Basic Documentation

Chapter 6–Minimizing HVAC Sound
The principle of the attenuation system is rather straightforward. The source input
microphone is located in the area where the offensive sound is present. The control unit
creates a near mirror image output signal of the sound sensed by the input microphone, but
180 degrees out of phase with the sound waves of the offensive sound. The control unit
amplifies the output signal to nearly match the strength of the offensive sound, and the output
speaker produces a sound that cancels much of the audible source sound. An error input
microphone picks up the remaining audible sound and enables the control unit to further
modify the output signal as may be required, to more nearly cancel the offensive sound at the
location where it is desired to have the maximum attenuation.
Figure 21. Active Sound Attenuation System.
The advantage of this type of approach is that no significant change in the existing HVAC
ductwork is normally needed, and the resulting sound is almost free of the most annoying
components such as the low frequency tones.
Active sound attenuation systems are best at attenuating the difficult low frequency sounds
from 250 Hz and lower, but are limited at higher frequencies. Thus, if the sound is generally
loud over the entire octave range, full range (broadband) attenuation may need to include
some passive attenuators to reduce the higher frequency sounds (500 Hz and up).
Using active sound attenuation is advantageous to fan sound on large existing HVAC
systems where it may not be possible or practical to change fans or make major changes to
the existing ductwork. However, if air terminal unit sound is the major problem, it may be
more cost effective to replace the terminals or add passive attenuation at those specific
locations.
102 Siemens Building Technologies, Inc.