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The sound of electricity comes from its frequency. Frequency is the rate something repeats itself over a period of time like a soundwave. Anything with a frequency that causes it to undergo repeated physical change has a sound. For example if you hit a cymbal with a drumstick you hear a crash because of the frequency of the cymbal’s vibrations. If you strum a guitar you hear it because the strings create vibration at different frequencies. We can hear electricity hum through a transformer because the electricity running through it affects its internal structure and causes vibrations. And we can hear the crack of a corona discharge in the air because it causes molecules in the air to become charged and collide vibrating the air around and making a sound.