Polyurethane is quite useful at attenuating high-frequency sound waves, but it does not store low-frequency isolation unless sufficient thickness is used. The porous quality of polyurethane greatly reduces acoustic reflection, but this low density also provides for the transmission of sound energy. Acoustic foams are chemically inert, so Polyurethane foams may not be used in industries. It is more suitable for installation within a room.
Felt is created by pressing and matting fibres together. Fibres use may be natural (mostly wool) or artificial. A combination of both is also common. Felt is strong and stable in the presence of moisture, lubricating oils, greases, salts, detergents, and is inert to many other chemicals. Its capability to bend to uneven surfaces prevents the unwanted invasion of foreign substances beneath the load-bearing area. Felt possesses almost permanent resilience, as it is made up of millions of individual fibres. The performance of felt in sound absorption is as a result of its optimum density and spring. The absorption of sound waves is achieved by the vibration of individual fibres within the felt. The energy is dissipated by frictional heat loss. Due to its method of absorption, too dense felt would not allow for sufficient vibrations. These qualities make it excellent for industrial soundproofing. Felt is also used as a damper in musical instruments.
Polyester fibre is a man-made fibre, made up of long-chain synthetic polymers. They are generally known as non-woven or bonded fibre fabric. They are used to make non-toxic, lightweight insulation products. Polyester fibre is spectacular for its unique blend of heavy density (approximately 2000g/m^3) and porosity. It's sound absorption increases with the frequency of the sound, hence it's most effective at high frequencies. It's NRC rating is between 0.8 and 1.Polyester fibre is also strong with high tensile strength. Other desirable properties are it's resistance to abrasion, fire, wrinkles, stretch, impact and wear. These properties make it an excellent soundproofing material in industrial and heavy machinery settings.
Fibre Glass. Acoustic fibreglass has a desirable combination of rigidity and being lightweight. Popularly dubbed, the shapeshifter of soundproofing, this material can be very easily customized, to allow for installation in the tightest of places. Glass fibre is mostly used in rooms and halls to prevent reverberations and echos.
Cork is an amazing natural alternative for soundproofing. It's the phellem layer of bark tissue, harvested from the cork oak. This material is fireproof, elastic and impermeable to an extent. Cork is so effective in soundproofing, that just 3mm of the material blocks 10decibels of sound. This amazing ability is as a result of the very cell structure and composition of the cork. Air is a great insulation material and cork is made up of 50% air. This makes it very light. When sound energy passes through the cork, the energy is converted to vibrational energy in by the air molecules. Cork can trap an immense amount of air molecules and this makes it an excellent insulator of sound.
Green glue is a viscoelastic compound that insulates sound using the constrained layer damping system. The glue is applied between two rigid materials like drywall. In CLD systems damping occurs when the viscoelastic material is sheared. When sound waves hit the rigid material, it causes it to shift in different directions. This movement results in shear forces within the green glue. The polymeric design of green glue, enables it to convert the energy from the shearing to frictional energy, and therefore heat. Green glue is not toxic; but despite the name, it does work as an adhesive.
Silicone is a good soundproofing option for tight spaces and corners. It's is mostly inert, has low thermal conductivity, is resistant to water, UV rays and provide airtight insulation. Silicone has applications in soundproofing as caulk. It's applied in a paste form and usually cures to form a rubbery coating. This coating is airproof and so stops sound propagation by air. It's also a great damping material and is excellent at damping mid-frequency sounds.
Epoxies' usefulness in soundproofing is as a result of its air resistance and damping properties. They are mostly used to supplement other soundproof materials. They can be used as an adhesive when setting up soundproof material as they are more advantageous than regular glue. They can also be applied as a coating.