Acoustic impedance
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| Sound measurements |
|---|
| Sound pressure p |
| Sound pressure level (SPL) |
| Particle velocity v |
| Particle velocity level (SVL) |
| (Sound velocity level) |
| Particle displacement ξ |
| Sound intensity I |
| Sound intensity level (SIL) |
| Sound power Pac |
| Sound power level (SWL) |
| Sound energy density E |
| Sound energy flux q |
| Acoustic impedance Z |
| Speed of sound c |
The acoustic impedance Z (or sound impedance) is the ratio of sound pressure p to particle velocity v at a single frequency. Plane, single-frequency traveling waves have acoustic impedance equal to the characteristic impedance, the product of longitudinal wave velocity and density of the medium. Acoustic impedance can be expressed in either its constituent units (pressure per velocity) or in rayls.
Distinction has to be made between:
- the characteristic acoustic impedance Z0 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.
Contents |
[edit] Characteristic impedance
| Temperature Influence | |||
|---|---|---|---|
| °C | c (m/s) | ρ (kg/m³) | Z (Pa·s/m) |
| -10 | 325.2 | 1.342 | 436.1 |
| −5 | 328.3 | 1.317 | 432.0 |
| 0 | 331.3 | 1.292 | 428.4 |
| +5 | 334.3 | 1.269 | 424.3 |
| +10 | 337.3 | 1.247 | 420.6 |
| +15 | 340.3 | 1.225 | 416.8 |
| +20 | 343.2 | 1.204 | 413.2 |
| +25 | 346.1 | 1.184 | 409.8 |
| +30 | 349.0 | 1.165 | 406.3 |
The characteristic impedance of a medium, such as air, rocks or water is a material property:
where
- Z0 is the characteristic acoustic impedance ([M·L-2·T−1]; N·s/m3 or Pa·s/m)
- ρ is the density of the medium ([M·L−3]; kg/m³), and
- c is the longitudinal wave speed or sound speed ([L·T−1]; m/s)
[edit] Impedance of acoustic components
The acoustic impedance Z of an acoustic component (in N·s/m3) is the ratio of sound pressure p to particle velocity v at its connection point:
where
- p is the sound pressure (N/m² or Pa),
- v is the particle velocity (m/s), and
- I is the sound intensity (W/m²)
