Transformers make a humming noise, which can be heard from a distance in large power transformers. This noise can be attributed to:
Magnetostriction: A phenomenon caused by the expansion and contraction of transformer core in response to varying magnetic flux.
Internal Forces: Forces are exerted on the laminations due to the presence of eddy currents and a time-varying magnetic field according to the relation:
As a result of multiplying two sinusoidal functions of the same frequency, the force has a constant component and a sinusoidal component at twice the line frequency. The time-varying component is responsible for vibrations in the core at two times the frequency of excitation. For example, a line frequency of would result in vibrations at , which is in human hearing range.
Insufficient Clamping: Even at the presence of the forces explained above, with sufficient clamping, the vibrations (and thus the noise) can be minimized. Insufficient clamping causes excessive vibrations and noise.