The fans in this dry cooler were the cause of multiple complaints about a 97Hz low-frequency hum outside a sports facility. The owners could not find a supplier willing to guarantee the problem would be resolved using conventional enclosures and silencers.
Why silencers are not effective for low-frequency dry cooler fan noise
Conventional silencers do not work well at low frequencies - unless you can afford:
- a substantial reduction in fan efficiency that puts up power consumption and running costs
- space for, in this case, silencers that are around 6m long on both the intake and exhaust
- the budget for a large enclosure
At 97Hz the wavelength of sound is around 3.4m. You would need a conventional silencer approaching 2 wavelengths long to not cut the fan efficiency. Fan noise is also radiated equally through both the intake and the exhaust, so both paths would have to be silenced.
Using aerodynamic fan noise control instead of silencers
This innovative technology modifies the airflow through the fan to cut the low-frequency tonal fan noise at source whilst, in many cases, simultaneously increasing fan efficiency and hence reducing running costs. This raises the prospect that rather than being a cost, the noise control project could well be a profitable exercise.
As the attenuation technology cuts the noise at source, it reduces the sound from both the fan intake and exhaust simultaneously. In this case, as illustrated by the sound file and this frequency analysis plot, the low-frequency tonal hum harmonics from the fans were reduced by 90% - 95% (10dB - 15dB), eliminating the source of the complaints.
The whole project was carried out remotely using smartphone video clips and photographs (on another continent without a site visit), making it a very rapid and cost-effective process.
This alternative to conventional attenuators is much lower cost, dramatically more effective and is applicable to similar applications where axial fan noise is a problem such as on chillers and cooling towers.
