Basic Documentation

Air Delivery Device Sound
Siemens Building Technologies, Inc. 45
Sound Breakout and Break-in
Aerodynamic sound that has sufficiently high energy within a duct can also be heard outside
of the duct. If you were on a ladder extending above a ceiling where HVAC equipment is
located, you might hear the sound of the air passing across a damper or even the rumble of
an upstream fan. The sound is radiated directly to us and is referred to as breakout sound.
Similarly, if we were doing some hammering nearby or talking loudly next to a duct, the
sound being produced could radiate to the inside of the duct system and travel along with the
sound already there. Sound that is radiated into a duct system is referred to as break-in
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 space and the duct, or if the
acoustical absorption capability of the ceiling is limited.
While breakout sound can adversely impact the sound level in a space where there is an
inadequate acoustical barrier, breakout sound also reduces the overall sound power level in
the duct since the breakout sound carries off a portion of the total sound power level energy.
Thus, the breakout sound phenomenon has an attenuating effect on the sound that would
otherwise be present in the duct system. However, due to the complexity of attempting to
quantify this effect, it is typically omitted from HVAC sound analysis calculations.
In applications where there are very long duct runs, the breakout sound phenomenon can
have a significant positive effect. For details on breakout and break-in sound transmission
loss and related calculations, consult the ASHRAE HVAC Applications Manual, Chapter 42.
Laboratory Elements
The primary source of a chemical laboratory’s HVAC ambient sound is on the exhaust side of
the ventilation system. The principles that apply to HVAC supply side produced sound
(airflow’s through duct fittings, dampers, etc.) also apply to the exhaust side and are
calculated in the same manner. Since fume hoods are the terminal point of the exhaust
system, the NC factors that apply to a particular fume hood operating at specific airflow’s
must be obtained from the manufacturer and can then be used as part of the overall exhaust
system analysis.
Later in this document, you will learn how to perform a sound analysis of varying HVAC
system configurations including a laboratory fume hood exhaust system.