The seat rows of the Hellenistic theater of Epidaurus play a significant role in the acoustics. The seats, which constitute a corrugated surface, serve as an acoustic filter that passes sound coming from the stage at the expense of surrounding acoustic noise. Reflections on the foreground of the theater result in a better distribution of sound throughout the cavea so that all positions become acoustically similar to one another. The installation of seat rows on a smooth cavea generates diffraction effects that change the acoustic properties of the theater. Second order diffracted sound for frequencies beyond a certain threshold plays an important role and causes sound to be backscattered from the cavea to the audience making the audience to receive sound from the front, but also backscattered sound from behind. The threshold frequency of the filtering effect also is mainly determined by the periodicity of the seat rows in the cavea of the theater.