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
Air Delivery Device Sound
In Table 11, the JLw values are all positive, so this takeoff adds to the sound power level
produced by the HVAC system. Since a takeoff is normally near an occupied area, its sound
power level will likely have an impact on the HVAC sound in the area.
Note that the higher the Strouhal number, the lower the net sound power level will be. The
Strouhal number increases as the duct size increases and also as the airflow velocity is less,
so the general approach to maintaining an acceptable sound level is always to avoid high air
velocities.
Air Delivery Device Sound
Just as dampers, elbows, junctions and takeoffs have the ability to add to or attenuate the
sound power level, so do the terminals and diffusers that function as the final air volume
control and distribution devices.
Sound power levels at rated airflow’s are normally available for these devices from the
manufacturers. Manufacturers typically provide tables for these devices that give the NC
rating at specific airflow’s (cfm) through the device. Recall from the earlier discussion in this
document on noise criteria (NC) curves, that a NC rating means that the sound power level
does not exceed the dB values of the respective NC curve at any of the octave band center
frequencies. Thus to determine the octave band dB values associated with a specific NC
rating, one may refer to a plot of the respective NC curve and note the corresponding dB
values at each of the octave band center frequencies. To save having to do this, Table 12
lists the dB values associated with each of the common NC curves for the octave band
center frequencies.
However, note that the dB values associated with the NC curve (and listed in the table) are
the maximum levels to be expected. Actual device sound levels in many of the octave bands
may be less thereby resulting in a lower room sound level.
Table 12. NC Curves vs. Sound Pressure Level Decibels.
NC 63 125 250 500 1,000 2,000 4,000 8,000
CURVE Hz Hz Hz Hz Hz Hz Hz Hz
15
47 36 29 22 17 14 12 11
20
51 40 33 26 22 20 17 16
25
54 45 38 31 27 24 22 21
30
57 48 42 35 31 30 28 27
35
60 53 46 40 36 34 33 32
40
64 57 51 45 41 39 38 37
45
67 60 54 49 46 44 43 42
50
71 64 59 54 51 49 48 47
55
74 67 62 58 56 54 53 52
60
77 71 67 63 61 59 58 57
65
80 75 71 68 66 64 63 62
Siemens Building Technologies, Inc. 43