Low absorption sustains strong modal variation in rooms at higher frequencies than predicted

Evidence

This expression is useful for making a rapid assessment of the likelihood of achieving a particular critical frequency in a given room. However, the real critical frequency may well be higher, because a room can have significant modal behavior at high frequencies if the absorption is low. Because of this, the accepted definition of critical frequency is based on the mode bandwidth, although this can result in a chicken-and-egg situation at the initial design stages; hence the earlier equation. … The main consequence of modal behavior is the frequency and spatial variation caused by it. This means that if a given frequency excites only one mode, then this variation will be very strong. However, if a given frequency excites more than one mode, both the spatial and frequency variation will be reduced. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2033

Categorization

• DEFINE: Defines critical frequency by mode bandwidth rather than a fixed number, and explains why the practical formula can understate the true value.
• 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