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 (p_ef) 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 v_ef. 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