Noise

	Overview   |   The basics of sound   |   Frequency   |   Measurement and rating of noise   |   Noise from workstations

Measurement and rating of noise
Attempts are made to quantify both the objective parameters of noise (such as sound amplitude, frequency, spectral content) and the subjective parameters (such as loudness and annoyance). Objective measurements
are relatively simple using modern measuring instruments, and the primary metric is the sound pressure level in decibels. Quantifying the human subjective responses to noise, particularly that of annoyance, is extremely difficult. Many schemes and metrics have been invented for various noises, and debates continue as to their effectiveness.

Sound pressure level measurements are taken for two principal reasons:
to characterize a source or to characterize an environment. The distinction must be kept in mind to avoid confusion in interpreting or defining noise level requirements.

A particular noise source (such as a workstation) emits a certain amount of sound power; sound pressure level measurements may be taken around that machine when operating alone in a special test room to determine its sound power level. On the other hand, sound pressure level measurements may be taken at a point in an ordinary room (such as the user's position in front of a workstation) to determine the noise level at that point in the environment. This measurement indicates what noise level the user may be exposed to, but it does not accurately characterize the sound power output of the workstation, because (i) there may be other noise sources in the
room contributing to the sound pressure level at the measurement point and (ii) the boundaries and obstacles in the room reflect the sound emitted from the workstation back to the measurement point.

Manufacturers want to lower the sound power levels of their products (regardless of the environment they are destined for), and customers or employees want to lower the sound pressure levels to which they are exposed (regardless of the source or sources of the noise).

In order to characterize the spectrum of a particular noise, the sound pressure level must be measured at different frequencies, or in different frequency bands. Thus, noise levels may be specified or reported in terms
of several octaveband or onethird octaveband levels. Alternatively, a singlenumber rating is commonly used to express the overall level of the noise. To do this, a standardized frequency weighting, called Aweighting,
has been internationally agreed upon and incorporated into almost all sound level measuring instruments. This weighting reflects the nonuniform sensitivity of the human ear by attenuating very low and very high frequencies relative to the middle frequencies. A measurement made using Aweighting will yield the Aweighted sound pressure level in decibels. Figure 59 shows the range of typical Aweighted sound pressure levels of some common sounds and environments (see, for instance, Harris, 1979).

In regards to standardization, there are many national and international standards specifying noise measurement methods, both for sound power levels of machines (e.g., ANSI S12.10 1985) and sound pressure levels in the environment (e.g., ANSI S1.13 1971). On the other hand, there are few standards specifying actual noise limits or criteria, due to the political and legal consequences of such specifications (monitoring, enforcement, employee or consumer compensation, etc.). In recent years, though, workers and consumers have begun to demand quieter environments and quieter products, and several new proposals for noise limits have been introduced into the standardization and regulatory process. For illustration only, most of these proposals are aimed at establishing maximum environmental noise limits similar to the following (measured at a typical point, or averaged over several points, none of which is close to any one noise source):

The first category might include manufacturing areas and construction sites; the second, retail stores and hotel lobbies; the third, customer reception and conference areas in financial or consulting agencies; and the fourth, private offices and scientific laboratories. It must be stressed that the levels listed are maximum values, and that the average noise level in the environment is expected to be several decibels lower.

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