Page 392 - Industrial Ventilation Design Guidebook

5,4 VENTILATION NOISE—CHARACTERISTICS, EFFECTS, AND SUGGESTED COUNTER-MEASURES 347

the ventilation noise, would thus be ineffective in these cases. This conclusion is
based on the fact that the sound pressure levels of the ventilation noise frequency
under 50 Hz were significantly below the threshold curve of auditory perception,
and as such were not audible. This situation is considered to be representative of to-
day's ventilation noise in offices. The same results were obtained in a study by
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Paakkonen. It must be pointed out, however, that levels above the perception
threshold in the infrasound range can, of course, be generated from heavy-duty ven-
tilation systems, for example in factories, stores, and department stores.
It should also be pointed out that the levels from ventilation noise in, for ex-
ample, workshop premises are often lower than those emanating from other
sources, for example machines of various kinds. It is not uncommon for the venti-
lation noise from industrial premises to cause more disturbance in adjacent offices
than in the industrial premises themselves. The fact that ventilation noise propa-
gates in this manner is due to its pronounced low-frequency character. The more
low-frequency components in the noise, the greater the propagation. Sound radia-
tion from industrial premises, via duct openings in facades and roofs, may in this
manner also cause disturbances in nearby residential accommodations,

5.4.4 Noise Generation
The fan is usually the main source of noise generation in a ventilation system.
Rotating fans always constitute a source of noise generation. However, aero-
dynamically designed fan blades may reduce the noise generated. Another im-
portant source of noise is the bearings of various kinds inside the fan motor.
Defective bearings add directly to the ventilation noise. Another common rea-
son for high noise levels may be the imbalances which easily arise in a fan sys-
tem or ductwork. Imbalances give rise to vibration and hence to noise
generation. The noise generated by a fan may also be due to poor impact
sound insulation in the fan mountings and duct connections. Noise from drive
motors for the fan may cause strong radiation from a fan room, especially if it
has poor sound insulation.
The noise from the fan is propagated in the duct to the openings inwards
or outwards in the premises. The ventilation noise often propagates into the
surrounding area from the supply air and exhaust air terminal units in. the
rooms. The ducts in themselves may also be important sources of noise, par-
ticularly if they are poorly insulated or otherwise designed in such a way that
noise generation may occur.
Noise generation in the ducts may occur as a result of unfortunately posi-
tioned bends, inappropriate geometry, or imbalances. Supply and exhaust air ter-
minal devices may themselves create noise if the air velocity is high or the devices
are designed in such a way as to cause air turbulence. The higher the air velocity,
the greater the risk of noise-generating turbulence occurring in the ducts.

5.4.5 Effects on Humans

5.4.5.1 Influence on Disturbance and Working Performance
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The office workers involved in the study mentioned above rated the venti-
lation noise as "somewhat disturbing" to "quite disturbing" at the two work-
places where the level of exposure was about 40 dB(A). At the two workplaces