Basic Documentation

Chapter 3–HVAC Sound Sources
HVAC aerodynamic sound is somewhat harder to attenuate since the ductwork provides a
direct conduit for its transmission to the conditioned spaces. In addition, some aerodynamic
sound is generated locally by HVAC system supply and exhaust components associated with
the room served by the system. On the supply side, this includes VAV box dampers, reheat
or cooling coils, air diffusers, and associated duct fittings. On the exhaust side, this primarily
involves the room exhaust terminals, laboratory fume hoods and other specialized room
exhaust units. Other sources of locally generated sound associated with HVAC systems
includes water flow through reheat coil valves, fan powered terminal units, and sometimes
even sound caused by bleeding or exhausting compressed air from the HVAC control
system.
Fan Sound Components
Fans are the predominant source of HVAC system sound. The fan sound power level must
be known to determine its contribution to the sound pressure level in a given space served by
the fan system. Fan sound is made up of several components. However, before we discuss
how to determine the overall fan sound power level, it will help to understand the nature of
each individual component affecting fan sound.
Fan Aerodynamic Sound
Aerodynamic sound is generated by air in motion. As you blow out a candle, an aerodynamic
sound is produced by the air rapidly passing through your lips. Since a fan imparts a high
level of motion to the air, it also results in significant aerodynamic sound.
Fans are tested for the sound power level produced by the manufacturers according to
standard tests covered by ASHRAE Standard 68-1986, and also by AMCA Standard 330-
1986. Virtually all fan manufacturers also send their fans to the AMCA laboratory for
certification of their test data. For greater accuracy of data, these tests cover the sound levels
produced in 1/3 octave bands. (Each of the eight octave bands is further divided into three
bands thus making 24 bands in all for the test. Three sound power level values are thus
obtained for each of the eight octave band. This data is then converted into the sound power
level for each of the eight octave bands and becomes the published data.)
Although fan manufacturers provide sound power level data for each of their different sizes
and types of fans, the data cannot cover each possible combination of operating conditions
(airflow, static pressure, etc.) in which a given fan may be applied. Therefore, fan sound
power level data is typically given at one set of standard operating conditions that also is a
common denominator for all fans. This consists of an airflow of 1 cfm and a static pressure of
1.00 in. WC. With this data, the fan sound power level at other operating conditions can be
determined through a calculation process that includes additional fan sound components.
Blade Frequency Increment
Before the arrival of electronic sound producing equipment, emergency warning sounds were
commonly produced by a mechanical device such as the unmistakable wailing sound of a fire
truck siren. The mechanical siren was very similar in design to a fan in that it had a rotor with
blades or slots that produced vibrations as it rotated.
24 Siemens Building Technologies, Inc.