In existing facilities, quantification of the noise problem involves identification of the source or
sources, determination of the transmission paths from the sources to the receivers, rank ordering
of the various contributors to the problem and finally determination of acceptable solutions.
To begin, noise levels must be determined at the locations from which the complaints arise.
Once levels have been determined, the next step is to apply acceptable noise level criteria to
each location and thus to determine the required noise reductions, generally as a function of
octave or one-third octave frequency bands (see chapter 1).
Once the noise levels have been measured and the required reductions determined, the next
step is to identify and rank order the noise sources responsible for the excessive noise. The
sources may be subtle or alternatively many, in which case rank ordering may be as important
as identification. Where many sources exist, rank ordering may pose a difficult problem.
When there are many sources it is important to determine the sound power and directivity of
each to determine their relative contributions to the noise problem. The radiated sound power
and directivity of sources can be determined by reference to the equipment manufacturer's data
(ISO 4871) or by measurement, using methods outlined in chapter 1. The sound power should
be characterised in octave or one third octave frequency bands (see chapter 1) and dominant
single frequencies should be identified. Any background noise interfering with the sound power
measurements must be taken into account and removed (see chapter 1).
NOTE: This is the ideal procedure. In reality, many people choose machinery or equipment
using only noise emission values according to ISO 4871 and they make comparisons according
to ISO 11689.
Often noise sources are either vibrating surfaces or unsteady fluid flow (air, gas or steam).
The latter are referred to as aerodynamic sources and they are often associated with exhausts.
In most cases, it is worthwhile to determine the source of the energy which is causing the
structure or the aerodynamic source to radiate sound, as control may best start there.
Having identified the noise sources and determined their radiated sound power levels, the
next task is to determine the relative contribution of each noise source to the noise level at each
location where the measured noise levels are excessive. For a facility involving just a few noise
sources, as is the case for most occupational noise problems at a specific location, this is usually
a relatively straightforward task.
Once the noise sources have been ranked in order of importance in terms of their
contribution to the overall noise problem, it is often also useful to rank them in terms of which
are easiest to do something about and which affect most people, and take this into account when
deciding which sources to treat first of all. This is discussed in more detail in Chapter 7.