Low frequencies cannot form diffuse fields because boundaries create standing waves

Evidence

The assumption behind the equations has been that the sound field is diffuse. However, if this is not the case, the equations are invalid. Although at mid and high audio frequencies a diffuse field might be possible, either by accident or design, at low frequencies, this is not the case due to the effect of the room’s boundaries causing standing waves. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1947

Categorization

• DEFINE: Defines the ‘diffuse field’ assumption that underlies every reverberation-time equation and specifies its low-frequency failure.
• XREF: Extends the model of room acoustics built in the reverberation-time sections of this chapter; limits where those formulas apply. — connects to: Sound waves reflecting between two boundaries can form standing waves — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 386; Standing waves arise when wall spacing equals half-wavelength multiples; Lower frequencies diffract more around obstacles than higher frequencies do (+4 more)
• corroborated: 3 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1947

Broader Connections

• Waves transitioning from bounded to unbounded space create interfering reflections #expands (0.85)
• True diffuse sound fields cannot form in small absorptive listening rooms #corroborates (0.85)
• Acoustic standing waves are termed modes with numbered order by frequency #expands (0.80)
• Even quarter-wavelength diffuser depths are too large for low frequencies #exemplifies (0.75)