Basic Documentation

Introduction to HVAC System Sound Analysis
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 duct airflow velocity = 15,000/12 = 1,250 ft/min.
S = the duct cross sectional area in square feet = 12 sq ft as calculated previously.
For elbows with turning vanes:
D = the cord length of a vane in feet (9.6 in. = 0.8 ft).
EC = a constant.
For elbows with turning vanes:
EC = 10 Log n - 107 (n =number of vanes) = 10 Log 15 - 107 = -95.
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 = 60FD ÷ U
St = 60 x 63 Hz x 0.8 ft ÷ 1,250 = 2.4,K = 48
St = 60 x 125 Hz x 0.8 ft ÷ 1,250 = 4.8,K = 50
St = 60 x 250 Hz x 0.8 ft ÷ 1,250 = 9.6,K = -55
St = 60 x 500 Hz x 0.8 ft ÷ 1,250 = 19.2,K = -62
St = 60 x 1,000 Hz x 0.8 ft ÷ 1,250 = 38.4,K = -72
St = 60 x 2,000 Hz x 0.8 ft ÷ 1,250 = 76.8,K = -85
St = 60 x 4,000 Hz x 0.8 ft ÷1,250 = 153.6,K = -102
St = 60 x 8,000 Hz x 0.8 ft ÷1,250 = 307.2,K = -123*
* Calculated by formula (K = - 47.5 - 7.69 (log St)
2.5
Table 27 sums up the factors comprising the ELw values for each octave band.
Table 27. Duct Elbow B ELw Values.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
K -48 -50 -55 -62 -72 -85 -102 -123
10 Log F 18 21 24 27 30 33 36 39
50 Log U 155 155 155 155 155 155 155 155
10 Log S 11 11 11 11 11 11 11 11
10 Log D -1 -1 -1 -1 -1 -1 -1 -1
Siemens Building Technologies, Inc. 71