Modal identification as a tool to increase the accuracy of acoustic absorption measurements in reverberation rooms

Measuring the acoustic properties of materials may be very challenging. At low frequencies, the sample may interfere with the room's modal behavior. When it occurs, the longest modal decays are influenced. Differences in reverberation time - empty Vs with-specimen conditions - are often emphasized by these modal effects, and, as a matter of fact, the absorption coefficient may be overestimated. The present work aims to increase the accuracy through data augmentation at low frequencies. Modal identification was performed on measured impulse responses of porous specimens, allowing the determination of modal-decay times within the third-octave bands of 80-160 Hz. A statistical analysis of natural modes highlights the modes whose decay exceeds the density function as outliers. This preliminary study ultimately provides some statistical considerations on the modal distribution of decay times and their relationship with the reverberation time measured through the Schroeder integral.