A demanding set of environmental noise specifications had been set for a proposed large (3m diameter) new exhaust fan and stack for a fibreboard manufacturing facility. These included meeting both an overall dB(A) figure and the absence of tones. As we had completed a number of highly successful fan noise control projects on the site, the client asked us to liaise with the supplier to design-in our technology during manufacture.
Fan noise data
Whilst the fan and stack OEM provided the standard octave band sound power levels, they had no narrow band data, a universal problem with noise information.
The smartphone audio sample (see above) from a similar installation anywhere on the planet that we requested revealed the fan to be highly tonal at 298Hz (fan blade pass) and harmonics as shown here.
As the tonality attracted a 5dB noise penalty, in addition to achieving the required 16dB(A) broadband noise reduction, it was also necessary to use aerodynamic technology to eliminate the tones.
Fan noise control design
We used Computational Fluid Dynamics (CFD) to model the fan so we could optimise the design modifications to provide maximum attenuation without affecting system efficiency. The modifications comprised:
- aerodynamic inserts to cut the tonal noise elements
- tuned broadband noise control elements in the base of the stack to attenuate the broadband dB(A) noise
The OEM was extremely surprised by the performance of the fan as they were skeptical that it would work. We are very pleased with the sound performance of this unit...
Cost and performance
This innovative approach to fan and stack noise control exceeded all the noise reduction requirements at a fraction of the cost of the conventional approach based on silencers. Moreover, our insistence on acquiring narrow band data from an OEM recording avoided the all-to-common (and very costly) problems associated with the discovery of a tonal noise issue after all the mitigation has been installed. This allowed us to use an aerodynamic engineering scalpel to remove the tones at source.
