# Acoustic impedance

Acoustic impedance Z (characteristic impedance or sound impedance) is the ratio of sound pressure p to particle velocity v. Also it is the product of the density of air ρ (rho) and the speed of sound c. See table: acoustic impedance vs. temperature.

The acoustic impedance Z in Pa·s/m (or, in the equivalent units common before the pascal was added to SI, N·s/m3) is:

[itex]

Z = \frac{p}{v} = \frac{J}{v^2} = \frac{p^2}{J} = \rho \cdot c [itex]

The acoustic impedance (characteristic impedance) of air is Z = 410 Pa·s/m at the temperature 25 °C (77 °F) or 413 Pa·s/m at 20 °C (68 °F). °C = degree Celsius and °F = degree Fahrenheit.

p = sound pressure in N/m2 = Pa = pascal
v = particle velocity in m/s
J = sound intensity in W/m2
F = force in N = newton
ρ (rho) = density of air in kg/m3
c = speed of sound (acoustic wave velocity) in m/s

Table - Acoustic impedance Z, Speed of sound in air c,
density of air ρ vs. temperature °C

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

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