The acoustic impedance Z (or sound impedance) is the ratio of sound pressure p to particle velocity v in a medium or acoustic component. Z can be either measured in its consituient units (pressure per velocity) or in rayls.

Distinction has to be made between:

• the characteristic acoustic impedance $Z\_0$ of a medium, usually air (compare with characteristic impedance in transmission lines).
• the impedance $Z$ of an acoustic component, like a wave conductor, a resonance chamber, a muffler or an organ pipe.

Characteristic impedance

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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 = \rho \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

Impact of temperature
°C	Z in Pa·s/m	c in m/s	ρ in kg/m³
− 10	436.5	325.4	1.341
−  5	432.4	328.5	1.316
0	428.3	331.5	1.293
+  5	424.5	334.5	1.269
+ 10	420.7	337.5	1.247
+ 15	417.0	340.5	1.225
+ 20	413.5	343.4	1.204
+ 25	410.0	346.3	1.184
+ 30	406.6	349.2	1.164

Impedance of acoustic components

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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 = \frac{p}{v} = \frac{I}{v^2} = \frac{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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• Impedance

External links

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• Ohm's law of the acoustics - calculation of acoustic impedance Z sound pressure p particle velocity v sound intensity I
• Connection of acoustic sizes for even progressive acoustic waves

Acoustics

Schallkennimpedanz | Impedancia acústica | 声阻抗