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.

The real and imaginary components are called, respectively, acoustic resistance and acoustic reactance.

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

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Subjects: Noise & Vibration