声阻抗

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Template:Sound measurements

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

[编辑] 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:

$Z_0 = \rho \cdot c \,$

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

[编辑] 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:

$Z = \frac{p}{v} = \frac{I}{v^2} = \frac{p^2}{I} \,$