Basic Documentation

SUPPLY AIR
TERMINAL
ITEM 2
SPACE EFFECT
SOUND ATTENUATION
ITEM 1
SUPPLY AIR
SOUND LEVEL
ITEM 3
MULTIPLE SOUND
SOURCES
ITEM 4
ALLOWABLE AIR
TERMINAL SOUND
AT DIFFUSERS
ITEM 5
DUCT END REFLECTION
AT SUPPLY AIR DIFFUSER INLET
ITEM 6
DUCT ATTENUATION
ALLOWABLE TERMINAL
DISCHARGE SOUND
ITEM 7
DUCT DIVISION
ATTENUATION
Figure 3. Example Laboratory Room Supply System Arrangement.
Item 1: Supply Air Sound Level
Figure 4 shows the maximum allowable room sound
level in dB's entered into a copy of the Room
Ventilation System Component Sound Levels chart
of Figure 2. The room dB levels for this example are
obtaine
d from Table 2, Row B. Only the 125 to 4000
Octave Band
dB's are listed since only those are
generally covered by manufacturers' sound data.
Item 2: Space Effect Sound
Attenuation
Assume that a nominal distance of 6 feet will exist
between the ceiling supply air diffusers and the mid-
point in the room where the sound level will be
measured
3
. This distance, in conjunction with the
overall room volume, decreases (attenuates) the
sound level that is heard or measured in the room
and is referred to as space effect sound attenuation.
Therefore, this sound level is somewhat less than
the sound level located at the ceiling supply air
diffusers. Table 3 lists Space Effect Sound
Attenuation levels for a 2500 cubic foot laboratory
room for various distances from a sound source.
These attenuation levels can therefore be added to
the desired room sound level to yield the allowable
sound level right at the supply air diffusers.
Siemens Industry, Inc.
Page 7 of 12
3.
3
Room sound measurement are generally measured at 5
feet off the floor (ear height) and in the middle of the room.
Table 3. Space Effect Sound Attenuation at
Various Distances (D) from a Sound Source.
Levels based on a room volume of 2500 cubic feet*
D
(Feet)
125
Hz
250
Hz
500
Hz
1000
Hz
2000
Hz
4000
Hz
3 3 4 5 6 7 8
4 4 5 6 7 8 9
5 5 6 7 8 9 10
6 6 7 8 9 10 11
8 7 8 9 10 11 12
10 8 9 10 11 12 13
* For a room volume between 1500 and 2500 cu ft,
decrease the above attenuation levels by 1 dB.
For a room volume between 2500 and 4000 cu ft, increase
the above attenuation levels by 1 dB.
For a room volume greater than 4000 cu ft, increase the
above attenuation levels by 1 dB and then add 1
additional dB for each 2000 cu ft over 4000 cu ft.
Table 3 values were calculated based upon
ASHRAE 2003 Applications Handbook, Chapter 47 -
Sound and Vibration Control, Point Sound Sources.
Item 3: Multiple Sources of Sound
In this example, two identically sized diffusers are
used with the same airflow at each diffuser. It is
assumed that each diffuser will generate the same
sound level. Two identical sound sources produce a
Document No. 1
49-979