Sound is undeniably a very important aspect of our existence. Aside from being a major form of communication, it's also put to task in geophysics; in aspects of signalling, location and positioning. As energy, it can be used to break down solids and degas liquids. Sound waves are as a result of vibrations between atoms or molecules. These vibrations are converted from kinetic energy to sound energy. It's this energy that enables the sound waves to be propagated.
Despite its vast applications and usefulness, this form of energy is sometimes unwanted and becomes a nuisance or hinderance. The noise of a motor engine, external noise in a music studio, noise from a shared apartment etc.There is therefore need for a means or technique to reduce or block it. Since they are waves, sound energy can refract, reflect and maneuver around edges. Although they cannot be completely blocked, they can be greatly reduced. The act of eliminating, reducing or blocking sound is referred to as sound proofing, which can be done in the following three basic ways:
- Distance: When traveling through a medium e.g air, the sound energy of the waves is gradually lost converted to vibrational (kinetic) energy in the atoms of the medium. As a result, the longer the distance the waves have to travel to reach the receptor, the more of its energy is lost.
- Absorption: Absorption and/or damping are complex forms of soundproofing, through the use of materials. In absorption, soundproofing is achieved by the reduction of the energy of the sound waves. When sound waves hit any surface, some get transmitted through the material, some get absorbed within the material and a percentage gets reflected (depending on the absorbing quality of the surface). The sound energy is converted to kinetic energy through vibrations, and finally to a miniscule amount of heat, through inter molecular friction.
- Damping: A form of absorption in which the oscillating sound waves are made to undergo resonance, with respect to the surface it hits. damping works best at low frequencies.
One might imagine that soundproofing can be achieved by any material, this is however, far from the case. There are a few materials that can effectively carry out soundproofing. Before studying some of these materials, we need to first get an understanding of what properties make these materials unique.
- Density. Density is the amount of mass per volume of a substance. It is measure of how packed together, the molecules of a material are. For a material to be sound proof it has to fall within a proper density range. High enough and sound waves get damped; low enough and they get absorbed. If the material's density is too low, the sound waves are transmitted through. If the density is too high, the waves get reflected off the material's surface.
- Porosity. This property involves the use of intersiticies to alter the energy of the sound waves by expansion, compression and change in the direction of flow; resulting in loss of momentum. Porosity is an advantage in absorption and a disadvantage in blocking.
- Flow resistivity. This is the flow resistance to noise per unit thickness of a material. It is the most important characteristic of sound absorbers. Resistivity is dependent on tapers of the sound waves.
- Cell size. The individual cells of the material, have to be adequately small for the material to qualify for soundproofing. The cell size of the material must be smaller than the wave length of the sound it is meant to absorb or block. Cell arrangement is also of importance. Open cell arrangement make better absorbers, while closed cell arrangements make better blockers.
- Tortuosity. This is a measure of the twists and turns in the material's cell arrangement. The more bends the sound waves have to maneuver, the more momentum they loose.
The above properties qualify a material as good for soundproofing. let us now take a look at some unique, soundproofing materials.
- Polyurethane foams. Acoustic foams were first used in the mid 1970's. Polyurethane foams are made through basic addition polymerization reaction involving a diol or polyol, a diisocyanate, and water.Acoustic foams have mostly open cells as a result of trapped gas bubbles which pop. Air passes easily, through this type foam. Polyurethane is designed as a flexible, open-cell, porous solid. Sound energy is propagated through the material by two major methods:
- Sound pressure waves move through the fluid within the pores of the polyurethane
- Elastic stress waves are created as a result of the pressure waves, which are carried through the frame of the polyurethane
Polyurethane is quite effective at attenuating high frequency sound waves, but it does not provide low frequency isolation unless sufficient thickness is used. The porous nature of polyurethane greatly reduces acoustic reflection, but this low density also allows for the transmission of sound energy. Acoustic foams are chemically inert but flammable. Because of its flammable nature. Polyurethane foams cannot be used in industries. It is more suitable for installation within a room.
