Noise
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Noise from workstations
There are a variety of workstations on the market today, all with different
noise levels and different spectra. From a human factors perspective, however,
it can be assumed that the majority of these are destined for a quiet office
type of environment, and that the maximum Aweighted sound pressure levels
about one meter away from the workstation (either caused by the workstation
equipment itself or by other sources in the room) should not exceed 50 dB.
This would mean that, to safely insure against customer or employee complaints
about excessive noise, an average Aweighted level of 40-45 dB should be the
design goal. To minimize the annoyance, the spectrum of the noise should
be as "smooth" as possible (so-called broadband noise, having a
sound similar to that of rushing water). Discrete tones, or narrowband noise
(perceived as hums, whistles, or whines), generally add to the annoyance
and should be avoided.
In the past, a common complaint about workstation noise concerned the highfrequency
tone emanating from the CRT display (usually around 16 kHz), which was generated
by the electron beam horizontal sweep circuitry. Although the actual sound
pressure level of such a tone was low, it was perceived as a shrill, "piercing" sound,
and was sometimes quite annoying. The sweep circuitry is still present in most
modern displays, but the sweep rate has been shifted to a much higher frequency
(typically 30 kHz or more) so that the resulting tone is above the range of
human hearing. High frequency tones are thus no longer a problem in modern
workstations and complaints of annoyance from such tones should be rare.
The keyboard is also a source of noise in a modern workstation. Although
a person is usually not annoyed by the clicking of his own keyboard, this noise
may be annoying or distracting to others in the vicinity. Still, complaints
about keyboard noise from workstations do not appear to be common, and should
not cause too much concern for the human factors specialist.
Rather than the display or keyboard, the primary noise source in a modern
workstation is the system unit, or the multiple system units in some of the
more powerful workstations. The small cooling fans and disk drives in these
units are responsible for most of the noise output, and the fans often emit
an annoying lowfrequency discrete tone in addition
(perceived as a
humming sound). Most complaints of excessive
noise from workstations can
be traced to the system unit.
Improving the work environment for users of modern workstations can be accomplished
in at least three stages. First, noise control engineering should be a major
concern at the design phase of development. Noise control is most effectively
accomplished when the source of the noise can be controlled. Indeed,
noise control is less successful after the product is built, when the attempt
is to control the path from noise source to the listener. Second, when
a workstation is chosen or purchased for a particular application, "low
noise emission levels" and "absence of discrete tones" should
be criteria in the selection process. Finally, when workstations are newly
installed, or modified in an existing installation, special attention to
the location of the workstation and acoustical
treatment of room surfaces can also have a significant effect on the
levels of noise at a user's position.
For those interested, Figure 59 shows the sound pressure levels for a
variety of sound sources that are found in the environment.
Figure 59. Range of typical A-weighted sound pressure levels.
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