• Some materials having hard, rigid including non-porous surfaces, provide the least absorption, whereas these which are flexible, soil porous also can vibrate, and absorb more extra sound
• The efficiency of the sound absorption depends upon the porosity of every material used essentially sound absorbent
• The term used to express the percentage of the incident sound that can be absorbed by a material is known as the absorption coefficient of the material
• If the absorption coefficient of a material is 0.75, means that the material is capable of absorbing 75 % of the incident sound
• Each absorption coefficient changes with the frequency of every incident sound
• low-density materials have a higher absorption coefficient

Classification of  Absorbents

1. Porous absorbents
2. Resonant absorbents
3. Cavity resonators
4. Composite type of absorbents

Porous absorbents

• When sound waves strike the surface of a porous material, a part of the waves make reflected while a component enters the pores of the material including dissipated within heat energy
• These efficiencies of this sort of absorbent increase with the increase in the resistance granted by the material
• It is used to absorb sound having a high frequency

Resonant absorbents

• Some absorbent material is fixed toward sound framing timber with an air space between each framing including the wall near the back.
• Such an arrangement runs most efficiently during absorbing sound waves at a base frequency
• This principle of sound absorption is that sound waves of the appropriate frequency cause sympathetic vibrations in the panel which serves to be a diaphragm
• Some absorption of sound uses placed under the dampening of some sympathetic vibration.
• The dampening effect of this system can be improved by placing a porous material in the air space.

Cavity resonators

• It essentially consists of a chamber with a narrow opening (entrance) in which absorption takes place by the resonance of the air in the chamber which gives a loss of sound energy
• Such an arrangement container work effectively over a single picked frequency.
• Application of cavity resonator is usually restricted to absorption from an individual machine or in similar cases

Composite type of absorbents

• They consist of perforated panels mounted on battens to leave a cavity between the panels and the wall at the back
• The panels may be of metal, wood (plywood) hard broad, plasterboard, etc
• The range of holes in the panel should differ between 10 to 20 % of each total area of the panel.
• When some sound strikes the panel, the sound waves pass through the holes and also get damped through the resonance of the air in that cavity.
• The effectiveness of that system can be increased by placing a porous material same mineral wood etc. in this cavity
• This type of absorbent is commonly used, as it is easy to install, economical and it can accommodate a wide range of frequencies

Characteristics of audible sound

There are three characteristics of sound 

1. Intensity & loudness
2. Frequency & pitch

• The intensity of sound is defined as the amount of flow of wave energy crossing per unit of time through a unit area taken perpendicular to the direction of propagation.
• Some loudness about sound is described as the degree of sensation.
• Which is not a wholly physical quantity but partly subjective & does not depend upon the ear & listener.

Frequency & pitch

• Frequency or pitch is defined as the number of cycles that a sounding body makes in each unit of time.
• It is a measure of the quantity of sound.