Noise Control

A Guide for Workers and Employers

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

Octave Band Analysis

• Obtain a frequency spectrum of the noise

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.

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.

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 wall’s ability to block transmission is indicated by its transmission loss rating, measured in decibels.

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.

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.

Source controls

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

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

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.

Damping material

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

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

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

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

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

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

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

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

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

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.