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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
- the determination of acoustic transmission and reflection
at the boundary of two materials having different acoustic impedances.
- the design of ultrasonic transducers.
- 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.
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