IM 725 (1-02)
35
Duct Design For Noise Control
Proper acoustics is often a design requirement for schools. Most of the
problems that are associated with HVAC generated sound can be
avoided by properly selecting and locating the components of the
system. There are some general do’s and don’ts:
The following suggestions can reduce the amount of sound that
reaches the occupied room:
• Use flexible duct connections.
• Make the discharge duct the same size as the unit discharge opening
for the first five feet.
• Line the first 5 feet of the supply duct.
• Make two 90-degree turns in the supply and return ducts.
• Keep duct velocity low and follow good duct design procedures.
• Mount and support the ductwork independent of the unit.
• Line the first five feet of the return duct.
• Locate the return air intake away from the unit discharge.
• Provide multiple discharges.
• Restrict use of high pressure drop flexible ducting.
• Size the outdoor air and return air ducts to handle 100% of the total
cfm to accommodate economizer or morning warm-up operation.
If a supply air duct with improper duct work is placed too close
to the unit discharge, it will result in substantial noise. Avoid
such forms of connections when designing ductwork where
sound attenuation is critical. Figures 109 through 111 shows
suggested duct considerations per SMACNA and ASHRAE.
Sound control applies to the return side of the duct design as well as
the supply side. Figure 110 illustrates suggested installation of return-
air duct. Note the return air opening, and the sizing and changes in
direction of the ductwork. Figure 111 illustrates suggested installation
of outside air ducting.
Duct System Considerations
NOTICE
Flexible Boot
5 Feet
Same Size As
Unit Discharge
Accoustically Lined
Two 90
°
Turns
Ductwork Sized
Based On Airflow
Ductwork Supported Independent Of Unit
Unit Ventilator Discharge
Accoustically Lined
Flexible Boot
Ductwork Supported Independent Of Unit
Two 90
°
Turns
5 Feet
Same Size As
Unit Return
Unit Ventilator Return
Figure 109 - Discharge Air Duct Work
Figure 110 - Intake/Return Air Duct Work