CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
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2. SIMULATIONS
Before the room acoustic computer models were developed the
best way to make a prediction for room acoustics was through
measurements in a scale model. Nowadays the room acoustic
computer model has reached a good level of reliability (Rindel,
2000; Lynge Christensen, 2001). Modifications can be made in
an easy way and in a short time and the results can be evaluated
by listening too. Different softwares are available for room
acoustic simulations. In this study the ODEON (Odeon
websites) software package is used.
2.1 The simulation principles
For the acoustic simulation, a three-dimensional model has to
be created (Fig.2). Materials with their absorptions coefficient
and scattering coefficient have to be assigned to each surface in
the model in order to obtain the desired acoustical behaviour.
When the simulation is based on an existing room, the materials
have to be assigned so that the model behaves like the real
measured room. This procedure is called calibration. Then
source and receiver positions have to be fixed. For the
simulation algorithm two classical geometrical methods, Ray
Tracing and Image Source Method, are combined for early
reflections. For the late reflections a hybrid between Ray
Tracing and Radiosity is used: a large number of rays are
emitted in all the directions from a source point. The rays are
traced around the room, loosing energy at each reflection
depending on the surface’s characteristics. At the same time a
secondary source is generated every time a ray hits a surface. Its
energy is a small portion of the primary source. The results at
the receiver positions are obtained by the reflections collected at
the receiver position. Through the impulse response, calculated
in each receiver position, the acoustics parameters are obtained
and shown in octave bands.
2.2 The simulation procedure
The calibrated room acoustic model of the Royal Theatre was
used for all the simulations. Because of its symmetry just one
side of the theatre was tested. The directional source was placed
on the stage in three different positions: the first position at 1 m
to the symmetry axis and 1 m to the fire curtain, the second
position at 1 m to the symmetric axis and 5 m to the fire curtain
and the third position at 4 m to the symmetric axis and 1 m to
the fire curtain, all of them at 1.5 m above the stage floor. These
positions were chosen as a singer’s significant positions. The
omnidirectional source was placed in the pit in three different
positions as well: the first violin position, the oboe position and
the brass position, all of them at 1.2 m above the pit floor. The
directional and the omnidirectional sources were simulated with
the same power level in each octave band in order to be
comparable. The sound pressure level simulated by the software
at each receiver position was the acoustic parameter used in this
study.
The energetic average of the sound pressure level obtained at
each receiver position (8 in the stalls and 13 in all the balconies)
for each single source, playing one at a time in the three
different positions on the stage and in the pit was calculated.
The balance was calculated as the difference between the
energetic average of the sources on the stage and those in the
pit. The results are showed as an average in two frequency
band, each covering two octaves: 500 - 1000 Hz and 2000 -
4000 Hz. This process was repeated for each simulation.
2.3 Acoustical design elements
Different changes were carried out based on geometries and
materials. The first attempt was to investigate the pit. This
element is historically important, but can be modified without
changing the identity of the theatre.
The pit depth was changed in order to see its influence on the
balance. Firstly a simulation was performed with a pit floor
depth level of 1.5 m from the edge of the barrier which
separates the pit and the stalls. In the results this change is
referred to as “1.5 m”. The second attempt was simulated with a
pit floor depth level of 2 m from the edge of the barrier. In the
results this change is referred to as “2 m”. The pit floor was
fixed at 2 m from the edge of the barrier in all the following
changes.
Subsequently the influence of changing the materials of the pit
walls was evaluated. Changes of the absorption coefficient of
the wall in the back of the pit, were tested. The new material
chosen was more absorbent than the previous one. The purpose
was not to look for a particular kind of material, but to search
for elements which could influence the balance. In the results
this change is referred to as “Pit back wall”. This material was
fixed for following simulations.
The covering of the pit fence facing the musicians was changed
too. A more absorbent material was chosen. In the results this
change is referred to as “Barrier”.
Other elements were tested. The barrier’s height and the stage’s
slope at 5%, 8% and 10%, but no significant aspect on the
balance were detected. Because of this these results are not
shown.
2.4 Results
The frequencies of main interest in this study are the octave
bands 2000 and 4000 Hz, where the formant of the singer’s
voice is located and where the emission spectrum of the voice
can be compared with that of the orchestra (Sundberg, 1977;
Meyer, 1986). In order to have a more complete overview, 500
and 1000 Hz have been considered too.
It seems that two different balances exist in an opera house: one
for the stalls and another for the balconies. That’s the reason
why the results have been divided by stall and balconies: two
completely different behaviours are observed. The results are
shown based on the distance between source and receiver
positions.
Figure 3 shows the calculated balance, obtained from the
simulations. Changing the pit level from 1.5 m to 2 m the
influence is noted in the stalls but nothing changes in the
balconies, for both the average frequencies considered. Keeping
