A MODAL APPROACH FORREVERBERATION TIME MEASUREMENTS IN NON-DIFFUSE SOUND FIELD

In recent years the extension of reverberation time measurements to frequencies below 100 Hz is becoming more and more important due to the increase of low frequency sources. In ordinary rooms with volumes smaller than 200 m3the low frequency sound field is non-diffuse due to the presence of modes, which are also the main cause of bad quality of listening in terms of clarity and rumble effects. Since standard measurements according to ISO 3382 fail to achieve accurate and precise values in third octave bands due to non-linear decays of room modes, a new ap-proach based on reverberation time measurements of single resonant frequencies (the modal re-verberation time) is introduced. Two measurementmethods based on the relation between mod-al decays and resonant half bandwidths are proposed: the direct method, based on the direct evaluation of modal sound decays with interrupted source signals, and the indirect method based on half bandwidth measurements. Proper measurement procedures, with microphones placed at rectangular room corners and anti-resonant sine waves and sweep source signals for direct and indirect measurement methods respectively, are proposed. Comparison between direct and indi-rect methods shows good and significant agreement. Comparing modal reverberation times with standard ones, the inadequacy of standard procedure to get accurate and precise values at low frequencies with respect to the modal approach becomes evident. In the future, further investi-gations are necessary in more rooms toimprove uncertainty evaluation.