Sound angled into absorbers absorbs more by traveling through more material

Evidence

Figure 12.21 is persuasive evidence that the angle of incidence matters greatly to the acoustical absorption of fiberglass panels. The low-bass performance is seriously degraded, and the absorption coefficients do not exceed 1.0. The absorption at normal incidence is lower than at 45° because the sound travels a shorter path to the reflecting surface. Lower incident angles experience greater absorption, explaining why random incidence measurements are highest because they include all angles up to grazing incidence, where the absorbing path length is maximum. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1372

Categorization

• XREF: Connects to Chapter 2’s Figure 2.10 documenting actual early-reflection incidence angles of 45-60 degrees.