ERATO Proceedings Istanbul 2006.pdf - Odeon

channels four different signals, one is omnidirectional and the others are figure of eight oriented as
the three cartesian component. The second one is a dummy head with two ¼” microphones inside
the ears.
The six different signals, coming from the two microphones, were digitalized by an A/D converter
Tascam MA-AD8 and then acquired by mean of a sound card MOTU 2048mkII linked via the PCI
bus to a portable PC. A dodechaedric source with a power amplifier was used to emit the sound
signals. A sine sweep from 50 Hz to 18 kHz was used as excitation signal. The recording and post
processing software was Adobe Audition with Aurora plug-in.
In the Aspendos theatre 4 position for the sources and 25 for the receivers were chosen. In Jerash
theatre 5 source positions and 17 receivers were placed. Considering the symmetry of the ancient
theatre, the receivers were positioned in one of the two semi-spaces, and only two of the receivers
were placed in a symmetrical position in the other part of the cavea.
4 DATA ELABORATION and RESULTS
For each combination of source and receiver in the theatres, a set of impulse responses were
obtained. In Tab. 1 a typical impulse response for each theatre with the related Schroeder plot are
reported. The position of the source is central (1 m from the symmetry axis) but behind the center of
the orchestra. The height of the source from the floor is almost the same since in Aspendos the
source is on the modern stage while in Jerash it is located on the original one (see Fig. 2 on the
right). In both theatres the receiver is situated in a central position of the cavea, with diazoma
running backward. The two responses are normalized for easier comparison.
In both cases the impulse response is made of the direct sound followed by a few outstanding peaks
and by a larger number of reflections having smaller amplitude. Such peaks are the first reflections
coming from the floor and the back wall. This latter contribute is not so evident in the Aspendos
theatre due to the modern structure. The group of smaller amplitude reflections is produced by the
scattering of sound on the steps of the cavea.
Table 1. Comparison of IRs and energy decays in the two theatres
Impulse response (first 120 ms) Schroeder plot (1.6 s)
Jerash
Aspendos
29