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 4–HVAC Sound Attenuation
Chapter 4 discusses the attenuating effect of common HVAC system elements (also referred
to as transmission loss or insertion loss). It includes the following topics:
• Plenums
• Duct Attenuation
• Duct Takeoffs and Divisions
• Duct Silencers
• End Reflection
• Environment Adjustment Factor
• Space Effect
• Radiated Sound Attenuation
Introduction to HVAC Sound Attenuation
Chapter 3, HVAC Sound Sources, established the means to calculate the sound power levels
generated as air passes through the duct system and some of its energy was converted into
sound pressure due to the resistance caused by damper blades, elbows, takeoffs, etc. Not
only do these duct elements cause additional generation of sound, but they also have an
attenuating effect on sound that was already generated by other duct system elements.
Therefore, at any point in a duct system, we have the phenomenon of an ever changing
sound power level and profile due to the simultaneous process of sound generation and
attenuation taking place.
So, even though the net sound power level at a given point in a duct system, such as
immediately upstream or downstream of an exhaust or supply fan may be quite high, the
sound power level will undergo a natural attenuation (reduction) with reference to more
distant areas of the duct system.
With respect to attaining an acceptable sound level in a given space or room, the key issue is
whether there will be sufficient natural attenuation to reduce the sound power to the proper
level, or whether the HVAC system needs to incorporate additional sound attenuation
elements to provide the required attenuation.
This chapter covers the attenuating effect of common HVAC system elements (also referred
to as transmission loss or insertion loss). With this information and the information provided
in Chapter 3, a given HVAC system or a portion of an HVAC system can be analyzed to
obtain the resultant sound power level at any given point. The next section provides
examples of how to perform this type of analysis.
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