Understanding fan noise and subsequent impact goes beyond a review of inlet and outlet sound power or pressure values supplied by a fan manufacturer. Careful consideration must be given to all the sources of noise from the fan. As important as the fan itself, is the environment where it’s placed, this will ultimately determine the solution and the degree we attenuate the noise! When reviewing an engineering specification and before installing a fan in a noise sensitive area, it is important to identify the various sources of fan “breakout” noise and understand the means to address and attenuate them. The attached illustration is of a belt driven fan that might be used in a typical process fan application. It is a centrifugal fan mounted on a unitary base with vibration isolators equipped with inlet and outlet flex connectors. Axial fan noise can be addressed in similar fashion.
Please hover over the image below to identify fan noise attenuation options:
A typical bid specification might state the maximum noise level for the outlet of the fan at some distance and reference point. Should the specified fan installation be ducted both in and out, the maximum noise level from the fan housing might be specified. But, do we have enough information to provide the noise attenuation necessary to meet the client’s expectations for a noise sensitive site?
The illustration shows noise values defined as a dBA sound pressure which is representative only and should be used as an order of magnitude. Actual noise levels can vary greatly depending on the fan and application.
Noise from axial and centrifugal fan inlets and outlets are typically addressed with attenuators or silencers. The level of noise to be attenuated depends upon the fan and wheel design as well as the speed or rpm of the wheel.
The typical sound level reduction would be 10 to 25 dBA depending on allowable pressure drop thru the silencer, the silencer length, type, and weight.
When we attenuate the inlet and the outlet, the resultant noise is defined as the housing breakout noise, and we can attenuate that with a flexible noise blanket or rigid cladding attached to the fan housing. Cladding attenuation would result in an 8 to 12 dBA reduction.
Now let’s look at some of the additional breakout points from our fan that are frequently overlooked. The fan shaft as it penetrates the housing leaves an open area around the shaft. Depending on the size of the shaft and opening, this can contribute major breakout noise. A simple shaft seal is usually enough to seal up the opening. Along with sheet metal gaps in the fan housing, review all gaps in the duct and gasketed flanges.
Looking further at our example, we expect belt and motor noise, both of which are contributors to the overall fan noise level. Depending on the noise level to meet, we can blanket the belt guard and specify a low noise motor resulting in a 5 to 10 dBA noise reduction.
We often use a direct driven fan and a variable frequency drive with the fan speed tweaked based on the CFM and static pressure requirement of the system. A 10% reduction in speed results in a two dBA reduction and a 20% rpm reduction, about five dBA.
Now that we’ve addressed a single fan what happens when we install multiple fans in the same space. Using logarithmic addition, we know that two equal sound sources produce a resultant noise level that is three dBA higher. As an example, the noise from two 85 dBA fans would combine to produce 88 dBA overall.
If you’re installing a fan in a noise sensitive space or adjacent to a residential community, specifying or purchasing a fan, considering all of the fan noise sources and the environment is a must.