Acoustic resistence

Acoustic impedance (inaccurately sometimes called acoustic resistance) is a physical quantity that describes the acoustic properties of an environment. It is calculated as the ratio of the effective value of the acoustic pressure (pef) and the effective value of the acoustic velocity, i.e. the speed of the oscillatory movement of the particles of the environment caused by the sound wave vef. It is a generalized term, therefore it is also used to interpret the phenomenon at wavelengths other than acoustic ones (e.g. ultrasonic,...)

$$ z = \frac{p_{ef}}{v_{ef}} $$ (Pa·s·m-1)

The effective values of sound pressure and wave speed can be calculated from the maximum values according to the relation:

$$p_{ef} = \frac{\sqrt{2}}{2}\, p_{max} \doteq 0.7\, p_{max} $$

respectively

$$v_{ef} = \frac{\sqrt{2}}{2}\, v_{max} \doteq 0.7\, v_{max} $$

Refraction and reflection of passing acoustic waves can occur at the interface of two environments with different acoustic impedance. This is exactly what ultrasound diagnostics uses.


 * another calculation: Z = ρ. c (Pa.s/m), where ρ – substance density, c – phase speed of propagation in the given substance

Related Articles

 * Ultrasound
 * Properties of Sound