Acoustic impedance
Encyclopedia : A : AC : ACO : Acoustic impedance
| 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 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
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]
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).
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
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 = \frac = \frac]
v = particle velocity in m/s
I = sound intensity in W/m2
See also
External links
- [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]
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.