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 [kg/m^3] and acoustic velocity V [m/s].

Z = pV

The acoustic inpedance Z can be expressed in:

• N.s/m^3
• Pa.s/m
• kg/(m^2.s) = 1 Rayl (S.I. unit of acoustic impedance is MRayl or Megarayls)

1 Rayl = 1 dyne.s/cm^3
1 Rayl represents the specific acoustic impedance when a sound pressure of 1 Pa produces a linear velocity of 1 m/s.

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.