Basic Documentation

Chapter 5–HVAC System Sound Analysis
When a fume hood sash is open, exhaust terminal discharge sound will emanate from the
fume hood opening and be the major source of ventilation sound heard by the fume hood
user. In this case, the discharge sound level rating provided by the exhaust terminal
manufacturer (at various airflows) should be reviewed for acceptability.
For a central laboratory exhaust system, the exhaust fan sound power level will typically be
substantially attenuated due to the length of the connecting ductwork and the numerous
junctions and takeoffs on the central exhaust system. Therefore, except for laboratory units
located close to the exhaust fans themselves, the sound power level of the exhaust fans can
be largely disregarded since they will likely be attenuated to a value less than that of the
fume hood exhaust terminals.
Since fume hood exhaust ducts cannot utilize an internal sound absorbing lining (such as,
fiberglass), there is typically only a minimal amount of attenuation possible for exhaust
terminals that are located within laboratory rooms or very close to the fume hoods. The only
alternative (which is not always possible) is to locate fume hood exhaust terminals above the
ceiling, and at a significant distance downstream from the fume hood.
Terminal Radiated Sound - Example Analysis
A 240 square foot laboratory room with two fume hoods will have an acoustical tile type of a
dropped ceiling 10 feet above the floor. It is decided to locate the two 8-inch diameter fume
hood exhaust terminals, that control the VAV fume hood face velocity, in a horizontal position
about 3 feet above the laboratory room ceiling and approximately 10 feet apart.
Figure 20. Example of Radiated Sound.
With reference to Figure 20, determine if the radiated sound pressure level and also the
discharge sound pressure level this arrangement is likely to produce for someone who
happens to be standing in the laboratory room midway between the two fume hoods. Assume
each fume hood sash is about 50% open. Also, consider how this design arrangement will
impact a desired room Noise Criteria of NC-40.
The following tables show the manufacturer’s exhaust terminal sound ratings at 50% of
maximum airflow (450 cfm).
90 Siemens Building Technologies, Inc.