Basic Documentation
Table Of Contents
- Getting a Handle on Decibels
- The Frequency Component
- The Octave Bands
- Sound Curves
- Now Come the Complications and Confusion
- NC Sound Curves
- Attaining Acceptable Ventilation Related Sound Levels in Laboratory Rooms
- Equipment Sound Ratings
- Fume Hood Sound
- Attaining an Acceptable Room Sound Level
- Example Analysis of a VAV Research Laboratory Room Supply Air System
- Item 1: Supply Air Sound Level
- Item 2: Space Effect Sound Attenuation
- Item 3: Multiple Sources of Sound
- Item 4: Allowable Supply Air Sound at Diffusers
- Item 5: End Reflection Sound Attenuation at Supply Diffuser Inlet
- Item 6: Supply Air Terminal Duct Attenuation
- Item 7: Duct Division
- Item 8: Allowable Supply Terminal Discharge Sound
- Allowable Room Sound Level
- Room General Exhaust Terminal
- VAV Fume Hoods
- Suggestions for Reducing Excess Room Related Ventilation System Sound
- Conclusion
Page 9 of 12 Siemens Industry, Inc.
Document No. 149-979
and IAQ concerns. Therefore, in this example
assume that a 9-foot long section of 12-inch
diameter unlined flex duct will be used between the
supply air terminal outlet and the diffuser inlets. The
attenuation levels for unlined flex duct are listed in
Table 7.
Table 7. Unlined Flex Duct Attenuation Levels
for Up to 10-Foot Lengths.
Dia.
(Inches)
125
Hz
250
Hz
500
Hz
1000
Hz
2000
Hz
4000
Hz
4-6 9 9 9 10 12 15
7-8 8 8 8 9 10 13
9 7 7 8 8 10 12
10 7 7 7 8 9 11
12-16 5 5 5 6 7 9
Levels in Table 7 are from ARI Standard 885-1998.
Item 7: Duct Division
When a duct fitting divides the duct into two equal
paths it results in a 3 dB reduction in sound level for
each path due to division of the sound energy.
Item 8: Allowable Supply Terminal
Discharge Sound
The item 7 resultant sound level is listed as item 8
and establishes the maximum allowable discharge
sound power level for the supply air terminal at the
required airflow (2000 cfm in this example) and static
pressure drop. This discharge sound power level
should be included in the project specifications and
cannot be exceeded in order to maintain the
maximum allowable supply air related sound level
for the room (listed in Item 1). If the air terminal's
discharge sound rating is higher than item 8, a
sound attenuator would be needed at the air
terminal outlet.
OCTAVE BANDS
Room Ventilation - Component Sound Analysis
2 3 4 5 6 7
Item Description
125
Hz
250
Hz
500
Hz
1000
Hz
2000
Hz
4000
Hz
1
Maximum allowable supply air related
sound level in room (Ref. Table 2)
Allowable Room Sound
Level
65 60 55 50 45 40
Attenuation
+6 +7 +8 +9 +10 +11
2
Space effect sound attenuation
Resultant
71 67 63 59 55 51
Attenuation
-3 -3 -3 -3 -3 -3
3
Multiple sound sources
Resultant
68 64 60 56 52 48
Attenuation
-5 -5 -5 -5 -5 -5
4
A
llowable supply air sound at diffusers
Resultant
63 59 55 51 47 43
Attenuation
+9 +5 +1 +1 +0 +0
5
End reflection sound attenuation at
supply diffuser inlet
Resultant
72 64 56 52 47 43
Attenuation
+5 +5 +5 +6 +7 +9
6
Supply air terminal to diffuser unlined
flex duct attenuation
Resultant
77 69 61 58 54 52
Attenuation
+3 +3 +3 +3 +3 +3
7
Supply air terminal outlet duct division
Resultant
80 72 64 61 57 55
Attenuation
8
A
llowable Supply Terminal Discharge
Sound
Resultant
80 72 64 61 57 55
Attenuation
9
Resultant
Attenuation
10
Resultant
Figure 4. Example Research Laboratory Supply Air System Sound Analysis.