Every room has a frequency below which modal effects break the diffuse-field assumption

Evidence

Because all rooms have modes in their lower frequency ranges, there will always be a frequency below which the modal effects dominate and the room can no longer be treated as diffuse. Even anechoic rooms have lower frequency limits to their operation. One of the effects of room modes is to cause variations in the frequency response of the room, via its effect on the reverberant field. The frequency response due to modal behavior will also be room position dependent, due to the spatial variation of standing waves. An important consequence of this is that the room no longer supports a diffuse field in the modal region and so the reverberation time concept is invalid in this frequency region. — David Howard, Jamie Angus, coustics and Psychoacoustics, 2066

Categorization

• DEFINE: Defines the modal region as the frequency band where reverberation-time analysis fails and modal decay analysis must take over.
• corroborated: 2 independent passages support this claim — David Howard, Jamie Angus, coustics and Psychoacoustics, 2066
• XREF: connects to: Modal behavior makes diffuse-field analysis fail in a mid frequency range — David Howard, Jamie Angus, coustics and Psychoacoustics, 2073; A defined frequency marks the transition between modal and diffuse acoustic behavior — David Howard, Jamie Angus, coustics and Psychoacoustics, 2078