Sound is created by a vibrating source and needs a medium to travel. If you talk, the source of sound is your vocal cords. They vibrate when you speak. The medium through which the vibrations travel is air.
It is similar to the following situation.
When you shake your hands submerged in water. The water forms waves. Similarly, when you speak, your vocal cords shake, and they create waves in the air. This shaking is also called vibrating.
The only difference is you are shaking your hands very slowly, maybe one or two times in a second.
While your vocal cords vibrate more than a hundred times in a second. A number of “shakes” or “vibrations” per second is called frequency. And it is measured in hertz. So one hertz means one vibration per second. Twenty hertz means twenty vibrations per second and twenty thousand hertz means twenty thousand vibrations per second.
The human ear can detect these vibrations if the frequency is between twenty hertz to twenty thousand hertz. And we call the detected vibrations as sound.
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