Basic Documentation

Chapter 5–HVAC System Sound Analysis
94 Siemens Building Technologies, Inc.
Terminal Radiated Sound -Example Analysis 2
If it is decided to locate the two exhaust terminals, from the previous example, directly atop
the fume hoods (approximately seven feet above the floor in the laboratory), you should ask
yourself the following questions:
• What sound pressure level would a person then experience when standing at the
front of a fume hood with the sash 50% open?
• Would the new sound pressure level experienced by a person at the fume hood be
noticeably different from the previous example?
The difference in this arrangement is that the radiated sound power level will not be
attenuated by the drop ceiling, and the discharge sound power level will not be attenuated by
a duct elbow.
As a result, the person standing at the fume hood, which would probably be within only 3 feet
or so from the exhaust terminal, would be exposed to the following full radiated sound level:
Exhaust Terminal Radiated Sound Power Level at 50% Maximum Airflow
125 250 500 1,000 2,000 4,000
Hz Hz Hz Hz Hz Hz
47 dB 41 dB 36 dB 32 dB 28 dB 23 dB
In addition, the sound pressure level at the fume hood sash would only be attenuated by the
fume hood’s End Reflection and not by the exhaust duct elbow, so it would be:
125 250 500 1,000 2,000 4,000
Hz Hz Hz Hz Hz Hz
50 dB 50 dB 50 dB 48 dB 45 dB 39 dB
Since these two sources of sound are physically very close together, we use the procedure of
Table 3 to come up with the combined results that become a bit higher at the lower frequency
as listed in the following chart:
Combined Fume Hood Exhaust Terminal Discharge and Radiated Sound
at 50% Maximum Airflow
125 250 500 1,000 2,000 4,000
Hz Hz Hz Hz Hz Hz
52 dB 51 dB 50 dB 48 dB 45 dB 39 dB
This is the sound pressure level that a user at the fume hood would experience while the
fume hood sash is at about 50% open.
Recall that a 3 dB increase in sound pressure level will be noticeable. Since all of the decibel
values listed above are about 2 to 3 dB higher than in the previous example (where
everything was located above the ceiling), the resulting sound pressure level would be
noticeable to the user.