The primary construction of the Cathédrale Notre-Dame de Paris began in the spring of 1163 CE and continued into the mid-14th century. Two decades after construction efforts commenced, the pre-existing liturgical community in Paris began its occupancy inside the incomplete building. To better understand the interrelationship between the occupants and the cathedral, the acoustical characteristics of key moments in the cathedral’s early development in the 12th and 13th centuries are examined using geometric acoustic modeling. Room acoustic parameters of the sacred regions of the cathedral are examined with attention to the experiences of active participants within the chancel of the cathedral.