Noise control methods

Every noise problem has three parts: source, path, receiver. Control is best applied at the source, then along the path, and last at the receiver.

  SOURCE  ---------- PATH ---------->  RECEIVER
  (reduce)      (barrier, enclosure,     (ear plugs,
                absorber, distance)       muffs)

1. Control at the source

• Choose quieter machines and processes (belt drives rather than gears, electric rather than pneumatic tools).
• Proper design, balancing of rotating parts, lubrication and maintenance to cut vibration and friction.
• Vibration isolation: spring mounts, rubber pads and flexible connections.
• Mufflers (silencers) for engine and fan exhaust: reactive (expansion chamber) and dissipative (absorptive lining) types.
• Reduce impact forces and speed, use damping materials on panels, and keep pipe velocity low.
• Legislation: limit noise from vehicles, horn restrictions, and no loudspeakers at night.

2. Control along the path

• Distance: noise falls 6 dB for each doubling of distance (point source).
• Barriers: a solid wall or earth bund between source and receiver gives 5-15 dB reduction for a wall that is high enough to break the line of sight; it should be dense (about 10 kg/m$^2$), without gaps.
• Enclosures: the machine is placed in a sealed box lined inside with absorbing material, with vibration breaks and silenced ventilation openings; gives 15-30 dB.
• Absorbing materials: porous materials (mineral wool, glass wool, foam, acoustic tiles, curtains, carpets) turn sound energy into heat and reduce the reverberant level inside rooms. Absorption coefficient $\alpha$ ranges from 0 to 1.
• Transmission loss (TL): heavy partitions and double walls with an air gap give high TL (mass law: 6 dB for each doubling of mass or frequency).
• Planning: zoning, buffer zones, trees (small effect, mostly psychological) and layout of buildings away from highways.

3. Protection at the receiver

• Ear plugs (reduce about 20-30 dB) and ear muffs (about 25-35 dB), limiting exposure time by job rotation, regular audiometric testing, and sound-proof control rooms for operators.