A bakery that specialises in manufacturing buns for the food industry started receiving complaints about noise. The complaints came from residents spread over a wide area of housing around the bakery factory. Residents were complaining about a very loud low frequency hum sound, which they found highly annoying during daytime and even more so at night.
The bakery worked out that the sound came from their bun oven, and they fitted an attenuator to the fan that supplies air to the oven combustion chamber. This did not reduce the sound at the houses and complaints continued. The bakery company then asked for our help.
Our initial questioning revealed that the bun oven has 11 fans. We asked the company for some basic information about each fan and obtained information about the main noise frequency from the local Environmental Health Officer who had carried out a noise survey at the houses. Using this information, within a few minutes we had identified the source of the annoying hum sound. We confirmed this with measurements at the houses.
The cause of the annoying hum was not the air combustion fan to which the bakery company had fitted an attenuator, but two small cooling extraction exhaust fans. We designed a solution to suppress the hum on these two fans. This was highly successful, making the cooling exhaust fans inaudible at the houses, even at night. The fan hum was completely eliminated such that it was no longer detectable - achieving a massive 27 decibel sound reduction. The residents were so relieved that the highly annoying hum had been removed that many of them wrote letters of thanks.