1. Sound is a kind of pressure wave: when playing musical instruments, knocking on the door or knocking on the table, their vibration will cause the medium-air molecules to vibrate rhythmically, which will change the density of the surrounding air and form longitudinal waves with alternating density, thus generating sound waves. This phenomenon will continue until the vibration disappears.
2. Sound, as a kind of wave, has become an important attribute to describe wave. Frequency corresponds to what we usually call pitch, while amplitude affects sound. Sound can be decomposed into the superposition of sine waves with different frequencies and intensities. This transformation (or decomposition) process is called Fourier transform.