Abstract
In everyday life people are simultaneously exposed to several sound
sources, which emerge from background soundscapes of considerable
variability due to building layouts, residential pattern, topography,
meteorology, and lifestyle.In contrast, noise regulation, planning,
and control treat the sound environment by separating it into pieces
and describe it by a one-number indicator. This practice ignores
the possibility of any effect modification (by inhibition, partial
or full additivity, or synergism). This effect modification can take
place not only between sound sources (multisource issue) but also
with simultaneously occurring environmental factors (vibration and
air pollution) from the same source or through other contextual factors
(multistressor issue).What people know about auditory perception
of combined sound exposure mainly rests on experimental work using
short-term loudness judgments in repeated designs in controlled settings.
Recent psychoacoustic experiments did not find full support for the
most prevalent models in practice (e.g., simple energy summation),
when the context of the assessment is more carefully distinguished
(sound heard within combined sound or alone). The findings are difficult
to compare with field studies where long-term judgments of annoyance
take place in the immediate context of the subject's living environment.The
combined noise paradox is such a finding. It describes the phenomenon
that total annoyance is often judged equal or even lower than the
dominant source alone. Some call it compromise judgments suggesting
them to result from ambiguous questions or misinterpretations of
the frame of reference when total annoyance should be assessed. In
experiments, compromise judgments were observed mainly with unequally
loud and time-separated sounds.But also increases in total annoyance
have been observed in the field studies. Although masking partly
explains lower annoyance, higher annoyance due to equally loud sources
is less well understood.Further effect modifications have been observed
with simultaneously occurring vibrations, low-frequency annoyance,
and tonal and impulsive components of heterogeneous sound sources
such as those from industry. The larger annoyance effects observed
can vary in terms of decibel equivalents between −3 and +15 dBA.
The highest values are associated with impulsive noise and low background
noise context.Eventually, air pollution and other contextual factors
can further contribute to total annoyance. Incomplete accountance
of effect modifications in environmental risk assessments will lead
to errors in planning and less than optimal noise control.
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