Acoustic Characterization of the Ancient Theatre at Syracuse

First author:
Marco Gullo
DREAM, University of Palermo,
mgullo@dream.unipa.it
 
Co -authors:
A. La Pica, G. Rodono’ and V. Vinci
DREAM, University of Palermo
 
Final title: 4pAAa3. Acoustic characterization of the ancient theatre at Syracuse
Paper code: 2354
Time, Date, Place of presentation: Thursday 3 July 2008, 2:00 P.M.
 

Introduction

Architectural acoustics concerns the interaction between the space and its acoustic quality. From the designer point of view the main task is to build a space where geometries and materials interact to produce a sound field suitable for the purpose the space is built for. If this target is achieved is a matter of subjective impression but there are “engineering tools,” indices, correlated with subjective impressions, that help to describe objectively the sound field. Finally, as we have many impressions when we subjectively describe a listening experience (volume, intelligibility, sound direction, etc.) we use several acoustic indices that objectively describe different sound energy features.

An ancient open-air theatre is an environment designed to hold thousand of people for drama performance: many examples are still preserved from Greek or Roman periods and some of them have remarkable subjective acoustic qualities. Characterizing objectively the sound field of such theatres offers the opportunity to correlate architectural and acoustical features and to correlate objective indices with subjective impression. Moreover, today such spaces are used not only for drama performance and very often they are not preserved in all their architectural components: on field objective acoustics data are needed to verify their performance and to calibrate models in order to simulate and predict acoustic performance versus architectural features.

An ancient open-air theatre is basically characterized by three architectural components: a circular or semicircular orchestra, a raised stage behind the orchestra and several rows of steps surrounding the orchestra (cavea). Very often the raised stage is not preserved and the cavea does not account for the original number of steps. Theatre’s size depends upon orchestra radius and the number of steps all around: in Segesta, for example, the theatre today can hold 2000 spectators while five times more the theatre in Siracusa.

The ancient theatre in Syracuse

The theatre is intensively used during the early summer season so the purpose of the survey was to record the theatre acoustics as a drama spectator would experience them. In fact the theatre place is strongly changed during classical representations: the cavea is covered with wood panels while the orchestra and the stage are prepared by the temporary stage manager. Usually the attitude is to follow the classical setup: a large and medium raised stage behind a large smooth semicircular plane laying on the orchestra (this was the case during our acoustic measurements). It must be pointed however that unfortunately main measurements were carried out in an empty state for the theatre and only few of them were performed in a partially occupied state.

The ancient theatre in Syracuse is a Greek-Roman type. The orchestra is an semicircle with a 29 meters diameter and the stage (scena) is not preserved. The cavea extends for 180° around the orchestra and continues with two wings for about 5 meters at the sides. It is radially splitted in nine sectors, kerkìdes, by eight stairs, klimàkes, 0.6 meters large: rougly each stair counts two steps every cavea step. First twelve cavea steps, from the bottom, constitute the ima cavea which has an average slope of 22,5°. This area is bounded on the upper part by a step 0.86 meters high, after which rows slope is about 20,8°. This area, media cavea, is bounded on the upper part by a large passage, diàzoma. Further on this passage up to the last row we have the summa cavea, with the same average slope of the media cavea. Most of the summa cavea is absent today and the grass appears after few rows.

During the 2007 drama festival most of the cavea was covered by wood planks hiding the stone below while there were two kind of stage. In fact two dramas were in program and alternated one day for each. The day of acoustic measurements Sophocles’s Trachinie was in program: a large and small rised wooden stage, covered with polystyrene, and a not too high wooden backing wall was prepared.

Acoustical characterization

In a closed environment the sound energy is bounded and travels changing direction and magnitude according to the distance covered and the objects along its path: so a relevant “reverberant” energy is summed to the “direct” energy in every point in the space. In an open-air theatre sound energy is weakly bounded and rather free to propagate away so that stage size and cavea slope and size strongly affect its acoustic performance. Ideally, in a completely boundary free environment there is no reverberant energy and only the direct energy is propagated (free-field condition).

An objective index accounts for the sound strength: the gain the space under test introduce respect to a free-field condition. In Syracuse the theater’s “gain” respect to a free field is on average about 3 dB while the decay rate with the distance is roughly the same (-6 dB doubling the distance between source and receiver).

The index “Reverberation Time,” RT, accounts for the reverberant energy (it is calculated in a simple way recording the sound decay rate after a noisy test signal abruptly muted): a small reverberant field exists and relatively high reverberation occurs in the mid-high frequency range.

Exciting the environment with an impulsive signal and recording the theatre’s response at several locations shows that the sound energy lies mainly in the first hundreds of seconds of the responses and that strong first reflections build up the weakly reverberant field.

Similar findings resulted at the ancient theatre in Segesta (except for smaller RT values because of the absence of the stage, thus reducing the boundaries of the weakly enclosed space).

Conclusion

Main results at the theatre in Syracuse are common in ancient open-air theatres. The reverberant field is quite weak and the theatre sounds “dry”: the theatre appears suitable for drama performance but needs a more enclosing stage for music performance, at first sight. Sound energy is due to direct propagation from the source and first reflections from boundaries, such as an hard covered orchestra: this cause a doubling of the energy (a 3 dB gain respect to a free field).

The stage preparation thus appears critical in this theatre in order to improve sound strength and avoiding electro-acoustic sound reinforcement at least for spoken performance.

Acknowledgements

The authors wish to thank Soprintendenza dei beni archeologici di Siracusa for the precious support. This work was carried out with part of the funds granted by MIUR (Italian Ministry of University and Research) to the project PRIN2005 ATLAS (Ancient Theatres Lighting and Acoustics Support).

Pictures

Cavea view

Stage and orchestra view

Cross-section view with sound sources and microphones