Night noise guidelines for Europe - WHO/Europe - World Health ...

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METHODS AND CRITERIA
1.3.6 BACKGROUND LEVEL
A simple definition of background level or “ambient noise” level is the noise that is
not targeted for measurement or calculation. Background noise can interfere with
the target noise in a number of ways. It can:
• mask the signal
• interact physically
• interact psychologically.
As this report is often dealing with low-level target noise, masking is an important
issue. The other two interactions are more important in the domain of annoyance.
Masking, however, is a complex process. The human auditory system is uncannily
good at separating signals from “background”. Microphones (and the software behind
them) have been slow to catch up, as the unsatisfactory results show when it comes to
automatically recognizing aircraft in long-term unmanned measuring stations.
The rule of thumb that a noise can be considered masked if the signal is 10 dB below
the background is only valid if the noises have the same frequency composition and
if they actually occur at the same time. This is particularly important to stress where
L Aeq levels are compared: even a relatively continuous motorway of 50 dB cannot
mask aircraft noise of 30 dB, because this may be composed of five aircraft arriving
at an L Amax of 57 dB. Neither can birdsong, because the frequency domains do not
overlap.
Another factor relevant for this report is that background levels are lower at nighttime
than they are in the daytime. This is true for most man-made noises, but also
for the natural background levels as wind speeds at night slow down.
Most levels mentioned in this report do not take background levels into account –
explicitly. Where long-term L Aeq levels are related to effects like hypertension and
self-reported sleep disturbance, background levels are ignored, but they could
obscure the effect at the lower end of the scale. This then influences the lowest level
where an effect starts to occur.
In sleep laboratory studies the background level is kept as low as possible, around
30 dB. The background of the instrumentation is 20 dB.
In semi-field experiments it has been found that background noise levels inside bedrooms
are very low, partly because people tend to choose their bedrooms on the
quiet side of the building. This may have the side-effect of exposing children to higher
levels.
1.3.7 CHOICE OF INDICATORS FOR REGULATORY PURPOSES
From the scientific point of view the correct choice for a noise indicator is its performance
in predicting the effect. There are, however, a number of additional criteria which
may influence the choice. Firstly, for different health end points different indicators
could be suitable. Further considerations are of a more political nature, as mentioned in
the Position Paper on EU noise indicators (European Commission, 2000). Indicators
should also be easy to explain to the public – intuitively understandable, avoiding
unnecessary breaks with current practice and enforceable. This is probably why in many
countries L Amax is a popular indicator: it has undeniable qualities in these areas.
NIGHT NOISE GUIDELINES FOR EUROPE