Basic Documentation

Chapter 5–HVAC System Sound Analysis
Duct Section I
Duct Diameter = 12 inches
With reference to Table 16, the following attenuation applies per foot of duct length:
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
0.03
dB
0.03
dB
0.03
dB
0.05
dB
0.07
dB
0.07
dB
0.07
dB
0.08
dB
Multiplying these values by the 8 feet yields the following attenuation in each octave band:
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
0.2
dB
0.2
dB
0.2
dB
0.4
dB
0.6
dB
0.6
dB
0.6
dB
0.6
dB
Duct Elbow J
Following the procedure for determining Elbow Airflow Noise, the basic formula is:
ELw = K + 10 Log F + 50 Log U + 10 Log S + 10 Log D + EC
Where:
ELw = the net elbow sound power level increase (or decrease if it is negative).
K = a factor that is dependent upon the elbow-operating conditions.
F = the octave band center frequency in Hz.
U = the airflow velocity = 1,528 fpm (from H).
S = the duct cross sectional area in square feet = 0.79 sq ft as calculated previously.
For elbows without turning vanes, D is the height of the elbow in feet (12 in. = 1 ft).
EC is a constant. For elbows without turning vanes EC = -107.
The K factor is determined by first calculating the Strouhal for each of the eight octave bands.
Figure 12 gives K factor values for the Strouhal numbers.
St = 60 F D ÷ U
St = 60 x 63 Hz x 1.0 ft ÷ 1,528 = 2.5, K = -21
St = 60 x 125 Hz x 1.0 ft ÷ 1,528 = 4.9, K = -28
St = 60 x 250 Hz x 1.0 ft ÷ 1,528 =9.8, K = -35
St = 60 x 500 Hz x 1.0 ft ÷ 1,528 = 19.6, K = -44
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