Absorption of Finite-Size Absorbers, as a Problem of Radiation

→ See also Mechel, Vol.?I, Ch.?8 (1989)

The surface impedance $Z_{{A}}$ of an infinite absorber is generally easily evaluated. The problem with finite-size absorbers in a baffle wall is the influence of border scattering. This influence can be taken into account by a simple equivalent network.

The absorbed effective power is:

The normalised absorption cross section is: $\begin{array}[]{rclr}\displaystyle\Pi _{a}&=&\displaystyle A\cdot\frac{|P_{i}|^{2}}{2\, Z_{0}}\;\frac{4\, Re\{ Z_{A}\}}{|Z_{A}+Z_{s}|^{{\, 2}}}\,.&\qquad(1)\\ \displaystyle\frac{Q_{a}}{A}&=&\displaystyle\frac{4\, Re\{ Z_{A}\}}{|Z_{A}+Z_{s}|^{{2}}}\,.&\qquad(2)\end{array}$

$A$	=	area of the absorber;
$P_{{i}}$	=	amplitude of the incident plane wave;
$Z_{{A}}$	=	surface impedance of the infinite absorber;
$Z_{{s}}$	=	radiation impedance of a radiator with the size and shape of the absorber,
		when its surface oscillation pattern agrees with that of the exciting wave at
		the absorber surface

This can be...