David French wrote:Note how absorption shifts the center frequencies of the resonances. BTW, if anyone has resources where I could further study this phenomenon, please let me know.

I'm a bit confused, because the addition of wall absorption should shift room modal frequencies higher rather than lower. As for a resource if you are really interested, the first (and best) theoretical analysis of this is by Morse from 1939 [Insert spooky time-travel theremin music here

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Philip M. Morse, "Some Aspects of the Theory of Room Acoustics," J. Acous. Soc. Am. 11, 56 (1939).

This paper derives one analytical solution to the wave equation, the thing that determines all the room modes, their damping, etc. It's a little slow-going mathematically, though not as hairy as Morse's definitive and monumental tome "Theoretical Acoustics."

One of the things that Morse does is look at the simple 1-dimensional case of two parallel walls. One wall is a poor absorber, while the other wall's absorption is changed continuously (via varying its impedance, which is taken to be purely real). What happens is that the standing wave frequencies shift upward, increasing in an S-shaped curve to 50% higher at lowest wall impedance (maximum absorption). I once confirmed this behavior with my own Finite Element Method axial mode analysis. Morse's analysis includes the 0th room mode (nominally "DC"), which always seems to get ignored in less rigorous room mode discussions.

Note that the extremes are exactly the same as the modes in a tube closed at both ends vs. a tube open at one end only. An open tube end, just like an open window, is a perfect absorber.

Regards,

Terry