Adding solid and heavy materials to surfaces can limit airborne sound. These are known as deflective treatments. Examples include drywall, plywood or concrete. These materials need to be dense and thick enough to stop noise from passing through them.
Many of these materials can be incorporated during an initial build or installed at a later stage, for example when renovating. They can also be used in conjunction with absorptive treatments to reduce reverberation and resonance.
Mass and Density
The more mass and density a soundproofing material has, the better it will perform. This is because it takes a lot of force to change the physics of vibration. To understand the principle behind this think of an aquarium filled with water. Would you use sponges for the walls of the tank or glass panels? It is the same with sound; a sponge allows the acoustic energy to seep through, whereas glass and good seals stop it.
Foam is a common soundproofing material and is often sold in sheets of different thicknesses to suit specific applications. However, it is important to note that foam does not add much in the way of mass. To improve its performance it must be used in conjunction with other methods of soundproofing such as decoupling or absorption.
Acoustic fabrics, which are often referred to as theatre curtains or blackout drapes, are extremely thick types of fabric which reduce noise passage as a result of their density. They are often hung in theatres and nurseries as a temporary partition or privacy material and can also be used as a wall lining to increase acoustic equalisation within rooms.
Mass loaded vinyl (MLV) is a limp type of vinyl that has been impregnated with heavy materials to increase its mass and density. It is typically used in music studios serious about sound control as it offers effective acoustic insulation and decoupling. It is best used in conjunction with acoustic absorbers such as rock wool or QuietFibre.
Deflection
The deflection of structures, such as walls and floors, has a huge effect on how much sound can pass through them. A structure’s deflection should be controlled to avoid vibration and to reduce sound transmission.
Deflection control can be achieved in many ways. Adding mass to the structure, such as using heavy materials like concrete or rock wool can increase its ability to stop sound. Another method is to break the path of a sound wave, such as installing resilient channels between studs to isolate drywall and minimize the transmission of sound.
Absorption can also be a very effective soundproofing technique. Using materials that can absorb and reflect at different frequencies, such as mineral fiber insulation or acoustic foam, can help reduce the amount of vibration that travels between spaces.
Resilient sound channels are one of the most common ways to decouple drywall from internal framing, and they can be used in walls, ceilings and floors. They are pieces of metal designed in a special shape that drywall can be attached to by placing them on top, and they allow the drywall to be screwed into place without making direct contact with the studs and joists. This can significantly improve the soundproofing of a structure as it allows for more isolation and less vibration. Also, installing acoustic clips on studs can help reduce sound transmission between them as well.
Absorption
Adding absorptive materials to a space prevents reverberation and reduces echo in a room. Porous materials such as carpets and curtains, acoustical foam and fiberglass insulation absorb sound instead of reflecting it. When absorbing materials are not feasible, decoupling barriers is an effective way to minimize sound transmission. Resilient channels and cross battens are made of resilient metal that are installed across wall studs or ceiling joists to create a gap that blocks vibrations from passing between walls.
To understand how the shape of surfaces and the thickness of materials affect sound absorption, imagine water flowing through a sponge. The sponge’s softness allows the water to flow freely – but it also traps some of the water as it flows through the material, much like how sound waves move through a soft acoustic foam wall.
While the physics of sound absorption is fairly complex, the principle is straightforward: the more porous the material, the more it will absorb and deflect sound. In order to maximize acoustic performance, building materials should be matched for their absorption and deflection properties.
Softening
When sound waves strike hard surfaces they cause vibration. The ability of some materials to dampen or weaken this vibration is the result of softening. Foam is one such material and is used in many applications for soundproofing purposes. Foam cannot block sound but it does absorb it and reduce its transmission within a room.
Foam insulation like spray foam does not add mass to the structure and it’s also not dense enough to stop sound from passing through it. To prevent this it is important to use a decoupling material to separate the wall studs from the joists. This can be done with resilient isolation channels or acoustic hangers.
Both are essentially a metal plate with a rubber compound or gasket between the two. The decoupling material converts the energy of the sound into heat and therefore stops the transmission of vibration from one side to the other.
There are many other ways to soundproof a room including the addition of door sweeps and draft stoppers for gaps around doors. Replacing hollow-core doors with solid core doors will also improve the acoustics. It is also important to seal outlets, light switches and electrical boxes. Finally, acoustic caulking can be used to fill any gaps. For a more comprehensive treatment, adding acoustic insulation such as QuietFibre Stone Wool Cavity Insulation will ensure effective sound absorption and deflection.