# What is acoustic impedance and how to calculate

Acoustic immittance refers to either acoustic admittance (the ease with which energy flows through a system) or acoustic impedance (the blockage of energy flow through a system).

Acoustic Impedance: The resistance to propagating ultrasound waves through tissues. Each tissue type has a unique acoustic impedance. Acoustic impedance is the product of the density and speed of sound in the tissue. Acoustic impedance( Z ) is a physical property of tissue.

Acoustic impedance depends on:

• the density of the tissue (d, in kg/m3)
• the speed of the sound wave (c, in m/s)

and they are related by

• Z = d * c

The acoustic impedance (generally used in discussing the radiation from a vibrating surface) is defined as:

\begin{align} \label{eq:11201a}
\textsf{Acoustic impedance} = \frac{\textsf{Wave pressure}}{\textsf{Particle speed * Area of radiating surface}}
\end{align}

The acoustic impedance is related to the specific acoustic impedance by —

\begin{align} \label{eq:11202a}
\textsf{Acoustic impedance} = \frac{\textsf{Specific acoustic impedance}}{\textsf{Area of radiating surface}}
\end{align}

Acoustic impedance is important in

1. the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances.
2. the design of ultrasonic transducer.
3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any material, so long as its density (p) and acoustic velocity (V) are known.  The applet also shows how a change in the impedance affects acoustic energy reflected and transmitted.  The reflected and transmitted energy values are the fractional amounts of the total energy incident on the interface.  Note that the fractional amount of transmitted sound energy plus the fractional amount of reflected sound energy equals one.  The calculation used to arrive at these values will be discussed on the next page.

Click here to run a JavaScript application on Acoustic Impedance.