Room mode density rises with frequency squared, so uneven mode counts create audible peaks

Evidence

In general, the number of resonances within a given frequency bandwidth increases with frequency. In fact, it can be shown that they increase proportionally to the square of the frequency, and in large well-behaved acoustical spaces, which sound good, this increase in mode density with frequency is smooth. This is the rationale behind a method for assessing the modal behavior in a room known as the “Bonello criteria.” These criteria try to ascertain how significant the modal behavior of a room is in perceptual terms. This is done by dividing the audio frequency spectrum into third-octave bands as an approximation of critical bands and then counting the number of modes per band. If the number of modes per third-octave band increases monotonically, there is a good chance that we will perceive the room as having a “smooth” frequency response despite the resonances. If the number of resonances per third octave drops as the frequency rises, there will be a perceptually noticeable peak in the frequency response. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1981

Categorization

• DEFINE: Introduces the Bonello criteria and the practice of binning room modes into third-octave bands as an approximation of critical bands.
• XREF: Connects to the critical-band concept from earlier psychoacoustics chapters and to any prior material on room modes and standing waves.