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Noise Reduction

Acoustical Barriers
  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.

Acoustical Absorbers
  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.

Vibration Damping
  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.