DIFFERENT TYPES OF ABSORBING MATERIALS AND ITS APPLICATION
POROUS MATERIALS
Basic acoustical characteristic of all porous materials is a cellular network of minute
interlocking pores. They convert the incident sound energy into heat energy by the frictional
and viscous resistance within these pores and by vibration of their small fibre.Good for
high frequency range.
Fibreboard, mineral wool, insulation blankets, etc. are some of the examples
ROCK WOOL GLASS WOOL
CAVITY OR HELMHOLTZ RESONATERS
They consist of an enclosed body of air confined within rigid walls and connected by a
narrow opening(called the neck) with the surrounding space in which the sound waves
travel. Cavity resonators can be applied
1. As individual units
2. As perforated panel resonators
3. As slit resonator panels.
GYPSUM BOARD
NON PERFORATED PANELS & MEMBRANE ABSORBERS
Any impervious material, installed on a solid backing but separated from it by an air
space, will be set to vibration when struck by sound waves. Good for low frequency range.
Wood and hardboard panelling, gypsum boards,suspended plaster ceilings, furred out
plasters, rigid plastic boards, windows,glazing,doors, wood floors and plat-forms, etc
Materials
DIFFERENT MATERIALS USED AS SOUND ABSORBERS
HAIRFELT
The material was used by prof. Sabin in his experimental works, made up of wool and fur.
The average value of coefficient of absorption of 25mm thick hairfelt is 0.60. It is used at
rooms and halls.
ACOUSTIC PLASTER
Also known as the fibrous plaster and it includes granulated insulation insulation material
mixed with cement.
The average value of coefficient of absorption of 20mm thick and density of 1kN/m3 acoustic
plaster is
0.30 at 500 cps. For acoustical plaster boards-0.15-0.30
ACOUSTICAL TILES
They are factory made, uniform sound absorbers, easily fixable but
expensive. Most suitable for rooms in which small area is available for
the acoustical treatment.
STRAWBOARD
Prepared from compressed straw and covered with thick paper or hardboard are used.
The average value of coefficient of absorption of 13ml thick and density of 12.4kN/m3 is
0.3 at 500 cps. It is used at rooms and medium capacity halls.
PULPBOARDS
These are the softboards prepared from compressed pulps.
The average value of coefficient of absorption of 13mm thick,is 0.17 at
500 cps. They are cheap and can be fixed by ordinary panelling.
COMPRESSED FIBREBOARD
Made up of compressed fibre, may be perforated or unperforated.
The average value of coefficient of absorption of perforated one with the density of
3kN/m3 is 0.30 and of other one is 0.
COMPRESSED WOOD PARTICLE BOARD
Prepared from compressed wood and provided with perforations and it can be painted also.
The average value of coefficient of absorption of 13mm thick is 0.40 at 500 cps.
PERFORATED PLYWOOD
Can be used by forming composite panels with mineral woc cement asbestos or with mineral
wool and hardboard. Generally suspended from trusses.
The average value of coefficient of absorption for the formed is as high as 0.95 and for the
latter one, it is about 0.20.
WOOD WOOL BOARD
Prepared from a mixture of portland cement and wood wool or wood shavings. A small
quantity of Gypsum is sometimes added.
The average value of coefficient of absorption of 25mm thick and density of 4kN/m3 is 0.20 at
500 cps.
QUILTS AND MATS
Prepared from mineral wool or glass wool and are fixed in the form of acoustic blankets
The absorption coefficients of such quilts and mats depend on the thickness, density,
perforations, mode of fixing, nature of backing and frequency of sound.
