Basic Documentation

Duct Configurations
• When designing an HVAC system, never try to specify the resulting sound pressure
level of a HVAC system component. Instead, specify the required sound pressure
level (room NC or RC) to be attained in the space(s) served and where in the room
the measurement should be taken.
• When considering a specific fan for a particular application, always obtain the
manufacturer’s certified sound rating at the standard rating condition of 1 cfm and
1.00 in. WC.
Duct Configurations
• Size the duct system so airflow velocity does not need to exceed 1,200 fpm to meet
relatively low room sound pressure level requirements (such as, NC 35). Velocities
up to 1800 can be used for areas where NC 45 is acceptable.
• Avoid sharp duct bends, transition pieces with steep angles, sharp edge takeoffs and
junctions, and in general any element that causes air turbulence and higher pressure
drops. Use gradual angular transitions (15 degree maximum), radius type elbows
and takeoffs with turning vanes, and in general whatever type of configuration will
provide a more streamlined airflow in the entire HVAC system.
• Round sheet metal ducts are the most efficient at conveying airflow, but offer less
sound attenuation per given length than equivalent sized rectangular ducts.
• Ducts with internal linings of 1- or 2-inch thickness offer significant sound attenuation,
especially for higher frequency sound.
• Avoid direct duct runs between noisy rooms (duplicating machines, operating
equipment, etc.) and areas requiring low sound levels, to prevent room noise from
being transmitted by the ductwork (duct borne crosstalk).
• Wrapping the exterior of ducts with insulation will reduce the radiated sound, but will
not reduce the discharge sound level. It may sometimes even increase the discharge
sound since it prevents duct borne sound energy from radiating out from the
ductwork (in much the same way as insulation retards thermal losses).
Terminal Equipment
• Moderate airflow velocities are the key to minimizing the sound generated by air
terminal units. Consider going to a larger size air terminal to reduce the airflow
velocity and provide a more acceptable level of radiated and discharge sound in the
space served.
• Terminals serving individual office areas should be located in corridors rather than
above the office served to lessen the effect of the terminal produced discharge and
radiated sound.
• Whenever possible, terminals serving larger general offices should be located above
areas that will be less sensitive to HVAC sound, such as over copy machines,
printers, supplies storage, etc.
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