Basic Documentation

Introduction to HVAC System Sound Analysis
Step 2. Blade Frequency Increment (BFI)
This calculation uses the Hz formula to determine the frequency at which the blade sound
component occurs.
Hz = Fan rpm x (Number of Blades ÷ 60)
Hz = 1,225 x (10 ÷ 60)
Hz = 1,225 x 0.167
Hz = 204
Referring back to Table 6, the 204 Hz is within the 250 Hz octave band. Therefore, the BFI
value of 3 dB (from Table 5) is added as a contributing component of sound power level of
the 250 Hz octave band.
Step 3. Efficiency Correction
Based upon an operating fan efficiency between 85 and 90%, Table 7 gives a 3 dB value that
must be added to all octave bands.
Table 26. Fan Sound Level Summary.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
Table 5
45 45 43 39 34 28 24 19
Step 1
(FLw)
51.5 51.5 51.5 51.5 51.5 51.5 51.5 51.5
Step 2
(BFI)
— — 3.0 — — — — —
Step 3
(EFF)
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
Final
99.5
dB
99.5
dB
100.5
dB
93.5
dB
88.5
dB
82.5
dB
78.5
dB
73.5
dB
The final values from Table 26 represent the expected supply fan generated noise level
(GNL). At the end of this analysis, these values are entered into the HVAC system - sound
analysis form, that is shown in Figure 17. This form provides a means to systematically
tabulate the individual element sound levels for an HVAC system or a portion of a system to
determine the resultant sound power level. A blank copy of this form (along with other forms)
is provided in the Appendix. You can use these forms when making sound level analysis of
actual systems.
Siemens Building Technologies, Inc. 69