Random-incidence absorption coefficients fail to predict actual reflected spectra

Evidence

These measurements again show increased absorption at 45° incidence, which forces that sound to travel through more fiberglass. But the most interesting observation is that the effect an absorber has on reflected sound spectrum is not predicted by the random incidence absorption coefficient. The effect, in reality, is to roll off the high frequencies, not to eliminate the reflection. The roll-off is quite uneven at 45°, making the early-reflected sound field different from what might be anticipated from off-axis anechoic loudspeaker data. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1374

Categorization

• XREF: Connects to the reader’s interest in psychoacoustic models — the standard published absorption data systematically mischaracterizes what a listener actually hears from an early reflection.
• SEED: Article angle: the gap between standardized acoustical measurements and what happens when an absorber faces a real room reflection.