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Why do humans hear sounds?

Sound is a form of "energy" generated by the vibration of an elastic medium that stimulates hearing.

The vibration of an object produces a continuous wave of compression and expansion in the air, which spreads in all directions through the air. Sound travels in the air at a speed of about 340 meters per second, but the exact speed is related to the temperature and humidity of the air. Sound propagates several times faster in dense media, such as water, than in air, but it cannot propagate in a vacuum.

It takes very little mechanical energy to produce sound, because the process of sound production is just the transfer of vibration energy from the source to the receiver. The volume produced by the sound source is called sound power, measured in watts. For example, the sound power generated by a symphony orchestra at the climax of its performance is about 90 watts. The sound emitted from the sound source spreads into a gradually increasing area. The so-called sound intensity is the sound energy passing through per unit area, measured in watts per square meter. Generally speaking, the sound intensity decreases with the square of the distance from the sound source.

The number of repetitions of sound vibrations determines the frequency of the sound, which can also be considered the pitch of the sound. The human ear can only respond to sounds with frequencies between 20 Hz and 20 kHz. And its response is different within this frequency band.