What is sound
A sound source causes changes in air pressure that travel in a wave motion through air. When such a sound wave reaches the eardrum, it is vibrated according to the frequency of the sound wave.
In most cases this is in the air (air noise: the human voice, TV and HiFi devices, musical instruments, …), but these vibrations can also propagate through contact surfaces (contact noise: high heels, a slamming door, a knocking hammer, scratches on a board, …).
Sound can be heard by people or animals with an organ of hearing when the eardrum of the ear is vibrated. The hearing organ processes these vibrations into signals that are interpreted by the brain.