Proposed £100k conventional attenuators for 10 axial mine water evaporation fans in Australia would have had zero effect in the noise as it was predominantly low frequency tones (hum or drone). A few emails and <£20k gave 23dB noise reduction and improved efficiency.
The Australian mining company had a series of 10 large axial fans that are used to evaporate water pumped from underground. However, these fans had caused complaints over a very wide area. The quote for £100k of mufflers would have provided zero attenuation as they would be fitted only to outlets (half the noise is from the intakes) and they would provide virtually no attenuation at the dominant 200Hz tone.
Our remote noise control service diagnosis allowed us to evaluate the problem using information and recordings provided by email, saving a visit from the UK to Australia... We designed and emailed aerodynamic fan noise control modifications for local manufacture. These reduced the tonal content by 23dB at a cost of <£2k/fan. As fan efficiency was also improved, this was a profitable, self-financing project for the client.