Theory
The incident wave is the plane wave travelling INTO the model through the port, not the total pressure at the port (which also contains the reflected wave). Its power follows from the amplitude, equation (1); an entered power is converted into an amplitude by the inverse relation, equation (2). The density and sound speed are those of the port's own gas (with a medium mapped from the CFD, at the port's area-mean temperature and pressure).
Because the model is linear, the amplitude does not change the transmission loss of a single inlet. With several excited inlets the reported TL is the solution with all of them driven together, so their RELATIVE amplitudes and phases matter. The TL of each inlet is its incident power over the power leaving each outlet, equation (3); the per-inlet TL sums over the outlets.
Equations
| where | meaning | unit |
|---|---|---|
| Incident sound power of the plane wave | W | |
| Incident pressure amplitude | Pa | |
| Port area (of the second-order mesh) | m² | |
| Density and speed of sound of the gas at the port | kg/m³, m/s | |
| Power leaving outlet j with every excited inlet driven | W |
Default and rule
- Default
- amplitude
- Rule
Amplitude (1 Pa).
- Source
The plane-wave power in a duct, , is the standard result for a uniform medium without flow. With mean-flow convection the program multiplies the modal powers by the convective factor of (Morfey, 1971).
Valid range amplitude | power
Amplitude or power
Use power when the sound powers of the inlets are known (an engine's source power); use amplitude for a unit excitation. Absolute levels matter only for the radiated SPL, which is computed from the insertion loss and an entered source spectrum, not from these values.