|MadSci Network: Engineering|
They do have a noticeable hum. It's due to the constant reversing of the AC current - 60 times per second in the US and 50 elsewhere. The transformer core and case actually are moving, imperceptibly at that frequency, and the hum you hear is composed of harmonic frequencies of that 60 cycles per second, which is referred to as 60 Hertz or 60 Hz. There are other harmonics at 120 Hertz (Hz), 180, 240, etc. with a decrease in the amplitude as the harmonic frequency goes up. The 60 Hz, 120 Hz, and 240 Hz harmonics are pretty close to the "B" note on a keyboard or other musical instrument; the 180 Hz third harmonic is close to an "F-sharp". The hum sounds the same no matter what the voltages, in and out, of the transformer are, but for a low power distribution system transformer that you see mounted on poles, there is still a hum, but its amplitude is so low it can't be heard. Actually, if you begin to hear it, something is wrong. Large DC transformers are really AC. For large DC transmission lines, the incoming power is AC and is transformed as AC, and then rectified before transmission. So large "DC" transformers hum for the same reason as their AC counterparts.
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