Chapter 8

# 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 :