Turn on a strong magnetic field near some metals and listen: you may hear a faint hum. Part of that hum is magnetostriction — materials changing shape slightly as they are magnetized.
A tiny shape-shift
When a magnetic material is magnetized, its internal structure rearranges a little, and the material changes dimension by a hair's breadth — usually a few parts per million. The effect is far too small to see, but it is real, and it happens with every cycle of the field. Special giant magnetostrictive alloys stretch much more, and those are engineered for real work.
Why transformers hum
Transformer cores and motor cores are made of magnetostrictive material, and they are magnetized and demagnetized dozens of times a second. Each cycle stretches the core by a microscopic amount and lets it relax, and that rapid flexing pushes the surrounding air — the hum you hear near substations and big electrical equipment. Engineers shape cores and add treatments to keep the noise down.
Magnetostriction at work
Controlled magnetostriction is genuinely useful. Sonar transducers use it to produce powerful underwater sound pulses. Precision actuators use it to move machine parts by fractions of a micrometer, with no gears and no backlash. It even appears in vibration sensors and tiny motors.
Practical tip: if a motor or transformer is noisier than it should be, magnetostriction is a prime suspect — but before replacing anything, check that loose mounting parts are not amplifying the hum.
Read on: eddy current losses → · magnets in motors →