When material vibrates (a string, a speaker, a rock) it displaces air molecules, creating pressure waves that propagate through spacetime. We call this sound.
Sound is also a sensation. Our ears turn the pressure waves into electrical signals that our brains can process, feel and understand.
Sound has physical characteristics that can be measured, like amplitude and frequency, and perceived characteristics that are subjective, like loudness and quality.
The wikipedia definition alludes to this dual nature:
In physics, sound is a vibration that propagates as a typically audible mechanical wave of pressure and displacement, through a medium such as air or water. In physiology and psychology, sound is the reception of such waves and their perception by the brain.
Sound is a type of energy that is created when objects vibrate, causing pressure waves in the surrounding medium (air, water). When sound waves reach our ears, they are turned into electrical signals that are processed by our brains, which interpret them as sound.
some key terms:
Vibration: Sound begins when an object vibrates, such as a guitar string, a vocal cord, or a speaker diaphragm.
Sound Waves: These vibrations create waves in a surrounding medium (air, water, or solid materials). Sound waves are longitudinal waves, meaning the particles in the medium move in the same direction as the wave itself (back and forth).
Frequency: This refers to the number of vibrations (or cycles) per second, measured in Hertz (Hz). .
Amplitude: This refers to the strength of the vibrations, which affects the loudness of the sound.
In summary, sound is a mechanical wave that requires a medium to travel through and is perceived by our ears when the waves interact with the eardrum and are processed by the brain.