The acoustic impedance Z (or sound impedance) is a frequency (f) dependent parameter and is very useful, for example, for describing the behavior of musical wind instruments. Mathematically, it is the sound pressure p divided by the particle velocity v and the surface area S, through which an acoustic wave of frequency f propagates. The characteristic acoustic impedance of a medium, such as air, rock or water is a material property and equals the product of the density of the medium and sound speed.