The present paper deals with acoustic computer simulations of Hagia Sophia, which is characterized not only by being one of the largest worship buildings in the world, but also by – in its 1500 year history – having served three purposes: as a church, as a mosque and today as a museum. The investigation is done as a part of the EU project – CAHRISMA. The room acoustic differences in the three periods have been investigated by creating three different computer models each including the particular furnishing as reported in the historical archives. For all the configurations simulations and auralizations have been made, with and without people occupying the rooms.