Modal behavior intensifies as wavelength approaches the room’s mean free path

Evidence

How can the critical frequency be calculated? There are two main approaches. The first is to recognize that when the wavelength of the sound approaches the mean free path of the room, the likelihood of modal behavior increases, because a sound wave is “in touch” with all the walls in the room. This approach can be used to set an approximate lower frequency bound on the critical frequency below which it is likely to be difficult to prevent modal effects from dominating the acoustics without extreme measures being taken. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2029

Categorization

• QUESTION: What exactly is the mean free path, and how does one compute it for a given room? Worth chasing the formula.
• DEFINE: Explains the physical mechanism (wavelength touching all walls) behind modal dominance rather than just naming the concept.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2029
• 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