Soundproofing
Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor (noise control). There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to block or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.
Soundproofing affects sound in two different ways: noise reduction and noise absorption. Noise reduction simply blocks the passage of sound waves through the use of distance and intervening objects in the sound path. Noise absorption operates by transforming the sound wave. Noise absorption involves suppressing echoes, reverberation, resonance and reflection. The damping characteristics of the materials it is made out of are important in noise absorption. The wetness or moisture level in a medium can also reflect sound waves, significantly reducing and distorting the sound traveling through it, making moisture an important factor in soundproofing.
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[edit] Distance
The use of distance to dissipate sound is straightforward. The energy density of sound waves decreases as they spread out, so that 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, the intensity of sound waves will be attenuated according to the inverse square of the distance from the source. Using mass to absorb sound is also quite straightforward, with part of the sound energy being used to vibrate the mass of the intervening object, rather than being transmitted. When this mass consists of air, the extra dissipation on top of the distance effect is only significant for typically more than 1000 meters (or 1 kilometer), depending also on the weather and reflections from the soil. The combined effect of distance and dissipation in air is implemented in this.
Damping is the process by which sound waves are converted into heat over time and distance. When travelling through a panel, these are actually called bending waves. This can be achieved in several ways. One way is to add a layer of material such as lead or neoprene which are both heavy and soft. These can be used as a sound deadening layer in the construction of sound studios, where levels of sound are targeted for reduction.
Acoustic foam can be used on the face of a wall or ceiling to reduce reverberation (echo), or inside a wall to damp cavity resinance. Such treatments must be fireproof to avoid disasters such as The Station nightclub fire which killed 100 in 2003.
Making a sound wave transfer through different layers of material with different densities assists in noise damping. Open-celled foam is not a good sound damper inside of a wall, as the foam is too dense and offers insufficient surface area for sound wave interaction. Improper use of foam tape as a stand-off for paneling can lead to problems with structural compliance enabling resonance of the panel. This process is analogous to a string holding wind-chimes: the string helps the chimes ring by isolating the vibration instead of damping it. Foam tapes may, therefore, be undependable in a soundproofing protocol.
Extruded polystyrene foam (XPS) and expanded polystyrene foam (EPS), commonly used for thermal insulation, are significant conductors of sound. Polystyrene use as a sound damper should be avoided except in applications where moisture resistance and buoyancy is necessary.
[edit] Room within a room
A room within a room (RWAR) is one method of isolating sound and stopping it from transmitting to the outside world where it may be undesirable.
Most vibration / sound transfer from a room to the outside occurs through mechanical means. The vibration passes directly through the brick, woodwork and other solid structural elements. When it meets with an element such as a wall, ceiling, floor or window, which acts as a sounding board, the vibration is amplified and heard in the second space. A mechanical transmission is much faster, more efficient and may be more readily amplified than an airborne transmission of the same initial strength.
The use of acoustic foam and other absorbent means is less effective against this transmitted vibration. The user is advised to break the connection between the room that contains the noise source and the outside world. This is called acoustic de-coupling. Ideal de-coupling involves eliminating vibration transfer in both solid materials and in the air, so air-flow into the room is often controlled. This has safety implications, for example proper ventilation must be assured and gas heaters cannot be used inside de-coupled space.
[edit] Noise cancellation
Noise cancellation generators for active noise control are a relatively modern innovation. 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 cancelling much of the noise.
[edit] Residential soundproofing
Residential soundproofing aims to decrease or eliminate the effects of exterior noise. The main focus of residential soundproofing in existing structures is the windows. Curtains can be used to damp sound either through use of heavy materials or through the use of air chambers known as honeycombs. Single-, double- and triple-honeycomb designs achieve relatively greater degrees of sound damping. The primary soundproofing limit of curtains is the lack of a seal at the edge of the curtain. Double-pane windows achieve somewhat greater sound damping than single-pane windows. Significant noise reduction can be achieved by installing a second interior window. In this case the exterior window remains in place while a slider or hung window is installed within the same wall openings.[citation needed]
[edit] Noise barriers as exterior soundproofing
Since the early 1970s it has become common practice in the United States (followed later by many other industrialized countries) to engineer noise barriers along major highways to protect adjacent residents from intruding roadway noise. The technology exists to predict accurately the optimum geometry for the noise barrier design. Noise barriers may be constructed of wood, masonry, earth or a combination thereof. One of the earliest noise barrier designs was in Arlington, Virginia adjacent to Interstate 66, stemming from interests expressed by the Arlington Coalition on Transportation. Possibly the earliest scientifically designed and published noise barrier construction was in Los Altos, California in 1970.
[edit] See also
- Acoustic board
- Acoustic transmission
- Anechoic chamber
- Hearing test
- Noise barrier
- Noise pollution
- Noise regulation
- Noise, vibration, and harshness
- Recording studio
- Sound masking
