Acoustic Impedance

After reading this section you will be able to do the following:

  • Describe and calculate acoustic impedance.
  • Explain the importance of acoustic impedance.

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid.

The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V).

Z = pV

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 transducers.
  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 the amount of acoustic energy that is reflected and transmitted.  The values of the reflected and transmitted energy 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. 

Change the settings for the applet and observe how the attenuation changes.

Review:

  1. The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V).