Noise exposure, in many cases, can be controlled. No matter
what noise problems may be in a particular workplace, technology
exists to reduce the hazard. It may be possible to:
Sometimes, the assistance of noise abatement design engineers
will be needed. In other cases, where the exposure is not
very severe and the control is straightforward, employers
working with a noise control products vendor can develop practical
and effective controls.
Either way, controlling the noise exposure reduces the reliance
on hearing protection and audiometric tests as a means of
protecting employees hearing. Where noise controls can reduce
exposure below 85 dBA, the need for a continuing Hearing Conservation
Program is eliminated.
Airborne sound is usually caused by vibration
in solids or turbulence in fluids.
For example, a circulation pump can produce pressure variations
in the water of a heating system. The sound waves are transmitted
through the pipes to the radiators, whose large metal surfaces
transmit airborne sound.
High frequency sound is strongly directional
and more easily reflected.
When high frequency sound strikes a hard surface, it is
reflected much like light from a mirror. High frequency
sound, unlike low frequency sound, does not travel around
corners easily.
Small vibrating surfaces give off less noise
than large ones.
An object with a small surface area may vibrate intensely
without creating a great deal of noise. The higher the frequencies,
the smaller the surface must be to prevent disturbance.
Since machines always will vibrate to some extent, noise
control will be aided if machines are kept as small as possible.
Densely perforated plates produce less noise.
Large vibrating surfaces cannot always be avoided. The
vibrating surface pumps air back and forth like a piston
of a pump, causing sound radiation. If the panel is perforated,
the "piston" leaks, and the pumping (of air) functions
poorly. Alternatives to perforated plates include mesh,
grating, and expanded metal.
Plates with free edges produce less low frequency
noise.
If a plate vibrates with free edges, pressure equalization
takes place between the two sides of the plate, thus reducing
sound emissions. Clamping the corners prevents pressure
equalization and the sound emission is greater, especially
at low frequencies. For example, speakers produce more bass
if they are enclosed in a cabinet.
A damped surface gives off less sound.
As vibration moves through a plate, it gradually decreases
as it travels, but in most plates, this reduction is rather
small. In such cases, the material is said to have low internal
damping. Internal damping in steel, for example, is extremely
poor. Good damping can be achieved by adding coatings or
intermediate layers with better internal damping.
Resonance increases noise but it can be damped.
Resonance greatly increases noise from a vibrating plate,
but it can be suppressed or prevented by damping the plate.
It may often be sufficient to damp only part of the surface,
and, in some rare cases, damping of a single point is effective.
Undisturbed air flow produces the smallest
amount of exit noise.
When a flowing gas mixes with a non-moving gas, such as
occurs at pneumatic tool exhausts, noise may be produced,
especially if the flow is disturbed before the outlet.
A lower outflow speed will produce a lower sound level.
Jet noise can be reduced by using an extra
air stream.
The term "jet stream" applies at flow speeds
in excess of 325 feet/sec. Turbulence outside the outlet
is great. Reducing the outflow speed by half may decrease
the noise level by as much as 20 dB. Since the noise level
is determined by the speed of the jet stream in relation
to the speed of the surrounding air, noise production can
be greatly reduced by using an air stream with a lower speed
outside the jet stream.
Fans make less noise if placed in smooth, undisturbed
flow streams.
A fan produces turbulence in air, which causes noise.
If turbulence is already present in the incoming air, the
sound will be more intense.
Noise Control Measures - Machinery & Equipment
The machines or machine parts to be controlled must be
identified. Methods of maintenance and servicing must be
taken into account in noise control design. Attempts should
be made to:
- prevent or reduce impact between machine parts.
- reduce speeds gently between forward and reverse movements.
- replace metal parts with quieter plastic parts.
- enclose especially noisy machine parts.
Designers should be encouraged to:
- select power transmission which permits the quietest
speed regulation; for example, rotation-speed-controlled
electric motors.
- isolate vibration-related noise sources within machines.
- provide proper TL and seals for doors on machines.
- provide machines with effective cooling flanges to reduce
the need for air jet cooling.
Noise of existing equipment can often be controlled as
effectively as new equipment without complicated procedures.
Common control measures include:
- providing mufflers for the air outlets of pneumatic
valves.
