MIL-HDBK-1191
23.4.2
Duct-borne Fan Noise.
23.4.2.1
Introduction. All rotating equipment, and most predominantly fans
in air distribution systems, generate noise because of their energy
consumption and inherent design. This noise is transmitted to occupied spaces
through the air distribution system, both supply and return. The selection of
quieter, initially more expensive equipment is generally more economical in
the long run than the selection of a less expensive type of fan that requires
considerably more noise and vibration control, detailing, materials,
isolators, and constructions for sound separation. When practicable,
equipment should be specified and selected on the basis of low noise level
output; designers should schedule the maximum sound power output, per each
octave band, for air handling unit fans.
23.4.3
Air-Generated Noise.
23.4.3.1
Introduction. The movement of air for heating and ventilating
systems generates noise related to turbulent airflow. Turbulent airflow is
created by uneven flow distribution, higher air velocities, obstruction in the
air flow, and the like. Acoustically, low or medium air velocity systems are
most appropriate for use in medical facilities because low velocity
distribution generally requires less energy to move the air and also because
low velocity air movement greatly reduces the generation and regeneration of
noise produced by high velocities.
23.4.3.2
Airflow velocities. Listed below are the approximate ranges of
airflow velocities at the face of the terminal devices (diffusers and grilles)
and in the last 1 to 2 meters (3 to 6 feet) of duct serving a space, required
to achieve specific ambient noise levels. These noise levels are represented
by NC (Noise Criteria) curves (ref. Figures 23.6) and assume no additional
noise contribution due to duct-borne fan noise or air turbulence.
Noise Criterion (NC)
Terminal Airflow Velocity
Range
meters per second
(feet per minute)
NC-25 to NC-30
1.8 to 2.2
(350 to 425)
NC-30 to NC-35
2.2 to 2.5
(425 to 500)
NC-35 to NC-40
2.5 to 3.0
(500 to 600)
23.4.4
Cross-Talk Between Spaces.
23.4.4.1
Introduction. Cross-talk is the transfer of sound, such as
intelligible speech, from one room to its neighbor by way of a common unlined
supply duct or by way of a common return air path (either a duct or plenum
return). These air paths are efficient paths for the transmission of sound,
and can negate the privacy provided by the intervening construction elements.
Examination rooms and patient bedrooms will require special consideration to
prevent such cross-talk.
23.4.4.2
Duct layout. Locate the outlets of connected ducts as far apart
as possible. Use diametrically opposing duct elbows to reduce cross-talk.
Internal acoustic lining should be avoided (is prohibited in inpatient areas),
in favor of utilizing the natural attenuating effect of longer separating duct
runs, elbows, and other duct fittings. To minimize cross-talk transmission in
supply and return ductwork, individual room runouts should be configured as
shown in Figure 23.7, where the outlets are spaced as far apart as possible.
When room return air is not ducted (plenum return), designers must consider
and provide construction details for an appropriate duct/fitting attachment to
achieve required attenuation.
23-12
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