Soundproofing
From Free net encyclopedia
Soundproofing is any means of to reducing the intensity of sound with respect to a specified source and receptor. 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, on the other hand, 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.
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Distance
The use of distance to dissipate sound is straightforward. The energy density of sound waves decrease 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.
Damping
Damping is the process by which sonic vibrations are converted into heat. This can be achieved in several ways. For example, use of a material such as lead that is both heavy and soft, with the softness allowing it to damp the noise rather than allowing transmission. Making a sound wave transfer through different layers of material with different densities also assists in noise damping. This is the reason why open-celled foam is a good sound damper; the sound waves are forced to travel through multiple foam cells and their cell walls as sound travels through the foam medium. Improperly done, however, structural compliance can make things worse, such as the string holding chimes help them ring by isolating the vibration instead of damping it.
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 180 degrees out-of-phase are output through a speaker, causing destructive interference and cancelling much of the noise.
Noise barriers as exterior soundproofing
- main article: Noise barrier
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 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.