Ancient Greek and Roman theatres are often considered acoustically perfect. However, the semicircular shape of the audience area in theatres built from hard and sound reflective materials may cause acoustic problems, and there is also evidence that the ancient architects were well aware of this. The Roman architect Vitruvius describes in his books on architecture four different kinds of sound reflections in a theatre, one of them called ‘circumsonant’ which is an acoustic phenomenon that we today would name a focused echo. Computer simulations of some examples of ancient Greek and Roman theatres confirm that this can be a real problem at some places in the audience area. For this study is used the echo criterion for speech as suggested by Dietsch & Kraak; this is implemented in the room acoustics software ODEON, and that makes it easy to identify positions with echo problems.
A possible solution to these echo problems could be the introduction of sound absorption in the vertical, concave surfaces in a way similar to that described by Vitruvius for the sounding vessels, i.e. in niches between the seats arranged in a horizontal range halfway up. Reading the description by Vitruvius in this light, it makes good sense if the vessels are supposed to act as sound absorbing resonators. Since they are efficient in a narrow frequency band, it also makes sense to apply different sizes with resonance frequencies distributed over two octaves, as described in detail by Vitruvius. Finally, it is noted that the principle of installing the sounding vessels in the theatre comes from older Greek references; obviously Vitruvius had no experience with the vessels himself, but he refers to a scheme based on music theory made by Aristoxenus (4th century BC), who was a famous Greek philosopher and expert in music theory.
