Basic Documentation

Allowable Room Sound Level
It is typically accepted that the room sound level in
the immediate area of a fume hood should not
exceed the NC 55 sound curve. Based on Table 5
the allowable room sound level in proximity to the
fume hoods is listed as item 1 in the following chart:
Siemens Industry, Inc.
Page 11 of 12
Item
125
Hz
250
Hz
500
Hz
1000
Hz
2000
Hz
4000
Hz
1 67 62 58 56 54 53
Room General Exhaust Terminal
Factors that impact the required sound rating for the
room general exhaust terminal include:
 Item 2: Space Effect Sound Attenuation which
yields the allowable sound level at the ceiling
exhaust grill (Table 3).

Item 3: Allowable Room Exhaust Terminal
sound at the ceiling exhaust grill.
 Item 4: Duct End Reflection (Table 6) for a 6-
inc
h duct.
Note that the dB levels of Item 2 in the chart
represent the allowable discharge sound rating for
the ceiling exhaust grill. Item 5 represents the
allowable room general exhaust terminal's discharge
sound level rating at the required airflow of 400 cfm
and the appropriate inlet static pressure.
VAV Fume Hoods
A fume hood's generated sound is the only
significant sound source that will be heard by a
person in close proximity to the fume hood. When a
person is in front of a fume hood there will not be
sufficient distance for any appreciable space effect
attenuation to occur. The sound level that a person
at the fume hood will experience is essentially what
is listed as the fume hood’s sound rating.
Testing that has been done on fume hood sound
indicates that a fume hood exhaust terminal's
discharge sound can be expected to be attenuated
by approximately 20 dB due to the fume hood's
baffles and the exhaust duct transition connection.
Therefore, using the same allowable room sound
level in proximity to the fume hood for Item 1 and
utilizing a uniform 20 dB attenuation factor, the
allowable fume hood exhaust terminal's discharge
sound rating is listed as Item 3 in the following
summation chart.
Item
125
Hz
250
Hz
500
Hz
1000
Hz
2000
Hz
4000
Hz
1 67 62 58 56 54 53
2 +20 +20 +20 +20 +20 +20
3 87 82 78 76 74 73
Thus, in order to not exceed the item 1 sound level
at the fume hood with the sash open, the fume hood
exhaust terminal's allowable discharge sound must
not exceed the dB level of Item 3 at the required
airflow and static pressure drop.
Suggestions for Reducing Excess
Room Related Ventilation System
Sound
Avoiding Excessive Supply System
Fan Sound
1. Use the lowest practical supply system static
pressure. This will enable the supply fan to
operate at a lower rpm and perhaps at a
more efficient point on the fan curve. This
can substantially lower the fan sound as well
as the discharge sound at the room supply
air terminals.
2. Ensure that the supply system ductwork is
well sealed. This reduces air leakage and
therefore reduces the overall required fan
airflow.
3. Minimize the supply fan system effect.
Utilize gradual inlet and outlet duct
transitions at the fan to reduce excessive
turbulence and static pressure loss which
increases fan sound.
4. Use a fan discharge plenum or consider
including a sound attenuator in the fan's
outlet duct.
Avoiding Excessive Room Supply Air
Sound
1. Ensure that the duct connections to the
room supply air diffusers will be in relatively
good vertical alignment. Ideally, the
connecting duct should be vertical for 1 to 2
duct diameters immediately above the
diffuser collar.
Document No. 1
49-979