Basic Documentation

Chapter 5–HVAC System Sound Analysis
JC = -107 + Δr + ΔT
Δr = Junction Radius/Branch Duct Diameter
Since there is no radius at the junction:
Δr = 0.0 and r ÷ D
BR
= 0.0
ΔT is a turbulence factor that doesn’t apply since there is no element within five duct
diameters upstream of Junction H.
Therefore:
JC = -107 + 0.0 + 0.0
= -107
The following chart summarizes the terms and results of the JLw formula to determine the
GNL of Junction H at each octave band.
Next, the Strouhal number is determined for each octave band center frequency. Then the
graph in Figure 13 will provide the K values.
Strouhal number (St) = 60 F D ÷ U
B
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
St =60 x 1,000 Hz x 1.0 ft ÷ 1,528 = 39.3, K = -53
St =60 x 2,000 Hz x 1.0 ft ÷ 1,528 = 78.5, K = -63
St =60 x 4,000 Hz x 1.0 ft ÷ 1,528 = 157.1, K = -74
St =60 x 8,000 Hz x 1.0 ft ÷ 1,528 = 314.1, K = -95*
*Estimated Value
JC is a constant that depends upon the configuration of the junction and relates to the
junction’s ability to attenuate sound.
JC = - 107 + Δr + ΔT
Δr = Junction Radius/Branch Duct Diameter
Since there is no radius at the junction,
Δr = 0.0 and r ÷ D
BR
= 0.0
ΔT is a turbulence factor that doesn’t apply since there is no element within five duct
diameters upstream of Junction H.
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