In my collage, We were doing an experiment of the air's Breakdown voltage . At the end of the experiment we were listening to the sound of corona (electric partial discharge). The power supply has the ability to vary voltage. The more the voltage increases, The higher the tone of the sound.
As you can hear in this video: https://youtu.be/KVANbkI8AmM?t=8m23s (At 8:23)
I know that the fundamental "hum" frequency will be twice the electrical frequency due to Magnetostriction effect. The frequency of the sound = 100 Hz (in Egypt). So, Why does the tune become higher while rising the voltage?
One more questions, What causes high harmonics in transmission lines? or Why do we hear frequency higher than 100 Hz?
Best Answer
To understand the answer to your first question, one needs to start with a voltage low enough that there is no discharge. This would give you an effective frequency of zero. As you increase the voltage, it becomes easier to ionize the air, so the frequency of the sound (of ionization) increases.
For the second and third questions... higher harmonics are always present, whether we hear them or not, depends on the sound level they produce. Since the power level of the harmonics decreases very rapidly, we may be able to "hear" the second and even the third harmonic, only if the fundamental frequency is loud enough.