Basic Documentation

Chapter 4–HVAC Sound Attenuation
A room’s ability to attenuate the sound to an acceptable level is dependent upon its size,
shape, ceiling height, acoustical properties, and other factors. The following equation
(referred to as the Schultz equation) provides the means to calculate the attenuation and the
resulting sound pressure level at a given location from a sound source.
RLp = 5 Log (V) + 10 Log (r) - 25 + 3 Log (f)
Where:
V = Room Volume.
R = Distance to the sound source (feet).
f = Octave band frequency Hz.
For example, assume that a laboratory room is 24 feet long by 12 feet wide by 10 feet high.
What attenuation would this room to provide at 10 feet from a supply air diffuser?
RLp = 5 Log (V) + 10 Log (r) - 25 + 3 Log (f)
= 5 Log (2,400) + 10 Log (10) - 25+ 3 Log (f)
= 16.9 + 10 - 25 + 3 Log (f)
= 1.9 + 3 Log (f)
Substituting each octave band frequency in the (f) term will yield the following room dB
attenuation values at each octave band:
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
7.3 8.2 9.1 10.0 10.9 11.8 12.7 13.6
The attenuation values determined for the room are then subtracted from the sound power
level value at the sound source to get the sound pressure level at the point of concern.
When multiple sound sources are present (as is the case with several supply air diffusers or
fume hoods), separate calculations are done to arrive at the sound pressure level due to
each separate sound source. Then the sound pressure levels can be combined according to
Table 3 to determine the net resulting sound pressure level at the point of concern.
Radiated Sound Attenuation
HVAC components served, such as VAV and CAV air terminal units that are typically located
close to the area served along with chemical fume hood exhaust terminals and similar
devices, have both a radiated sound and a discharge sound rating. A discharge sound rating
applies to the sound power level generated by the device that may add to the existing sound
power level already within the ductwork. This sound energy ultimately enters the room
through the air diffusers (or exhaust grilles).
64 Siemens Building Technologies, Inc.