Modal decay differs from diffuse decay because reflection paths are fixed, not random.

Evidence

The decay of sound energy in modes is in many respects identical to the decay of sound energy which is analyzed in Appendix 4. The main differ­ ence is that the absorption coefficient is sometimes smaller, because the modal sound wave does not have random incidence; it will also be specific to the surfaces involved instead of being an average value for the whole room. In addition the time between reflections will be dependent on the length of the modal path rather than the mean free path. This means that the decay time for a mode is likely to be different to the diffuse sound. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2044

Categorization

• DEFINE: Explains why mode decay time is computed using modal path length rather than the statistical mean free path — a definitional contrast that is key to psychoacoustic room analysis.
• SEED: Article angle: why singing a sustained note in a small room ‘rings’ differently than in a hall — the physics of modal paths versus diffuse fields as felt experience.