulation is the prevention of transmission of sound or alternatively, a reduction of sound energy transmitted into an adjoining air space.

ound Insulation:

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:

1. Porous absorber for high frequencies.

2. Panel absorber for lower frequencies.

3. Cavity absorber for specific lower frequencies.

1. 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.

2. 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:

9. The mass or density of the panel.

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

3. 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.

Sound Insulation

• Sound insulation is the most useful method for controlling noise in buildings.

• Insulation is the principal method of controlling both airborne sound and impact sound in the buildings.

• Two types of sound insulation are to be dealt with in building construction:

i.  Airborne Sound Insulation:: the insulation against noise originating in air, e.g. voices, music, motor traffic, wind.

ii. Impact Sound Insulation: the insulation against noise originating directly on a structure by blows or vibration e.g. footsteps above, furniture being moved, drilling and hammering the structure.

• The Building Regulations 1991 call for ‘an adequate resistance to the transmission of airborne sound’ for walls and for airborne and impact sound in the case of floors.

• Means of complying the Regulations are set out in Approved Document E : 1992 Airborne and impact sound.

• The standard of airborne sound reduction are specified in term of ;

• weight of walls and floors and

• soft coverings and floating layers for impact sound,

• together with special requirements for joints and the elimination of air passengers.

Construction for sound insulation

1. Walls

Heavyweight wall

• Heavyweight walls generally provide high levels of airborne sound insulation.

• Construction of cavity in the heavyweight wall can increase sound insulation.

Lightweight wall

• Lightweight wall can provide adequate levels of airborne sound insulation with airtightness.

• Each side of the wall is built on separate timber frames and an absorbent blanket of mineral fibre is used to provide acoustical isolation between the sides of the wall.

2. Floors

• The insulation of a floor must be maintained at all junctions with the surrounding walls in order to prevent flanking transmission.

• The separation of 2 parts of a floating floor must continue around all edges by the use of resilient materials and airtight techniques.

Concrete floor

• The natural mass concrete floors provide insulation against airborne sound but concrete can transfer impact sound.

• Therefore a resilient layer (mineral wool) is also needed to provide insulation against impact sound.

Timber floor

• Thick layer of composite floorboard or multiple layers of plasterboard is used in timber floor construction to increase the mass of timber floor for better sound insulation.

3. Windows

• Glass has relatively high density so increasing the thickness of glass provides increased of sound insulation.

• Air cavity can be provided between 2 panels of glass for sound insulation.

• Window must design to shut with a good seal in order to provide air tightness.

• The acoustic insulation of insulating glass can be achieved by using glasses of asymmetrical thickness, as well as laminated glass with special PVB (Polyvinyl-Butyral) layer which is highly considerably reduces noise.