An exact solution for the sound energy absorbed by an absorbing panel set in the surface of a large, rigid, plane wall is obtained. The ability of the strip for absorbing sound is given by a “shape factor” A, defined as the ratio of the energy retained by the panel to the energy which would be absorbed, at normal incidence, by the same section of an infinite wall, covered uniformly with the same material. Curves of A are given for three angles of incidence, and for a wide range of wave‐lengths and of normal acoustic impedance of the absorbing material. The “shape factor” reveals that a given material composing a panel absorbs most efficiently when the panel‐width approximates, or slightly exceeds, a half‐wave‐length of the incident sound. For such widths, the factor is larger than two in the case of very “soft” materials, and reduces to unity for less and less absorbent panels.