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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.”
There are two types of 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 motor).
5) Air leakage. This can allow sound leakage and also generate a whistle-type noise.
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 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 problem.
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 reduction apparatus.
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 original.
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.
For additional information please refer to http://www.chicagoblowercanada.com/mixflowfans.html
Industrial Process Ventilation Engineer
CBC Canada Co.
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