St John Baptistery in Pisa is a huge space with well-known reverberation effects. The building, recognised as UNESCO world heritage, is a complex space that shows the coupled volumes of galleries - the so-called "matroneum" - and the conical dome. Previous works concerned the calibration of the numerical model of the space. In the present work, a more detailed survey was conducted on acoustic coupling, echo fluttering, and whispering-gallery effects, basing on measurements and simulation. Moreover, the sound decay of the space does not fit the prediction model of scholars. A further aim of the study is to investigate the peculiar typology of free path distribution.