Basic Documentation

Chapter 5–HVAC System Sound Analysis
Junction F
Since the takeoff at Junction F has the same physical dimensions and airflow velocity as the
takeoff at Junction D, calculating the GNL is identical to that previously done for Junction D,
and the same GNL values apply. Therefore, the same values are entered into the HVAC
system - sound analysis form (Figure 17) as Junction F GNL entries.
Note that there is a difference in determining the attenuation values for Junction F since the
ratio between the branch duct and the main duct are different at junction F than they were for
Junction D.
Attenuation = 10 Log [Branch Duct ÷ Total Duct Area Leaving Junction]
= 10 Log [4 sq ft ÷ 8 sq ft]
= 10 Log [0.50] = -3.0
3.0 dB attenuation is entered into the HVAC system - sound analysis form (Figure 17) for
Junction F.
Duct Section G
Duct Perimeter = (16 in. + 36 in. + 16 in. + 36 in.) ÷ 12
= 8.67
Duct Area = (16 in. x 36 in.)/144
= 4
P/A = 8.67/4
= 2.17
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
.260
dB
.175
dB
.117
dB
.079
dB
.052
dB
.035
dB
.023
dB
.016
dB
Multiplying these values by the 14 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
3.6
dB
2.5
dB
1.6
dB
1.1
dB
0.7
dB
0.5
dB
0.3
dB
0.2
dB
76 Siemens Building Technologies, Inc.