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There are two types of fan
noise problems — those that we anticipate from our sound ratings and
those emanating from some abnormal condition in the fan. Some of the
more common sources of abnormal or unanticipated noise are:
1) Fan wheel unbalance.
2) Resonance of fan or attached components.
3) Rotating components rubbing on stationary parts.
4) Failing, misaligned, or contaminated bearings (on the fan or on the
5) Air leakage. This can allow sound leakage and also generate a
6) Belts slipping.
7) Coupling misalignment.
8) Motor noise, especially with improper power supply. Inverter drives
may increase motor noise at certain speeds.
9) Air turbulence.
10) Operation in surge.
11) Loose components.
12) High velocity air blowing over fixed components which are not part
of the fan.
The phenomena of both sound and vibration are very similar and related.
The term “sound” is used for air or other gas while “vibration” is used
for a similar disturbance of motion in a solid. The sound pressure
disturbance impacting on a solid can impart a vibration while the
vibration of a solid can result in sound. Some prefer to call the
concept of vibration which results in sound “structure borne noise.”
The other class of noise problems are those we have anticipated because
of normal fan sound ratings. Some cures for these problems are:
- Select a different fan. Computerized selection routines allow us to
examine many fan types looking for the quietest. Use a custom fan
design, if required.
- Relocate the fan to where sound is not a problem.
- Vibration isolation and flexible connectors on the inlet and
discharge will reduce structure-borne noise.
- Insulate or acoustically enclose the fan housing if housing radiated
noise is a problem.
- Add silencers or duct lining to inlet and / or discharge to reduce
sound in these directions. However, a silencer on the outlet does not
reduce the housing radiated noise or inlet noise; and an inlet silencer
does not affect the housing radiated and/or outlet noise.
- Look for ways to reduce system resistance since sound output is
proportional to fan static pressure.
One final tip which can help to avoid noise problems is to select lower
RPM fans. Fans exceeding 3000 RPM are much more likely to tune in to an
attached structure resulting in structure borne noise. Structure borne
noise easily propagates an entire system and can become a problem at
many locations. Also, people tend to become more annoyed with higher
frequencies than with lower, increasing the likelihood of a noise
Recently, several changes are developing in the technology of fan
noise. Some of these are:
1. The latest codes define testing for fan inlet noise, fan outlet
noise, and noise radiating from the housing (or casing). The inlet
noise can no longer be assumed to apply to the outlet and vice-versa.
2. A new test code which uses sound intensity is near adaption. In
theory, integrating a series of measured sound intensities over an
enclosed area yields the sound power directly.
3. Sound criteria is playing an ever more important role in the
selection process. Many fans are insulated for sound or use other sound
4. Active noise cancellation continues to be difficult to apply to most
fan installations. If it ever proves practical, this technique can
cancel fan noise by adding a second pressure wave out of phase with the
5. Many more fan specifications are requiring AMCA sound certification.
Certified sound ratings mean that AMCA has verified that the ratings
are generated in accordance with the codes and that at least one sample
has been tested in the AMCA Laboratory to verify the ratings.