Noise in Industry - Fundamentals and Assessment Tools for Occupational Ergonomics - page 635

Noise exposure records may be used as feedback to identify machines that need maintenance attention,
to assist in the relocation of noisy equipment during plant layout efforts, to provide information for
future equipment procurement decisions, and to target plant areas that are in need of noise control inter-
vention. Some employers plot noise levels on a “contour map,” delineating floor areas by their decibel
levels. When monitoring indicates that the noise level in a particular contour has changed, it is taken
as a sign that the machinery or work process has changed in the area and that further evaluation may
be needed.

31.5.2.2.6 Engineering Noise Control
While OSHA does not stipulate the level of effort to be devoted to engineering noise controls or the types
of controls that should be applied, the physical reduction of the noise energy, at its source, in its path, or
at the worker, should be a major focus of noise management programs. Hearing protection or admin-
istrative controls should not supplant noise control engineering; the best solution, because it does not
rely on employee behavior, is to reduce the noise itself, preferably at the emission source. However, in
many cases where noise control is ineffective, infeasible (as on an airport taxi area), or prohibitively
expensive, HPDs become the primary countermeasure.
There are many techniques used in noise control, and the specific approach must be tailored to the
noise problem at hand. A noise control engineer is typically consulted to assist in the measurements,
usually taken from spectrum analyzers, and in the selection of control strategies. Example noise
control strategies include: (1) isolation of the source via relocation, enclosure, or vibration-damping
using metal or air springs (below about 30 Hz) or elastomer (above 30 Hz) supports; (2) reduction at
the source or in the path using mufflers or silencers on exhausts, reducing cutting, fan, or impact
speeds, dynamically balancing rotating components, reducing fluid flow speeds and turbulence, absorp-
tive foam or fiberglass on reflective surfaces to reduce reverberation, shields to reflect and redirect noise
(especially high frequencies), and lining or wrapping of pipes and ducts; (3) replacement or alteration of
machinery, examples including belt drives as opposed to noisier gears, electrical rather than pneumatic
tools, and shifting frequency outputs such as by using centrifugal fans (low frequencies) rather than pro-
peller or axial fans (high frequencies), keeping in mind that low frequencies propagate further than high
frequencies, but high frequencies are more hazardous to hearing; and (4) application of quieter materials,
such as rubber liners in parts bins, conveyors, and vibrators, resilient hammer faces and bumpers on
materials handling equipment, nylon slides or rubber tires rather than metal rollers, and fiber rather
than metal gears. Further discussion of these techniques may be found in Bruce and Toothman, 34 and
an illustration of implementation possibilities in an industrial plant appears in Figure 31.9. A final
approach that has just recently become available to industry is active noise reduction (ANR) in which
an electronic system is used to transduce an offensive noise in a sound field and then process and rein-
troduce the noise into the same sound field such that it is exactly 180 out-of-phase with, but of equal

amplitude to the original noise. 22 The superposition of the out-of-phase “antinoise” with the original
noise causes physical cancellation of the noise in a target zone of the workplace. For highly repetitive,
predictable noises, synthesis of the antinoise, as opposed to transduction and reintroduction, may also
be used. At frequencies below about 1000 Hz, the ANR technique is most effective, which is fortuitous
since the passive noise control materials to combat low-frequency noise, such as absorptive liners and
barriers, are typically heavy, bulky, and expensive. At higher frequencies and their corresponding
shorter wavelengths, the processing and phase relationships become more difficult and cancellation is
less successful, although the technology is rapidly improving.
In designing and implementing noise control hardware, it is important that ergonomics be taken into
account. For instance, in a sound-treated booth to house an operator, the ventilation system, lighting,
visibility outward to the surrounding work area, and other considerations relating to operator
comfort and performance must be considered. With regard to noise-isolating machine enclosures,
access provisions should be designed so as to not compromise the operator

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machine interface. In this
regard, it is important that production and maintenance needs be met. If noise control hardware

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