- AHU
panel vibration “couples” to the lightweight, flexible
gypsum wall just a few inches away.
This coupling lets low frequency noise pass easily through
the wall.
- The
counterclockwise rotation of the fan’s discharge is forced
to change direction at the downstream elbow.l
The change in the direction at the elbow causes turbulence
resulting in excessive low frequency noise, duct rumble and pressure
drop.
- Problem
2 is aggravated if the elbow’s turning vanes do not have
long trailing edges to straighten the air flow and control the
turbulence.
- The
sound trap is too close to the elbow.
This compounds the turbulence problem.
- Rectangular
ductwork and sound traps do not control the rumble produced by
turbulent air flow.
- The
AHU’s air inlet is too close to the wall.
This causes two acoustical problems: unstable fan operation leading to surge and rumble, and direct
exposure of the inlet noise to the mechanical room wall.
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- The
lack of a sound trap in a mechanical room return air opening allows
fan noise to travel into the ceiling cavity, then through the
lightweight acoustical ceiling into the occupied space.
- The
unit is resting on thin cork/neoprene isolation pads that are
too stiff to adequately isolate the fan vibration.
- The
poorly isolated unit is resting on a relatively flexible floor
slab without sufficient structural support.
This arrangement allows unit vibration to enter the slab.
- The
chilled water piping is rigidly attached to the slab above, thereby
letting unit vibration enter the slab.
- Ductwall
vibration in the sound trap (or any other part of the trunk duct
system) touching the drywall partition can cause the partition
to act as a sounding board and radiate low frequency noise into
the occupied space.
- Suspending
the ceiling from the supply duct causes it to be a sound radiator.
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