Basic Documentation

Chapter 5–HVAC System Sound Analysis
Commentary on HVAC System Sound
Based upon the preceding example of a sound analysis for an HVAC supply system to a
room, we can form these general observations regarding HVAC system sound sources and
attenuation:
• The major source of sound in a system is typically the supply fan. In this example, it
has a peak sound power level of 100.5 dB at 250 Hz.
• Duct fittings such as rectangular elbows, and takeoffs (such as, B & H), provide a
major attenuating effect on sound power.
• Unlined sheet metal duct, especially the larger sizes (such as, A, C, & E), offers very
limited sound attenuation.
• Although duct fittings generate sound, their GNL effect typically does not adversely
impact the sound power level at moderate airflow rates (up to 2,000 fpm).
• Unless a fan is relatively close to a room, the major source of sound in rooms with
higher air change rates (such as laboratories with fume hoods open) will be caused
by the terminal unit and/or the air diffusers.
• Lined ducts offer considerable sound attenuation. (Compare the unlined duct section
L to lined duct section L.)
Room Sound Analysis
Laboratory rooms with high air change rates and high chemical fume hood exhaust rates are
particularly prone to higher ambient sound levels. The sound generated by HVAC
components such as supply and exhaust terminal units that must be located in close
proximity to the room, and supply air diffusers is mainly dependent on the airflow velocity
through these units. The higher the airflow velocity, the higher the sound power level
generated by the units.
Because it is necessary for higher ventilation airflow rates in rooms such as chemical
laboratories, it is not typically feasible to maintain a ventilation ambient sound level on a par
with an office environment (RC or NC Criteria of 30 to 35). Depending upon the purpose of
the laboratory, somewhat higher sound levels are normally acceptable. A two person
laboratory intended for occupancy by individuals who normally concentrate on their separate
research projects may be acceptable with a 45 to 50 Noise Criteria sound level. On the other
hand, academic laboratories where the instructor must almost continuously communicate and
be heard by the students should preferably not exceed 40 dB.
Laboratory
As discussed earlier, HVAC generated sound that has sufficiently high energy, will not only
become part of the sound within the duct system (discharge sound), but will also be audible
outside of the device that is generating the sound. This externally audible sound is radiated
sound and is also referred to as breakout sound. Breakout sound can have an adverse
impact on the overall HVAC system sound in a space particularly if there is no drop ceiling
between the room space and the HVAC components, or if the acoustical absorption
capability of the ceiling is limited.
88 Siemens Building Technologies, Inc.