We hear them with nerve-endings in our ears. When spreading out they have a finite speed that characterizes medium compressibility. For adiabatic and isentropic processes , where k is adiabatic exponent. In each elementary volume there is oscillations of overpressure. An energy of an acoustic wave is characterized by an acoustic pressure and an acoustic intensity.
Acoustic waves have all wave properties.
It is proved in occurrence of interference and diffraction during their spreading. It is a relative value showing how much the volume get increased or decreased. According to the psychophysical law of Weber-Fechner, this value for a human is in direct proportion to a subjective perception of volume change. The sounds will have some different characteristics. Their collection is called a timbre.