Acoustic Impedance

The total reaction of a medium to the transmission of sound through it, expressed as the ratio of sound pressure to particle velocity at a given point in the medium. The acoustic impedance Z is useful in describing the acoustic radiation from sources such as surfaces and ducts.

where

Z = acoustic impedance [rayls or kgm^-2s^-1]

p = acoustic pressure [Pa]

U = particle velocity [ms^-1]

The acoustic impedance of air is the density ρ times the speed of sound c

Also known as characteristic impedance.

The acoustic impedance of a duct is

where

S = cross-sectional area of the duct [m²]

For a typical air intake duct of internal radius 0.03m the impedance Z₀=1.47x10⁶.

See also: Acoustic Admittance, Acoustic Ohm, Duct Acoustics, Impedance, Rayl.