Basic Documentation

Sound Attenuation Devices
A sound trap is typically larger in external dimensions than the duct it is intended to be used
with, since it must incorporate internal sound absorbing material and baffles. Therefore, it can
require an angular duct transition piece at the inlet and outlet of the silencer. The transition
element should not utilize angles greater than 15 degrees when connecting to fan outlet
ducts.
Duct silencers come in many configurations and lengths and their pressure drops vary.
Typically, the smaller the silencer, the greater will be the resulting pressure drop.
Ceiling and Wall Absorbers
In some applications, such as mechanical equipment rooms and large plenum areas above
ceilings, ambient sound may be attenuated by adding absorbers to the wall and ceiling (if
sufficient open wall space exists) or hanging sound absorbers from the ceiling. When applied
to hard wall areas (such as, masonry walls) they may provide up to 10 dB of attenuation to
the ambient sound pressure level in the room.
Enclosures
When it is not practical to reduce the sound level of certain equipment (pumps, compressors,
etc.), the next best approach is to consider building a sound isolating enclosure around the
sound source or adding an acoustical type of barrier between the sound source and the area
affected. Normally, conventional drywall construction on studs with insulating material
between the two inner surfaces of the drywall, will handle the majority of equipment noise
problems. In some of the more severe instances, it may be necessary to use two layers of
drywall on each side of the studs.
Aside from the obvious access considerations when enclosing equipment in this manner, the
enclosure will have maximum effectiveness if ample space exists between the inner wall of
the enclosure and the equipment. This obviously results in a larger enclosure, but enables
the enclosure to be more effective at absorbing the sound energy.
Active Sound Attenuation Devices
Active sound attenuation is a more recent technical innovation for attenuating sound. One of
the more common methods consists of sensing the sound profile and generating opposing
sound waves that are nearly equal to that of the original sound source, but are 180 degrees
out of phase with the source.
This results in a cancellation type of effect that can dramatically decrease or almost eliminate
the offensive sound that location. Figure 21 shows an active sound attenuation application.
With reference to Figure 21, note that the sound source can be anything, although
attenuating fan sound, particularly because of the low frequency components, is a most
common HVAC application.
Siemens Building Technologies, Inc. 101