Introduction to Sound Absorbing Materials
Sound-absorbing materials absorb most sound energy, striking them and reflecting very little, making them very useful for the control of noise. The major uses of absorbing materials are almost invariably found to include the reduction of reverberant sound pressure levels and, consequently, the reduction of the reverberation time in enclosures or rooms. Although all materials absorb some incident sound, the term “acoustical material” has been primarily applied to those materials that have NRC of at least 0.5 — in other words absorbing at least 50% of sound
Porous sound absorbing materials can be classified as cellular, fibrous and granular
Porous Cellular Sound Absorbing Material is a solid that contains cavities, channels or interstices so that sound waves are able to enter through them. To be effective sound absorber, the cells have to have an opening in order for sound waves to enter the materials through a multitude of small holes or openings. Those pores “open” into continuous twisted channels which have a great importance for the absorption of sound. Open pores can be “blind” (open only at one end) or “through” (open at two ends).
Porous Cellular absorbers, typically open cell rubber foams or melamine sponges, absorb noise by friction within the cell structure.
Porous Fibrous Sound Absorbing Materials are composed of a set of continuous filaments that trap air between them. Fibers can be classified as natural or synthetic (artificial). Natural fibers can be vegetable (cotton, kenaf, hemp, flax, wood, etc.), animal (wool, fur felt) or mineral (asbestos) and are essentially completely biodegradable. Also, natural fiber processing is more economical and environmentally friendly. Porous Fibrous sound absorbers absorb noise by vibrating the fibers within the material and also by air friction trapped between the fiber filaments of the material.
Absorbing sound spontaneously converts part of the sound energy to a very small amount of heat in the intervening object (the absorbing material), rather than sound being transmitted or reflected. There are several ways in which a material can absorb sound. The choice of sound absorbing material will be determined by the frequency distribution of noise to be absorbed and the acoustic absorption profile required.
As mentioned above, there is an important difference in the sound absorption mechanism between porous cellular materials like acoustic foam and sound absorbing fibers like cotton,
Acoustic foam absorbs the sound energy by trapping sound waves in a pores and channels of the foam. Diameter of the channels, its tortuosity (shape) and length of the channels are all contributing factors in the sound absorption. Sound waves get into the open cells of the foam and lose their energy through friction between the air particles and the void walls of the material it is passing through. Foam attenuates airborne sound waves by increasing air resistance, thus reducing the amplitude of the waves.
In Fibrous Sound Absorption, panels sound reduces its energy by trying to vibrate the fibers within the panels. When the air gets into fibrous thickets – the waves are trying to move and shake each of these fibers and dissipate its energy.