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At a given surface, the complex ratio of effective sound pressure averaged over the surface to the effective flux (volume velocity or particle velocity multiplied by the surface area) through it. The unit is the N x (s/m5) (newton- second/meter or the mks acoustic ohm. In the cgs system the unit is the dyn x (s/cm5) (dyne-second/centimeter5)
Specific acoustic impedance is the complex ratio of the effective sound pressure at a point to the effective particle velocity at a point. The unit is the N x (s/m3) or the mks rayl. In the cgs system the unit is the dyn x (s/cm3) or the rayl. The difference between specific acoustic impedance and acoustic impedance is in the specification of impedance at a point, as compared to the average over a surface.
Characteristic acoustic impedance is the ratio of effective sound pressure at a point to the particle velocity at that point in a free, progressive wave. This ratio is equal to the product of the density of the medium times the speed of sound in the medium. The characteristic impedance of a sound wave is analogous to the characteristic electrical impedance of an infinitely long, dissipationless transmission line. It's common in acoustical analyses to represent specific acoustic impedances in terms of their ratio to the characteristic impedance of air.
Acoustic impedance, being a complex quantity, can have real and imaginary components analogous to those in an electrical impedance. In applying this analogy, the real part of the acoustic impedance is termed acoustic resistance, and the imaginary part is termed acoustic reactance.
