Page 396 - Industrial Ventilation Design Guidebook

5 4 VENTILATION NOISE—CHARACTERISTICS, EFFECTS, AND SUGGESTED COUNTER-MEASURES 3 5 i

5,4.5.5 Effects on Hearing
The most usual effect of exposure to ventilation noise, as previously
mentioned, consists of annoyance and disturbance of various kinds. Such
effects may occur as a result of the relatively low levels of exposure occur-
ring in offices, schools, etc. In industrial environments, workshops, ware-
houses, etc., however, the levels from a fan system may sometimes even
reach the level of risk of hearing damage or of speech-masking. The risk of
hearing damage and the speech-masking effect arise at levels around 70
dB(A), Pronounced or well-defined health effects expressed as a function
ot long time or repeated exposure to ventilation noise have not been dem-
onstrated. However, the possibility that repeated exposure to ventilation
noise may cause increased stress and in this way may have an effect on
health cannot be ruled out. An increased risk of stress-related complaints
may occur, not least because human ability to acclimatize to low-frequency
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noise seems very limited.

5.4.6 Measures
The significant differences in disturbance, when evaluated with regard to the
average noise levels, shows that the noise level is a decisive factor with regard
to disturbance. Measures to limit the disturbance reaction due to ventilation
noise should, therefore, naturally be directed in the first instance at lowering
the noise level.
The extent to which a ventilation noise is perceived as disturbing depends
not only on its dB(A) level, but also on the spectral distribution and the pres-
ence of tones or intermittent components in the noise. From an experiment
carried out on respondents exposed to ventilation noises with different charac-
teristics in a simulated office room, it emerged that the highest acceptable level
was about 7 dB higher for ventilation noise with a superimposed tone at 30
8
Hz than for other types of noise. In another experiment, it was found that the
tolerance level was much higher for a tone than for a noise at 100 Hz, whereas
the opposite tendency applied at 1000 Hz. 8
Earlier experiments indicate clearly that a lowered sound pressure level
can be an effective measure to reduce the inconvenience reactions due to a
ventilation noise, provided that it is targeted at the most critical frequency
range from the point of view of influence or that the measure results in a gen-
eral lowering over the entire spectral range of the ventilation noise,

5.4.6. I Elimination of Different Ventilation Noise Sources
Efforts to reduce the noise from a ventilation system may be concentrated
on measures concerning the fan, the fan room, the fan ducts, and the supply
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and exhaust air terminals. '
5.4.6.2 The Fan
Fans with poorly designed or excessively simple straight blades should
be replaced with quality fans with lower noise generation. As the accumu-
lation of dirt on impellers often causes imbalance, leading to vibration and