Soundproofing is any means of impeding sound propagation. There are several methods employed including increasing the distance between the source and receiver, decoupling, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles for absorption, or using active anti-noise sound generators. Acoustic quieting and noise control can be used to limit unwanted noise. Soundproofing can reduce the transmission of unwanted direct sound waves from the source to an involuntary listener through the use of distance and intervening objects in the sound path (see sound transmission class and sound reduction index). Soundproofing can suppress unwanted indirect sound waves such as reflections that cause echoes and resonances that cause reverberation.
Techniques
Absorption Sound-absorbing material controls reverberant sound pressure levels within a cavity, enclosure or room. Synthetic absorption materials are porous, referring to open cell foam (acoustic foam, soundproof foam). Fibrous absorption material such as cellulose, mineral wool, fiberglass, sheep's wool, are more commonly used to deaden resonant frequencies within a cavity (wall, floor, or ceiling insulation), serving a dual purpose along with their thermal insulation properties. Both fibrous and porous absorption material are used to create acoustic panels, which absorb sound reflections in a room, improving speech intelligibility.
Porous absorbers Porous absorbers, typically open-cell rubber foams or melamine sponges, absorb noise by friction within the cell structure. Porous open-cell foams are highly effective noise absorbers across a broad range of medium-high frequencies. Performance can be less impressive at lower frequencies. The exact absorption profile of a porous open-cell foam will be determined by a number of factors, including cell size, tortuosity, porosity, thickness, and density. The absorption aspect in soundproofing should not be confused with sound-absorbing panels used in acoustic treatments. Absorption in this sense refers to reducing a resonating frequency in a cavity by installing insulation between walls, ceilings or floors. Acoustic panels can play a role in treatment, reducing reflections that make the overall sound in the source room louder, after walls, ceilings, and floors have been soundproofed.
Resonant absorbers Resonant panels, Helmholtz resonators and other resonant absorbers work by damping a sound wave as they reflect it. Unlike porous absorbers, resonant absorbers are most effective at low-medium frequencies and the absorption of resonant absorbers is matched to a narrow frequency range.
Damping Damping serves to reduce resonance in the room, by absorption or redirection through reflection or diffusion. Absorption reduces the overall sound level, whereas redirection makes unwanted sound harmless or even beneficial by reducing coherence. Damping can be separately applied to reduce the acoustic resonance in the air or to reduce mechanical resonance in the structure of the room itself or things in the room.
Decoupling Creating separation between a sound source and any form of adjoining mass, hindering the direct pathway for sound transfer.
Distance The energy density of sound waves decreases as they become farther apart, so increasing the distance between the receiver and source results in a progressively lesser intensity of sound at the receiver. In a normal three-dimensional setting, with a point source and point receptor, the intensity of sound waves will be attenuated according to the inverse square of the distance from the source.
Mass Adding dense material to treatment helps stop sound waves from exiting a source wall, ceiling or floor. Materials include mass-loaded vinyl (MLV), soundproof sheetrock or drywall, plywood, fibreboard, concrete or rubber. Different widths and densities in soundproofing material reduce sound within a variable frequency range.
Reflection When sound waves hit a medium, the reflection of that sound is dependent on the dissimilarity of the material it comes in contact with. Sound hitting a concrete surface will result in a much different reflection than if the sound were to hit a softer medium such as fiberglass. In an outdoor environment, such as highway engineering, embankments or paneling are often used to reflect sound upwards into the sky.
Diffusion If a specular reflection from a hard flat surface is giving a problematic echo, an acoustic diffuser may be applied to the surface. It will scatter sound in all directions.
Active noise control In active noise control, a microphone is used to pick up the sound that is then analyzed by a computer; then, sound waves with opposite polarity (180° phase at all frequencies) are output through a speaker, causing destructive interference and canceling much of the noise.
Applications
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