Page 397 - Industrial Ventilation Design Guidebook

352 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS

unnecessary noise, these should be cleaned regularly. Imbalance, whether
due to dirt or to other causes, should be corrected by adjustment. Defective
bearings should be adjusted or replaced. Struts and sharp edges in front of
an impeller should be avoided. Impact noise insulation should be intro-
duced between fan room and floor structure and between fan room and
connecting ducts. The fan unit should be enclosed, with satisfactory air-
borne sound insulation as the objective.

5.4.6.3 The Fan Room
The roof and walls of the fan room should be lined internally with absor-
bent materials in order to reduce the sound level in the fan room. The wall in-
sulation should be sufficient to reduce the transmission of sound to adjoining
silent premises.

5.4.6.4 The Fan Ducts
Straight, internally smooth ducts should be avoided as these give very
little noise reduction. Fan noise can pass virtually unobstructed. Silencers
should be installed inside the ducts by covering the walls with absorbent
material. In this way noise in the higher-frequency range may be reduced.
The low-frequency components of the fan noise are more difficult to at-
tenuate. Very thick absorbent linings are needed to reduce such noise. Al-
tering the area of the duct produces a damping of the noise because parts
of the sound are reflected back into the duct. This kind of damping,
which may also be achieved at the openings of a ventilation duct, is most
effective for low-frequency noise. By inserting internally smooth bends in
the duct, damping of the noise may be obtained. The larger the duct
widths, the better the damping for low frequencies. Narrow ducts
dampen very little. Sound-absorbent bends may produce very sharp re-
ductions in noise level. High frequencies are dampened most easily with
duct bends. To obtain damping in the low-frequency range with bends,
wide ducts are required. Larger spaces with absorbent walls, "absorption
chambers," built into the ductwork, also give effective damping in the
lower frequency ranges. The ventilation duct often consists of large
noise-generating surfaces which may need to be insulated or enclosed.
Counter noise may be an alternative method of reducing the noise level in
a ventilation duct. However, this method is relatively costly compared
with other technical solutions.

5.4.6.5 The Supply and Exhaust Air Terminals
There is usually a certain damping of the fan noise at the opening of the
duct. The damping is greatest if the opening consists of a pipe projecting clear
of the wall. The noise radiation is also lowest if it is on the level of the roof or
the wall in a corner of the room. Excessive air velocities in the opening may
also cause noise in the terminal device, as may inappropriately shaped devices
with sharp edges, etc. Supply and exhaust air terminal devices may be fitted
with silencers or absorbents.