The ancient Greek theatre in Kalydon (Αρχαία Καλυδώνα), Aitolia, has a special shape with a square orchestra and a Pi-shaped koilon. The seat rows of the lower section (rows 1 to 9) are straight and meet at a right angle between the wings and the central part, whereas the seat rows of the upper section (rows 10 to 31) are connected by curved rows in the shape of a quarter circle. The skene building had ramps on both sides, and the height of the proskenion was approximately 2.5 m above the orchestra. The theatre dates from the late 4th or early 3rd century BC.

The acoustical properties of the theatre are analysed using a computer model of a reconstruction of the theatre. Two sound sources are used: an omnidirectional source and a source with the directivity and spectrum of a speaking person. The acoustical parameters used for the analysis are sound pressure level, the clarity index C50, the Speech Transmission Index (STI), and the Dietsch echo criterion. Another analysis is carried out on a theatre model with a semi-circular koilon, but using the same seat-row slope and the same skene building. This theatre has a circular orchestra with the same area as the square orchestra in the Kalydon theatre (approximately 245 m²). By comparing the acoustical results from the two models, it is discussed whether the acoustical properties may have been a major reason for the evolution of ancient Greek theatres from the Pi-shape to the semi-circular shape.