ERATO Proceedings Istanbul 2006.pdf - Odeon

5.3 The Sound Level
In Figure 6 the strength parameter G is reported vs distance as an average of the middle frequency
bands (500 Hz, 1 kHz and 2 kHz). The graphic is in a semi-logarithmic scale for the distances and
the regression lines are reported together with the free field values. For a better comparison only the
values measured with the source in the orchestra are showed.
In general the G mid values are quite low compared to closed theatres and the trend is constantly
decreasing with a slope close to the free field. In particular RO and HE have a very similar trend,
and are about 5 dB higher than the free field values. The small differences visible at the single
positions are imputable to the different dimensions of the front stage in the two theatres. GE has
lower values probably due to a less efficient reflection from the stage building caused by the
presence of the porch.
6 DISCUSSION
All the results reported above point to the close investigation of the reflection pattern of a typical
impulse response measured in the modelled theatres. An example of the first 200 ms of an IR
recorded in RO (source in the orchestra, receiver at the diazoma minor) is shown in Figure 7.
From the analysis of the reflections it appears that after the direct sound there are only two major
reflections: the former comes from the orchestra floor and the latter, less loud and delayed too, is
from the stage building. Some edge scattering is also recognised in the small peaks following the
floor and the stage building reflections.
Then in the IR one can see a group of distributed reflections with much smaller amplitude which are
mainly caused by the scattering of sound on the tiers of steps in the cavea. Keeping this figure in
mind, the relation between the measured acoustical parameters can be explained.
Amplitude (dB)
Time (s)
Figure 7. Example of an Impulse Response recorded in Roman theatre (first 200 ms)
At each position the energy it is concentrated in the very first part of the IR, including the direct
sound and the two outstanding reflections, and just a very small amount of energy comes from the
tail. Therefore the clarity it is very high and the sound level strongly depends on the distance of the
receiver mostly from the source but also from the stage building. This reasoning can explain the
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