Sheets of magnetic steel in the transformer vibrate in a magnetic field, generating an audible hum. Transformers operate by changing an electric current with one voltage and current value to a magnetic field, which is then transformed back into an electric current with a different voltage and current value.
Step down transformers for utility power usually hum at the frequency of alternating power used, 60 Hz in the United State. Audible humming also results when the transformer components expand and contract in the magnetic field, a process known as magnetostriction, but this only occurs in larger transformers with higher currents and stronger magnetic fields.
Measures taken to limit this humming include using a special, rigid steel that resists magnetically induced expansion and contraction, increasing the number of steel sheets in the transformer to reduce the amount of expansion and contraction per sheet, and fastening the transformer components securely to a rigid frame to restrict their vibration. The noise generated by a transformer can also be dampened by placing the transformer in an insulating liquid such as oil, which absorbs both the mechanical vibrations and heat generated during operation. Because transformers are usually operated at set input and output voltages, reducing the magnetic field is not a viable way to limit the vibrations.