Sound basics
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There is a thin layer of skin on the inner side of the human ear, which we call the eardrum. When the eardrum vibrates, the brain interprets this vibration as sound, which is hearing. The sharp change in air pressure is the most common cause of eardrum vibration.
When an object vibrates in the air, it will make a sound (sound can also be transmitted through liquids and solids, but air is the medium through which we hear the sound from the speaker). When an object vibrates, it causes the surrounding air molecules to move. These air molecules will squeeze the air molecules around them, thereby spreading vibrations in the air in a way of spreading disturbances.
To understand this working principle, let us study a simple vibrating body-electric bell. When ringing, the metal vibrates quickly back and forth. When it bends to one side, it pushes out the air molecules around that side. Then, these air molecules collide with the air molecules in front of them, which causes the air molecules in front to collide with the air molecules in front of them, and so on. When the electric-bell metal sheet bends, it sucks in the surrounding air molecules, causing the surrounding pressure to drop, causing more air molecules to be sucked in, so that the pressure around the particles further away also drops, and so on. This pressure drop is called thinning. In this way, the vibrating body transmits pressure pulsating waves through the atmosphere. After the pulse wave is transmitted to the ear, it will vibrate the eardrum repeatedly. The brain interprets this movement as sound.
