Noise Control

1
Noise Control

• A Guide for Workers and Employers

2
Steps used to solve Noise Control Problems

• Define the problem
• Establish a noise reduction target
• Determine noise contributors and relative
  contribution of each
• How many sources must be reduced to realize the
  target?
• Construct model or mockup, as necessary
• Determine effect of materials handling and
  operational compromises

3
Octave Band Analysis

• Obtain a frequency spectrum of the noise

4
Resonance

• Each object will resonate, or strengthen a
  sound, at one or more particular frequencies.
  The frequency depends on the size and the
  construction of the object.

5
Sound reduction by distance

• Sound spreading in open air and measured at a
  certain distance from the source is reduced by 6
  dB for each doubling of that distance. Sound is
  reduced less when spreading inside a room.

6
Sound Transmission Loss

• TL When a wall is struck by sound, only a small
  portion of the sound is transmitted through the
  wall, while most of it is reflected. The walls
  ability to block transmission is indicated by its
  transmission loss rating, measured in decibels.

7
Sound Absorption

• Sound is absorbed when it strikes a porous
  material. Commercial sound-absorbing materials
  usually absorb 70 percent or more of the sound
  that strikes them.

8
Noise Reduction

• Noise reduction is the number of decibels of
  sound reduction actually achieved by a particular
  enclosure or barrier. This can be measured by
  comparing the noise level before and after
  installing an enclosure over a noise source.

9
Source controls
10
Dual flow mouthpiece

• Jet noise (flow streams in excess of 325 f/s) can
  be reduced by using an extra air stream
• Reducing the outflow speed by half may decrease
  the noise level by as much as 20 dB
• See page 61 cleaning with compressed air

11
Larger belts produce lower frequency noise

• Several narrow belts separated by spacers will
  produce less noise.
• When a large belt vibrates, excess air pressure
  forms on one side of the belt and then the other.
  Sound comes from both sides. The pressure
  difference balances out close to the edges, so
  the radiation there is slight. Therefore,
  several narrow belts will produce less noise than
  one larger belt

12
Resonance

• The glass shown on page 63 resonates. One finger
  can stop this noise.
• Dampening material such as shown below the glass
  can be used to help reduce resonance of
  reciprocating machines.
• Saw blades are known for the ringing sound that
  is produced when cutting. A urethane rubber
  coating clamped to the saw can help reduce this
  ringing.

13
Damping material

• Page 65 Notice how the addition of a damping
  panel lowered the peak resonances.

14
Damping material in a hopper

• Covering surfaces of materials conveying
  operations with resilient damping material can
  help reduce noise levels of parts hitting large
  solid metal hoppers

15
Low speed dropping of material

• When a plate is struck by an object, the plate
  vibrates and makes noise. The sound level is
  determined by the weight of the material and the
  striking speed. If the dropping height of an
  object is reduced from 16 feet to 2 inches, the
  sound level drops about 20 dB.
• Page 67

16
High frequency noise is reduced by passing
through air

• If low frequency noise is causing a problem with
  residential areas, the noise can be shifted to a
  higher frequency by replacing the rooftop fan
  with another fan of similar capacity but with a
  larger number of fan blades. This will produce
  less low frequency noise and more high frequency
  noise
• It is also easier to insulate and shield

17
Structure borne noise

• Sound through solid connections can be blocked.
  The flexible connection will help reduce the
  noise that transfers through a rigid connection
  from the pipe to the wall.
• Page 69

18
Vibration Isolators

• Machines should be vibration isolated.
• Vibration isolators are made of various materials
  and in various shapes
• Foam, rubber, plastic, mineral wool, cellular
  material, dense rubber/plastic, cork
• Wire coils, springs, leaf spring, or plate springs

19
Lower repetition produces lower frequency noise

• The greater the number of teeth, the greater the
  frequency of the noise that is produced. Since
  the ear is less sensitive to lower frequency
  noise, there may be times where producing a lower
  frequency noise is advantageous.
• Page 71

20
Force, pressure or speed produces noise

• Louder noises will be produced if a task is
  carried out with great force for a short time
  than with less force for a longer time.
• Tools that fit the operation can still be used,
  it does not have to be a manual operation.
• Page 72

21
Mufflers

• The exhaust air from a compressed air-driven
  grinding machine produces a loud noise. A porous
  sound-absorbing muffler replacing the handle can
  break up the turbulence, and the less disturbed
  exhaust produces a weaker noise.
• Page 73

22
Enclosures

• High frequency noise is strongly directional and
  more easily reflected.
• Sound absorbing material placed within the
  enclosure will reduce the noise. The safety
  glass blocking the direct radiation of noise from
  the rivetting hammer will reflect the noise back
  into the enclosure.