Acoustic impedance is the way in which the medium, through which the sound wave propagates, reacts to the movement of a vibrating surface. Vincenzo Landi, in “La linea di trasmissione, principi di funzionamento” (AUDIOreview n.104, Aprile 1991), describes it as follows:
“Radiation impedance is the ratio between sound pressure (i.e. the variation of pressure with respect to atmospheric pressure, measured in N/m2) and volume velocity (i.e. the velocity of oscillation of air particles multiplied by the equivalent piston surface of the loudspeaker, measured in m3/s) and is a complex quantity, composed of two real quantities: the radiation resistance, which is responsible for the effective transmission of acoustic power (and therefore for sound generation), and the radiation reactance, which, while not generating sound, takes into account losses due to the inertia and elasticity of air particles”.