Acoustic Impedance

Acoustic impedance indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a given frequency. The acoustic impedance Z (or sound impedance) is frequency (f) dependent and is very useful, for example, for describing the behaviour 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. If the impedance is calculated for a range of excitation frequencies the result is an impedance curve. Planar, single-frequency traveling waves have acoustic impedances equal to the characteristic impedance divided by the surface area, where the characteristic impedance is the product of longitudinal wave velocity and density of the medium. Acoustic impedance can be expressed in either its constituent units (pressure per velocity per area) or in rayls per square meter.

Note that sometimes vS is referred to as the volume velocity.

The specific acoustic impedance z (also called shock impedance) is the ratio of sound pressure p to particle velocity v at a single frequency and is expressed in rayls. Therefore

where D is the shock wave velocity.

Distinction has to be made between:

• the characteristic acoustic impedance of a medium, usually air (compare with characteristic impedance in transmission lines).
• the impedance of an acoustic component, like a wave conductor, a resonance chamber, a muffler or an organ pipe.