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 2–Physics of Sound
= 60 dB
Therefore, the sound pressure level of normal conversation for the listener is approximately
60 dB.
To give a “feel” for the common range of values for both sound power and sound pressure,
Table 1 gives some typical values for the common levels in our environment.
Table 1. Common Sound Power & Sound Pressure Levels.
Source Sound Power Level
dB
Sound Pressure Level
dB
Jetliner takeoff 160 140* (at 100 ft away)
Very loud sound (race car engine, gun fire) 120 -130 110*
Very noisy room (loud machinery) 100 -110 100*
Somewhat noisy room (computer printout
room)
80 80
Normal voice conversation 70 60 (at 3 feet)
General office 45 45
Soft whisper 30 30 (at 5 feet)
Leaf rustling 20 20
Hearing threshold 0 0
* Hearing protection should be used in sound pressure levels of 90 dB or more since permanent
hearing loss will occur after exposure of 8 or more hours at 90 dB. OSHA limits the permissible
exposure time for unprotected workers in a sound pressure level that averages 90 dB or more.
Remember, even though the decibel values in Table 1 are almost the same for the sound
power level and the sound pressure level, they represent different physical parameters. In
HVAC design, we are mostly concerned with the sound pressure level experienced by an
occupant in an area served by an HVAC system.
Even though you will find that the sound power level of a sound source such as an HVAC
system supply fan may be considerably high (such as, 90 or more dB), the objective is to
design and configure the HVAC system so the sound pressure level will be attenuated down
to an acceptable level, perhaps 35 dB, when heard by an occupant of the area served by the
HVAC system.
Table 2 lists a few rules that generally predict how a person perceives loudness of sound as
changes occur in the sound pressure level.
Table 2. Effects of Sound Pressure Level Changes.
dB Change Effect
0 to 2 None
3 Just noticeable
8 to 10 Increase Twice as loud
8 to 10 Decrease Half as loud
12 Siemens Building Technologies, Inc.