Q: When I increase the absorption coefficient in the ceiling reverberation time gets longer?

A: First a general comment on how the reverberation time is calculated from the impulse response. ODEON follows closely the ISO 3382 standard for measurements, i.e. the T30 parameter is found from the slope of a linear regression line that follows the decay curve from -5 dB to -35 dB. In any room the decay curve is a mix of 1-, 2-, and 3-dimensional modes, and generally they have different decays, i.e. the total resulting decay curve is not a straight line, but can be more or less bent. (The 1-dimensional modes have typically the longer reverberation time, and the 3-dimensional modes have the shorter reverberation time).
When the ceiling absorption is high and the walls are reflecting this can give a strong one- and two-dimensional horizontal reverberation. When making the ceiling even more absorbing it means that the one- and two-dimensional sound field gets relatively stronger, and so the reverberation time gets longer. Normally the sound pressure level will decrease. Therefore we recommend that you use auralisation and other acoustical parameters, e.g. STI, SPL and for large rooms DL2 to analyze the acoustics.

 Non-diffuse room - example case

 Claus Lynge Christensen, Gry Bælum Nielsen, Jens Holger Rindel, "Danish Acoustical Society Round Robin on room acoustic computer modelling". ODEON A/S, 2008. 20 pages.