It is well documented that power and distribution transformers produce a low-frequency hum. The overall sound level can be quite low, but due to its tonal characteristic, it is perceived as extremely annoying. Noise arises due to magnetic field variations in the laminations of the core. Magneto restriction effects cause the laminations to change in length as the magnetic field changes. Although the total change in length is of the order of micrometers, this small dimensional change is sufficient to produce the transformer hum at frequencies of 100 Hz, 200 Hz and 300 Hz which are harmonics of the 50Hz excitation current of the transformer. The level of such noise increases with a greater magnetic flux density or with the kVA rating of the transformer.
Sonobex Limited, a spin out company from Loughborough University, was founded in 2012 to undertake research and development into next generation noise control technologies and solutions for industrial noise mitigation. View.