Theory

The wall impedance is the hole impedance divided by the open-area ratio, equation (1) (Lee and Ih, 2003): at a given particle velocity through the wall, a smaller open area forces a faster flow through each hole, so the same per-hole losses act more strongly.

The value the acoustics uses, equation (2):

a perforated tube: the open-area ratio of the flow card (Open-area ratio);
a plate whose flow loss is computed from the open-area ratio (Defined by (perforated plate) = open-area ratio): that same value;
a plate whose flow loss is a typed coefficient $K$ : this field.

The acoustics-only page has no flow card; its "Open-area ratio" field is Open-area ratio, and the acoustics uses it.

Equations

\zeta_p = \frac{\zeta_h}{\sigma}

(1)

\sigma = \begin{cases} \sigma_{flow} & \text{perforated tube, or plate with mode = open-area ratio} \\ \sigma_{ac} & \text{plate with mode = loss coefficient } K \end{cases}

(2)

where	meaning	unit
$\zeta_p$	Normalised wall impedance	-
$\zeta_h$	Normalised impedance of one hole	-
$\sigma_{flow}$	Open-area ratio of the flow card	-
$\sigma_{ac}$	This field	-

Default and rule

Default: the wall's open-area ratio when it has one; otherwise as entered
Rule: The field starts with the flow card's open-area ratio; it is stored only when typed.
Source: The division by $\sigma$ is Lee & Ih's eq. (25) (Lee and Ih, 2003); the program applies it to every impedance model.

Valid range 0 < σ ≤ 1

Values

$0 < \sigma \le 1$ .

Ignored in some cases

On a plate in open-area-ratio mode the field is shown on the card, but the acoustics uses the flow card's value; type the open-area ratio there.

Model ranges

Lee & Ih: 2.79-22.3 %; Kirby & Cummings: fitted for 20-27 %.

References 1

Lee, S.-H. and Ih, J.-G. (2003). Empirical model of the acoustic impedance of a circular orifice in grazing mean flow. The Journal of the Acoustical Society of America, 114(1), 98-113. DOI 10.1121/1.1581280

Theory

Equations

Default and rule

Valid range 0 < σ ≤ 1

Related 5

References 1