Basic Documentation

Chapter 6–Minimizing HVAC Sound
• Terminals should be mounted as high above the room’s ceiling as practical. At least
three feet above a dropped ceiling should be maintained whenever possible.
• Lined metal duct between a supply terminal and the air diffusers will best attenuate
the air terminal unit sound and allow the least sound breakout. Purposely locating
terminals to use longer runs to diffusers will enable more attenuation of terminal
sound.
• Minimize the use of flex duct as it is likely to generate sound at bends and wherever
sagging or compression occurs.
• Air diffuser sound is also directly dependent upon the type of diffuser and the airflow
velocity. For instance, a perforated diffuser has a higher sound level than a louver
type. Use an ample number of diffusers in a space to maintain a lower airflow velocity
and thus a lower sound level. Also, if a supply diffuser is used in a return/exhaust
application, airflow should be reduced to maintain the desired sound level.
• Use dampers to enable system balancing as far upstream as possible from the
spaces served by the system. Throttling dampers at diffusers should only be used for
small volume adjustments not requiring more than 0.10 in. WC pressure drop.
Table 29. Existing Ventilation System - Noise Troubleshooting and Potential Remedy.
Noise Problem Potential Remedy
Excessive Fan Sound
This is one of the most common noise problems and
many times is due to a multitude of factors. Often there is
a poor inlet and outlet duct arrangement resulting in
excessive air turbulence and pronounced system effect.
There can also be higher pressure drops in the duct
system than originally estimated. Sometimes there will be
a need for more airflow than the design intended. This all
leads to attempting to handle these conditions by
“speeding up the fan” and results in a significant increase
in the generated sound.
• Ensure vibration isolation mounts are present and
properly installed so the fan floats on its mounts.
• Determine the fan’s operating point on the fan curve.
Determine if the system static pressure drop can be
reduced and thus enable the fan operation to be
moved to a more efficient point (higher) on the fan
curve.
• Determine if the system airflow and fan RPM can be
reduced. Seal up all sources of system air leakage.
• Improve fan inlet and outlet airflow if the present
arrangement causes excessive turbulence and
pressure loss.
• Install a sound attenuator in the fan outlet duct (see
Noise Attenuation Devices)
Excessive Air Supply Terminal Discharge Sound
This is usually caused by a relatively high airflow through
the terminal and insufficient attenuation after the
terminal.
• Install lined flexible duct between the terminal and the
diffusers.
• Install more diffusers with longer lined flexible duct
after the terminal to deliver the same air change rate
for the space. (This lessens the airflow per diffuser
and increases the attenuation by utilizing more
terminal discharge paths.)
• Consider installing a larger air supply terminal.
98 Siemens Building Technologies, Inc.