Basic Documentation
Table Of Contents
- About this Application Guide
- Chapter 1–Introduction
- Chapter 2–Physics of Sound
- Chapter 3–HVAC Sound Sources
- Chapter 4–HVAC Sound Attenuation
- Introduction to HVAC Sound Attenuation
- Plenums
- Duct Attenuation
- Duct Takeoffs and Divisions
- Duct Silencers
- End Reflection
- Environment Adjustment Factor
- Space Effect
- Radiated Sound Attenuation
- Chapter 5–HVAC System Sound Analysis
- Chapter 6–Minimizing HVAC Sound
- Appendix
- Glossary
- Index
Chapter 3–HVAC Sound Sources
Note that although a fan is normally the major sound producer, you should also analyze the
entire system of ductwork, terminal devices and the room diffusers to determine what their
contribution to the HVAC system aerodynamic sound they may make, as well as what
attenuation they may provide.
Analyzing a duct system’s components for the purpose of determining the attenuation factors
of each separate element is covered later in this document.
Damper Airflow Noise
Dampers generate aerodynamic sound and also attenuate a certain amount of aerodynamic
sound. The result can be either a net gain or reduction in sound pressure level, depending
upon the airflow velocity through the damper, pressure drop and several other factors. Like
the previous discussion on determining a fan’s sound power level, it is necessary to
determine the actual sound power level at each octave band for a damper at the HVAC
system’s design operating conditions.
The following formula provides the actual sound power level increase or decrease at each
octave band for a damper at specific operating conditions:
DLw = K + 10 Log F + 50 Log U + 10 Log S + 10 Log D -107 dB
Where:
DLw = the net damper sound power level increase (or decrease if it is negative).
K = a factor (characteristic spectrum) dependent upon the damper’s operating conditions.
F = the octave band center frequency in Hz.
U = the damper velocity factor dependent upon the damper pressure drop.
S = the duct cross sectional area in square feet at the damper location.
D = the duct height in feet.
107 dB = a constant for any damper and relates to a damper’s ability to attenuate sound.
U (Velocity Factor)
Determining the U (velocity factor) requires a separate series of calculations. The following
procedure will yield the U value. However, in lieu of following this lengthy procedure, Figure 9
can also be used to obtain approximate U values for common airflows and damper pressure
drops.
Calculate Pressure Loss Coefficient C
To calculate pressure loss coefficient C, use the following formula:
30 Siemens Building Technologies, Inc.