Mechanical waves
Acoustic impedances
Fondamental : Calculate the acoustic impedance
Consider a harmonic progressive plane sound wave (HPPSW) propagating in ascending order .
The speed and pressure may be written :
And :
With the dispersion relation :
Where is the speed of sound waves.
The following equation (see course sheet on sound waves) :
allows to determine a relation between the speed and the acoustic pressure.
Indeed :
Is :
Attention : Definition of the acoustic impedance

For HPPSW moving in the direction of :
With :
is called acoustic impedance.

For HPPSW moving in the direction of :
Remarque : Other definition of acoustic impedance
The acoustic impedance can be defined according to :
Where is the cross section of the sound conduct.
This definition is similar to that of the electric resistance : the flow rate is comparable to the electrical current (charge flow rate).
The acoustic impedance is defined as :
See also the lesson about "Thermal resistance".