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The acoustic impedance Z (or '''sound impedance''') is the ratio of Sound Pressure ''p'' to Particle Velocity ''v'' in a medium or acoustic component.

Distinction has to be made between:
  • the characteristic acoustic impedance Z_0 of a medium, usually air (compare with Characteristic Impedance in Transmission Line s).

  • the impedance Z of an acoustic component, like a wave conductor, a resonance chamber, a muffler or an organ pipe.



CHARACTERISTIC IMPEDANCE


The characteristic impedance of a medium (usually air, but also for instance exhaust gases in a muffler) is the ratio of Sound Pressure ''p'' to Particle Velocity ''v'' in open field (i.e. in a condition of no reflecting waves).
This impedance is a material constant and equals the product of the density of air ρ (rho) and the Speed Of Sound ''c'':

:
Z_0 = ho \cdot c


''ρ'' (rho) = Density Of Air in kg/m3

''c'' = Speed Of Sound ( Acoustic Wave Velocity ) in m/s

Note: Z_0 is expressed in Pa·s/m. Before the pascal was introduced by the SI , impedance was expressed in N·s/m3.

The characteristic acoustic impedance of air at one Atmosphere pressure Z_0 equals:
  • 410 Pa·s/m at a Temperature of 25 °C (77 °F)

  • 413 Pa·s/m at 20 °C (68 °F).


°C = degree Celsius and °F = degree Fahrenheit .

Table - Acoustic impedance Z_0, Speed Of Sound in air ''c'', Density Of Air ''ρ'' vs. Temperature °C at 1 Atm pressure


IMPEDANCE OF ACOUSTIC COMPONENTS


The acoustic impedance ''Z'' of an acoustic component (in Pa·s/m) is the ratio of Sound Pressure ''p'' to Particle Velocity ''v'' at a transition with a previous component:
:
Z = rac{p}{v} = rac{I}{v^2} = rac{p^2}{I}


''p'' = Sound Pressure in N/m2 = Pa = pascal

''v'' = Particle Velocity in m/s

''I'' = Sound Intensity in W/m2



SEE ALSO




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