Highly tonal drone / hum from onion dryer fans
We were recommended by a noise consultant to help their client mitigate the high levels of low frequency tonal noise from 32 axial dryer fans that were the cause of environmental noise complaints.
Our analysis showed the problem to be a low frequency 73Hz fan blade pass hum plus harmonics. Conventional silencing could not be an option due to the low frequency (silencers would have to be extremely large and incredibly expensive).
Noise analysis and diagnosis
This overlay plot of the unweighted (lin) and "A" weighted analyses close to the building illustrates why the "A" weighting is deprecated for low frequency noise problems as it progressively filters-out the lower frequencies. It also shows why octaves (or even 1/3 octaves) are of no practical use compared with narrow band frequency analysis.
This FFT analysis not only allows us to identify the precise cause of the tonal noise problem (the fan blade pass frequency and harmonics), but when zoomed at a much higher resolution as in the 2nd plot, you can see that there are actually multiple tones grouped around each frequency as each fan is running at a very slightly different speed.
Our evaluation of the fan geometry and air flow path coupled with the narrow band analysis allowed us to determine the precise aerodynamic causes of the low frequency tonal noise elements.
Note: whilst accurate FFT analysis was once confined to high-cost sound level meter analysis software, this is no longer the case. It is now available at no cost using free smartphone apps. Whilst uncalibrated, they provide extremely accurate information on tonal frequency and level relative to broadband components i.e. tonal audibility and precise diagnostic data. See our our recommended frequency analysis app with a complete set of instructions >
We provided support for the Irish Environment Agency to update their noise guidance to include our diagnostic techniques, noise mitigation case studies and the practical use of smartphone analysis apps i.e. our approach should be considered mainstream good practice.
Dryer fan noise control
We modelled the fan airflow and used our aerodynamic fan noise control technology to determine the optimum, minimal cost modifications (it would be copied to 31 other fans...).
These comprised retrofit aero modifications that not only attenuated the tones, but also significantly improved the fan efficiency and airflow as shown here. This approach is also more generally used for grain dryer and air conveyor system noise control.
More drying at a lower running cost, a win-win solution that was not only low cost, but would actually pay for itself over time...