- changing the pump type in hydraulic systems.
- changing to quieter types of fans or placing mufflers
in the ducts of ventilation systems.
- providing mufflers for electric motors.
- providing mufflers for intakes of air compressors.
In a new plant, it is often sometimes possible to make
more extensive changes, such as:
- installing quiet electric motors and transmissions.
- selecting hydraulic systems which have remote oil tanks
and accumulators at pump discharges, and designing pipelines
for low flow speeds (maximum 5 meters/sec).
- designing ventilation ducts with fan inlet mufflers
and other mufflers to prevent noise transfer in the ducts
between noisy and quiet rooms.
Maintenance
In some cases, a noise hazard will be created or made worse
by a lack of maintenance. Parts may become loose, creating
more noise because of improper operation or scraping against
other parts. Grinding noises may also occur as the result
of inadequate lubrication.
It is especially important to provide proper maintenance
of noise control devices which are added or built into machinery.
If a muffler becomes loose or worn out, for example, it
should be fixed or replaced as soon as possible.
Noise Control Measures - Barrier and Enclosures
Shields and Barriers
An acoustical shield is a solid piece of material placed
between the worker and noise source. It is often mounted
on a machine. An acoustical barrier is a larger piece of
solid material, usually free-standing on the floor. Both
the barrier and shield work by deflecting the flow of acoustical
energy away from the worker. They are most effective when:
- The worker is close to the noise source;
- The smaller dimension of the shield or barrier is at
least three times the wavelength contributing most to
the noise exposure received; and,
- When the ceiling and other nearby reflecting surfaces
are covered with sound absorptive materials.
The materials usually selected for shields and barriers
provide considerably greater transmission loss than 8 to
10 dB. Typical materials used are light-gauge sheet metal,
1/2 inch plywood, 1/4 inch clear plastic, or safety glass.
Refer to Figure A-1.
Partial Enclosures
A partial enclosure is a barrier that is wrapped around
a machine, with its top more or less open. Such an enclosure
can be effective in reducing noise to workers nearby. The
noise, however, escapes through the top and contributes
to the reverberant sound in the workroom. These spill-over
noise effects can be reduced by covering the inside of the
enclosure with acoustically absorbent material. Also, suspended
acoustically absorbent baffles may be placed over the openings
to reduce the escaping noise. The enclosures must be designed
to allow access to the equipment they enclose for maintenance
purposes. Partial enclosures can provide as much as 12 to
15 dB of noise reduction. Refer to Figure A-2.
Total Enclosures
Total enclosures are used when more than 12 to 15 dB of
noise reduction is required. By virtue of their design,
total enclosures can cause a heat build-up problem. This
is handled by adding a ventilation blower, together with
silencers for both supply and exhaust air. As a general
matter, enclosures should not touch any part of the machine
and should be vibration-isolated from the floor. As with
the partial enclosure, the total enclosure should be designed
to allow access to the equipment for adjustment and maintenance
purposes. Refer to Figure A-3.
Construction Materials
Shields, barriers and enclosures are typically constructed
out of a combination of Transmission Loss materials and
sound absorptive materials. The Transmission Loss materials
absorb and reflect noise energy. Absorptive materials actually
absorb noise by converting the sound pressure energy into
heat. Absorptive materials are incorporated into barriers
and enclosures to reduce reflected noise.
Absorption Materials
With absorption, small amounts of sound energy are changed
into correspondingly small amounts of heat energy. Suitable
materials are usually fibrous, lightweight, and porous.
The fibers should be relatively rigid. If a cellular material
is used, the cells must intercommunicate. Examples of
absorbent materials include acoustical ceiling tile, glass
fiber, and foamed elastomers.
Transmission Loss Materials
When a wall or barrier is struck by sound, only a portion
of the sound is transmitted through the barrier, while
the balance is reflected. The ability to block sound transmission
is indicated by the Transmission Loss (TL) rating, measured
in decibels. The TL of a wall or barrier does not vary
regardless of how it is used. Common materials used in
construction, such as gypsum wall board and concrete block
have significant TL characteristics (20 - 50 dB, depending
on the frequency). Special acoustic panels designed for
industrial noise enclosures are typically a metal sandwich
which provides for a significant amount of sound absorption
in addition to the transmission loss from reflection.
