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
NC Curves
Avoid using A-Weighted sound criterion when designing HVAC systems or conducting a
detailed analysis of the sound pressure level in a room with the intent of improving the room
ambient sound profile. Using A-Weighted values should be limited only to general noise level
comparison measurements or when involved in ensuring against exceeding permissible
occupational sound levels.
NC Curves
In an effort to come up with ambient sound pressure level curves that provide a good balance
between the sound frequency spectrum and the acceptable loudness for various room
applications, standard Noise Criterion (NC) curves have been developed. Figure 5 shows the
family of NC curves.
Until now, this document has avoided using the term noise since there is no scientific way to
define it. It is merely a term that each individual subjectively applies to a sound profile that for
them ranges from unwelcome or bothersome to very annoying. With regard to the Noise
Criterion curves, they establish balanced sound (noise) levels that are generally acceptable
for specific room applications. In other words, the sound produced by an HVAC system
serving a specific application would be acceptable by the vast majority of occupants if the
sound pressure level that it produces does not exceed the dB level of the appropriate NC
curve at any point, and it also has the same general shape as the referenced NC curve.
When analyzing a given room sound profile, it is also acceptable to visually interpolate
between the NC curves. For instance, if the highest penetration of a listed curve (NC 45 in
this case) is 52 dB at 500 Hz, then the measured sound can be stated as having an NC 48
rating.
The NC curves were developed in 1957 and are still widely used today. However, note that
they do not include any dB values for frequencies below 63 Hz. In general, the most
objectionable HVAC noise is the low frequency rumble that is produced by HVAC fans. The
bulk of this sound occurs below the 63 Hz octave band.
Siemens Building Technologies, Inc. 17