Phase of the incident wave

The phase of the incident wave at this inlet relative to the other excited inlets. It matters only when several inlets are excited: the reported transmission loss drives them together, and their waves add with these phases.

Last updated 2026-11-03

Theory

The model is linear: the outgoing waves are $B = S\,a$ , with $S$ the scattering matrix of the sweep and $a$ the vector of incident amplitudes. With several excited inlets, $a_i = |A_i| e^{i\phi_i}$ , equation (1), and the power leaving each outlet is the power of the superposed wave.

Two equal inlets in phase add at an outlet.
In opposition (180°), a symmetric muffler can cancel at the outlet, which appears as a very high TL.
For a single inlet the phase has no effect.

Equations

\mathbf{B}_{out} = \sum_i \mathbf{S}_{:,i}\,|A_i|\,e^{i\phi_i}, \qquad W_{out,j} = \sum_{m \in j} w_m |B_m|^2

(1)

where	meaning	unit
$\phi_i$	Phase of inlet i (this field)	rad
$\mathbf{S}_{:,i}$	Column of the scattering matrix for the plane mode of inlet i	-
$B_m$	Outgoing amplitude of mode m	Pa
$w_m$	Modal power weight $S\,\mathrm{Re}(k_z)/(2\rho\omega)$	m⁴ s/kg

Default and rule

Default: 0°
Rule: 0° (all inlets in phase).
Source: Linear superposition of the port waves, with the time convention $e^{+i\omega t}$ of the acoustic solver. With mean-flow convection the modal powers carry the factor of (Morfey, 1971).

Valid range any value (degrees)

Values

Any number of degrees; 360° is the same as 0°.

Correlated sources only

Driving inlets with fixed phases assumes coherent sources (one engine, two pipes). For uncorrelated sources, evaluate the TL of each inlet alone in the port-power file.

References 1

Morfey, C. L. (1971). Acoustic energy in non-uniform flows. Journal of Sound and Vibration, 14(2), 159-170. DOI 10.1016/0022-460X(71)90381-6

Theory

Equations

Default and rule

Valid range any value (degrees)

Related 5

References 1