- Felt. Felt is produced by pressing and matting fibers together. Fibres use may be natural (mostly wool) or synthetic. A blend of both is also common. Felt is durable and stable in the presence of moisture, lubricating oils, greases, salts, detergents, and is inert to many other chemicals. Its ability to bend to uneven surfaces prevents the unwanted intrusion of foreign substances beneath the load-bearing area. Felt possesses almost permanent resilience, as it is made up of millions of individual fibers. The performance of felt in sound absorbtion 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 absorbtion, too dense felt would not allow for sufficient vibrations. These qualities make it excellent for industrial sound proofing. Felt is also used as a damper in musical instruments.
- Polyester fibre. Polyester fibre is a man made fiber, 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, light weight insulation products. Polyester fibre is spectacular for it's unique blend of heavy density (approximately 2000g/m^3) and porosity. It's sound absorbtion 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.
- Fiber Glass. Acoustic fiberglass has the desirable combination of rigidity and being light weight. Popularly dubbed, the shape shifter of sound proofing, 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.
- Mass Loaded Vinyl has been applied in soundproofing since the 1960s. It is basically a viscoelastic material, such as polyvinylchloride, that is infused with an inert material, like calcium carbonate or barium sulfate. Viscoelastic materials exhibit both viscous and elastic properties. They will flow, but when the force is removed or reversed, they will go back to their original shape. This attribute, known as hysteresis , contributes to the ability of MLV to attenuate sound. It is this mechanism by which many claim that MLV “converts sound to heat”, although it is not the fundamental mean by which MLV attenuates sound. MLV basically acts like a tuned mass damper, capable of reducing the frequency of waves. When vinyl is exposed to low temperatures, it becomes very hard, but when it is exposed to high temperatures it becomes very compliant. When the elastic component becomes very stiff, the vibration modes of the sheet are tuned to a higher frequency, affecting its ability to attenuate sound at lower frequencies. As is the other way round, as the temperature increases, the MLV becomes limp, to the point where it doesn't possess enough compliance to vibrate. Plasticizers can be used to ensure the proper compliance of the MLV over the operating temperature range. This material is very flexible and suitable for corners and bends. It's however, expensive.
- Cork. Cork is an amazing natural alternative for soundproofing. It's the phellem layer of bark tissue, harvested from the cork oak. This material is fire proof, 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, with a density of 0.16g per cubic centimeter. The cells of these material are arranged as in the honey comb with each cubic centimeter meter containing an average of 40 million tiny air sealed cells. When sound energy passes through cork, the energy is converted to vibrational energy in by the air molecules. Cork is able to trap an immense amount of air molecules and this makes it an excellent insulator of sound.
- Green glue. Green glue is a viscoelastic compound that insulates sound using the constrained layer damping (CLD) system. The glue is applied (sandwiched) between two rigid materials like dry wall. 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 totally work as an adhesive.
- Silicone. Silicone is a good soundproofing option for tight spaces and corners. Silicone, also known as polysiloxanes has many desirable properties. It's is mostly inert, has low thermal conductivity, is resistant to water, UV rays and provide air tight 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 air proof and so stops sound propagation by air. It's also a great damping material and is excellent at damping mid frequency sounds.
- Epoxy. 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 adhesive when setting up soundproof material as they are more advantageous that regular glue. They can also be applied as coating.
The ratings for classifying and comparing these various materials are the noise reduction coefficient (NRC), which is for absorbers and the sound transmission class (STC), which is for blockers. The NRC rating is between 0 and 1 and its an average of how absorptive a material can be at these four frequencies - 250, 500, 1000 and 2000. Because it's an average, two material with the same NRC may work well at different applications. STC is a measure of how well a material blocks sound. The higher the rating the better.
Check out Phelps product portfolio of soundproofing materials