The equivalent incidence angle for porous absorbers backed by a hard surface

An equivalent incidence angle is defined as the incidence angle at which the oblique incidence absorption coefficient best approximates the random incidence absorption coefficient. Once the equivalent angle is known, the random incidence absorption coefficient can be estimated by a single experiment using a free-field absorption measurement technique with a source at the equivalent angle. This study investigates the equivalent angle for locally and extendedly reacting porous media mainly by a numerical approach: Numerical minimizations of a cost function that is the difference between the oblique incidence absorption coefficient at a specific incidence angle and the random incidence absorption coefficient. The equivalent angle is found to be around 55° under local reaction conditions, and 45° for extendedly reacting porous absorbers. As practical guidelines for measuring absorption coefficients by free-field techniques, a broad incidence angle range can be suggested: 20° ≤ θi ≤ 65° for extended reaction and θi ≤ 65° for locally reacting porous absorbers, if an average difference of 0.05 is allowed.