2.1. The “Huygens” principle
“If from a point S of a homogeneous isotropic medium a spherical
wave is emitted, one can imagine the procedure of the individual
wave reproduction in that a particle brought into oscillation by
external forces, transfers its movements to its neighboring particles.
This procedure then continues symmetrically in all directions
and in this way gives cause to a spherical wave...” [7].
If a sound source S (the so-called “primary source”) emits
spherical wave fronts, one can imagine in accordance with the
“Huygens” Principle each emitted wave front (refer to Figure 1a
blue), through the addition of all participating “secondary sources”
(which also emit spherical waves) on the surface O. Due to the
knowledge of the wave front on surface O (Figure 1a red) the state
of the oscillation can be determined at an arbitrary point P of the
sound field. The wave front through point P is constructed through
the summation of all participating secondary source signals.