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Spécial “ Noise at work 2007 ”
Acoustique
&
Techniques n° 49
Acoustical evaluation and planning of offices
and disappointment caused by permanent distraction
at workplaces can be avoided. The designer of those
workplaces should make use of every possibility in order to
provide employees a suitable environment for their work.
Requirements
Normally the aims of an adequate acoustic design are
specified as the maximum allowable or aspired A-weighted
sound pressure level or rating level [6] respectively noise-
exposition level. These kind of limiting values mostly
are orientated on levels, whose shortfall is at limited
risk only for the occurrence of undesirable effects. The
criteria “hearing loss” according Directive 2003/10/EC or
“understandability of speech” – according to VDI-Guideline
2569 “Noise protection and acoustic design in offices”
[7] are insufficient for the formulation of objectives for
working places. They do not assure a desirable acoustic
atmosphere at workplaces.
Suitable requirements take into account the human
perception and the reaction of persons concerned. As a
matter of principle any kind of noise exposure that is not
caused by the considered person itself is to be regarded as
disturbance when performing a concentrative activity. Self-
induced noise, such as the own speech and that of one's
conversational partner, the noise of one's keyboard and
expected communicational noise (e.g. one's own telephone
ringing) are perceived differently than noise from technical
devices (e.g. computers), noise caused by other people
and that which one does not have influence on. These
noises are perceived as annoyance.
In order to describe emission and immission of noise
the most important indicator for annoyance by noise is
the average sound pressure level in dB(A) respectively
rating level at the workplace. The term noise level will be
used in the following as an abbreviation for the average
A-weighted sound pressure level related to a specific
time interval. At a given noise level a monotonous noise
which is not assignable to a specific source, is perceived
as less distracting than noise which is relatable to a
source because of its temporal structure, frequency
spectrum or its level difference to the background
noise. The degree of annoyance is especially high, if noise
is caused by understandable speech. The following three
complains, which were condensed of numerous statements
of everyday's life at offices meet the requirements in a
pragmatic way.
- Criterion 1 (Annoyance caused by a single
dominating source):
The difference between the part of the noise level caused
by an individual source and the lower part of the noise
level caused by the remaining sources should not exceed
4 dB. Otherwise the workplace is to be classified as
“unfavourable”.
-
Criterion 2 (Annoyance caused by speech):
Speech from workplaces of other functional groups and
from beyond should not be understandable. The difference
between the part of noise level caused by the speech of
people who are not linked to the same functional area and
the higher part of noise level caused by the remaining
sources has to be at least 3 dB. Otherwise the workplace
is to be classified as “unfavourable”.
-
Criterion 3 (continuous noise load):
The noise level caused by all sources together should be as
low as possible, in times, when the person at the considered
workplace is not working, but prepared to start working,.
The qualification can be made according to table 1
These three requirements are applicable for different office
sizes. If the criteria 1 till 3 come to different estimation the
most unfavourable applies for the characterisation of the
workplace.
Noise sources
The requirements refer to the noise level at workplaces
caused by all sources. In order to estimate the noise levels
at workplaces inside a planned office, a calculation based
on the emission values of all single sources have to be
done according to the methods for the calculation of sound
propagation [8].
In large scaled offices with many workplaces a lot of
sources contribute to an overall level which in the following
will be called also background level Lb.
In large scaled offices with many workplaces a lot of
sources contribute to an overall level which in the following
will be called also background level Lb.
- Noise from sources which are located inside
the room
:
These are machines and equipment such as computers,
printer and copier. Further sources that often cause
significant acoustical problems inside offices are other
people talking as well as other noise, connected to their
communication (e.g. ringing of telephones).
- Other noises particularly from outside the room
:
However the bigger the floor space of the office, the
smaller is the importance of noises from outside. They are
primarily caused by
:
- noise from outside the building (e.g. traffic noise)
- noise from neighbouring rooms (e.g. other offices,
production or plant rooms)
- noise from ventilation and air condition (this has to be
observed especially if the regarded office itself is ventilated
and therefore air discharge openings are present) o noise
from other building services such as heating installations,
elevators, gates of underground parking lots.
Noise Level (rating level at
the workplace)
Acoustical quality of office
workplaces
≤
30 dB(A)
excellent
> 30 dB(A) and
≤
40 dB(A)
very well
> 40 dB(A) and
≤
45 dB(A)
well
> 45 dB(A) and
≤
50 dB(A)
acceptable in large offices
> 50 dB(A) and
≤
55 dB(A)
unfavourable
> 55 dB(A)
Noise load too high
Table 1 : Acoustical quality of office workplaces