Basic Documentation

Introduction to HVAC System Sound Analysis
Perforated Diffuser
The perforated diffuser GNL NC rating is listed as NC 35 by the manufacturer and are
expanded in the chart below based on the values from Table 12 and are then entered into
Figure 17.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
DIFFUSER
NC 35
(GNL)
60
dB
53
dB
46
dB
40
dB
36
dB
34
dB
33
dB
32
dB
End Reflection
When ducts terminate into a ceiling air diffuser, a significant amount of low frequency sound
energy is reflected back as end reflection.
From Table 24, the end reflection attenuation values for 12-inch diameter round duct is
shown in the following chart.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
12-inch
Diameter
12.6
dB
7.6
dB
3.6
dB
1.3
dB
0.4
dB
0
dB
0
dB
0
dB
These values are now entered into the HVAC system - sound analysis form (Figure 17) as
End Reflection ATTENUATION.
Space Effect
The last factor regarding attenuation of the sound power level is the ability of the room to
absorb and attenuate the sound power.
The following equation (referred to as the Schultz equation) provides the means to calculate
the resultant room sound pressure level (Rlp) at a given octave band level five feet above the
floor, from the sound power level present at the diffuser(s).
RLp = 27.6 Log (H) + 5 Log (A) + 3 Log (f) -1.3 Log (N) - 30
Where:
H = ceiling height in feet = 10 ft.
A = (floor area ft
2
÷ number of diffusers) ÷ H
2.
= (12 ft x 24 ft ÷ 2) ÷ 100.
= (144) ÷ 100
Siemens Building Technologies, Inc. 83