Basic Documentation

Commentary on HVAC System Sound
Discharge Sound
Since the terminals are 8 inches in diameter and will be horizontal, we can assume that there
will be one 90 degree radius type of elbow and some modest length of unlined round duct
between each of the exhaust terminals and the fume hood.
With respect to an 8 inch diameter 90 degree radius type of elbow, there will be some small
amount of sound power generated as 450 cfm of air (1,290 fpm) passes through it, and there
will also be some attenuation. Using the procedures in Chapter 3 and Chapter 4, we would
find that the sound power level generated by the elbow is too low to impact the exhaust
terminal sound power. However, the elbow would still attenuate a few decibels at the upper
frequencies and would result in the following discharge sound power level:
125 250 500 1,000 2,000 4,000
Hz Hz Hz Hz Hz Hz
61 dB 55 dB 51 dB 46 dB 42 dB 36 dB
Since a round unlined duct provides very little attenuation, we can assume that the above
discharge sound power level will exist at the fume hood connection.
The primary attenuation of the fume hood exhaust air terminal will occur at the junction with
the fume hood due to end reflection, and also the baffle arrangement inside the fume hood.
As previously stated, there is very little information on the properties of fume hoods with
respect to sound power generation or attenuation, so we will base our final values on
attenuation by means of end reflection alone.
Table 24 lists end reflection attenuation values. Subtracting the appropriate end reflection
values for an 8-inch round duct from the preceding chart results in the following final
discharge sound power level values:
Fume Hood Sound Discharge Sound at 50% Maximum Airflow
125 250 500 1,000 2,000 4,000
Hz Hz Hz Hz Hz Hz
50 dB 49 dB 48 dB 45 dB 42 dB 36 dB
We can expect this sound power level to exist at the open sash of each fume hood. With
respect to the Noise Criterion curves (Figure 5), these dB values would not exceed those of
the NC-40 curve. With respect to the person standing some distance away, the resulting
sound pressure level would further decrease in accordance with the Space Effect equation.
Note that this analysis assumed that the fume hoods were 50% open. The sound power level
would increase as fume hood sashes are opened further. But the 50% open value is a
reasonable level to use for general ambient noise analysis, so the conclusion would be that
the room noise level would be very acceptable for a chemical laboratory.
Siemens Building Technologies, Inc. 93