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| Acoustical Barriers
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An energy source produces noise as a radiating sound pressure wave which moves
through the air in all directions. The most effective, economical and flexible method
of noise reduction is usually the construction of a barrier (or enclosure) between the
noise source and the receiver.
Barriers prevent the transmission of sound, but do not absorb sound. With a
barrier, the sound is reflected back in the direction of its source.
The essential physical characteristic of a sound barrier is mass. Heavy, dense
materials are good barriers, while soft, porous materials are poor barriers.
The second important characteristic of a good barrier is limpness. A rigid
barrier material can transmit vibration and regenerate noise on the other side of the
barrier, while a limp material will not shake or vibrate in a sound field.
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| Acoustical Absorbers
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Sound absorption is necessary to reduce the intensity within a room or enclosure.
The process of absorption depends on the sound wave entering the material and being
converted to heat by a frictional process on the porous material surface and cells.
The essential physical characteristics of absorbers is controlled porosity. Sound
absorption is intended to reduce noise reverberation from reflective surfaces. Since the
sound wave must flow through an absorbing material, its effectiveness as a sound barrier
is very limited.
It is important to consider the use of sound absorbing material on the inside
surfaces of a noise barrier, especially when a full or partial enclosure is being
designed. The lack of sound absorbing materials causes a highly reverberant condition
inside the enclosure, thus defeating the effectiveness of the design.
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| Vibration Damping
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To control vibration, it is necessary to prevent the structural transmission of
vibrational energy between the source and the surface. Vibrating surfaces are frequently
damped by applying visco-elastic materials directly to the surface converting the
vibrational energy to heat.
The minute flexing of the damping materials provides the energy dissipation and
"decay" to reduce noise. Metal no longer "rings" when struck. Damping materials are
primarily used on light guage vibrating metals, but may also be effective on wood or
plastic.
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