Significant architectural modifications were made to Notre-Dame de Paris between the 1230s and the middle of the 14th century despite the completion of the central volume in the 1220s. These changes include the restructuring of the windows and attics along the clerestory and the gradual outward expansion of the building. Mainly, the transept arms were widened and side chapels were constructed between the buttresses along the perimeter of the cathedral. This study explores the acoustic effect the modifications to the transept and side chapels may have had on sound produced in the cathedral.