Basic Documentation

Chapter 6–Minimizing HVAC Sound
• Anticipate potential noise problems when difficult situations or configurations leave
no choice but to depart from good design practice.
• Consider utilizing additional sound isolating equipment enclosures and additional
sound absorbing elements.
• Rooms without a dropped ceiling and exposed ductwork are especially prone to
radiated sound from terminal units, and other duct elements. Consider lowering
airflow velocity by using larger sized duct and acoustical covering over terminal units
and sharp elbows to reduce sound breakout.
• Require ductwork to be well sealed. Much sound is due to air leaking out of supply
ducts and also leakage into exhaust ducts. In addition, reducing air leakage will
reduce the amount of the overall required airflow in the system, which increases
system efficiency and lessens the sound.
• Room size and furnishings have a large effect on the final ambient sound level.
Larger rooms absorb and dissipate sound better than smaller rooms. It is difficult to
achieve a low sound in smaller rooms since there is less sound energy absorption by
the furnishings and less overall Space Effect (ambient sound reduction due to the
volumetric effect).
• Low frequency noise is the hardest to attenuate and typically the most annoying.
Effective low frequency sound attenuation normally requires duct linings of at least
two inches thick.
Fans
• Backward inclined centrifugal fans are recommended for the lowest noise generation.
• Axial fans also have a higher BFI (blade frequency increment) component that
typically contributes the most to annoying fan sound.
• Always select the most efficient fan for the purpose and strive for operation near the
top of the fan curve.
• Variable speed drives are also a contributing source of noise in equipment rooms
and consequently to overall fan system sound. Of the types of variable speed drives
available, the current source is typically the quietest, while a voltage source is
typically the noisiest.
• Avoid VAV fan operation at the point where variable inlet vanes must be nearly
closed. Inlet vanes tend to generate much low frequency sound as the vanes near
the closed position.
• Use a fan configuration that minimizes the system effect as much as possible.
Airflow into and out of the fan should avoid swirling and turbulence.
• Plenums can help attenuate considerable fan sound. This is a particularly effective
means of lowering the sound of return or exhaust systems. For supply systems,
plenums with one or two inches of sound absorbing material are especially effective
at attenuating fan sound.
96 Siemens Building Technologies, Inc.