Identifying 60-Hz hum from 120-Hz

[dcriner]

A 60-Hz hum typically comes directly from the power xfmr or the AC supply. A 120-Hz hum comes from the DC power supply with a full-wave rectifier. It's useful to be able to immediately recognize the difference by ear.

I am a tenor, and cannot comfortably match a 60-Hz tone. Matching a 120-Hz tone is very easy for me. Much quicker and easier than dragging out my 'scope.