CIPA 2003 XIX th International Symposium, 30 September-04 October, 2003, Antalya, Turkey
Figure 6. The measured values of the parameter strength G.
The deviation bars of one standard deviation are included.
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V
, Mosques
i Churches
Mosques:
y = 118.27Ln(x) - 943.27
R 2 = 0.9224
Churches:
y = 113.63Ln(x) - 832.49
R 2 = 0.9452
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V [m3]
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> Mosques
i Churches
Mosques:
y = 75.947Ln(x) - 510.72
R 2 = 0.9435
Churches:
y = 114.51 Ln(x) - 835.44
R 2 = 0.904
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V [m3]
Figure 7. The centre of gravity of energy. Top: all pass values; Bottom:
averaged values of 500Hz, 1kHz and 2kHz octave bands.
Also the so-called center of gravity of energy was measured
(Ts) and this parameter is selected for being reported in this
section. Actually Ts is highly correlated with the former indexes
C or D but it is not as scattered from position to position as C
and D are.
In Fig. 7 one can see that there is a sort of bias in the parameter
between mosques and byzantine churches. The slope of the two
lines is quite similar so that the values of a church are
reproduced by a mosque of larger volume. In general the Ts
values are higher for greater volumes and inappropiate for the
comprehension of speech. In any case the sound field inside
mosques is more suited for speech showing lower values. The
graph can also be interpreted in terms of useful listening areas.
That is, both in mosques and churches the positions close to the
sound source are remarkably better than the others, where also
the parameter Ts is out of range for both speech and music. In
the case of mosques the good listening area is usually wider that
in a byzantine church of comparable volume.
4. THE ACOUSTICAL PERFORMANCE OF THE
FLOOR
Many features of the sound field inside mosques are governed
by the sound absorbing characteristics of the floor. This surface
can be found in different arrangements from mosque to mosque.
In particular it can be set by simply laying carpets on the floor
or by building a wooden structure of some centimeters height,
where the carpets are rested. Moreover this wooden structure
defines an air-backing of a few centimeters with respect to the
floor, which can be either left void or filled with material for
thermal insulation.
In order to study the acoustical properties of mosque floor
layouts a simple project was prepared. Then the mosque floor
was built as a modular structure summing up to a sample of
12m 2 . The wooden structure consists of 25mm thick panels and,
in order to model the air-gap underneath the structure, two
heights were chosen: 40mm and 150mm.
The mosque floor model was then taken to a reverberation
chamber and measurements of acoustical absorption were made
(Prodi, Marsilio, Pompoli, 2001). Four configurations were
tested, namely: carpet alone, structure (40mm) alone, structure
(40mm) with carpet, structure (150mm) alone. The
measurements consisted in the placing of the dodecaedric sound
source in three positions in the chamber and sampling the sound
field in six positions each time with omnidirectional
microphones. Impulse responses were calculated and later
parameters regarding reverberation time were extracted. The
measurements followed the specific standard and the alfa
absorption coefficients in the diffuse sound field were obtained.
From the results of these measurements reported in Fig. 8 it can
be seen that the behaviour of the floor is a combination of the
absorption of the carpet and of that of the wooden panel
structure.
100 160 250 400 630 1000 1600 2500 4000
Freq. [Hz]
Figure 8. The absorption coefficient for each of the four tested
configurations.
In particular the carpet works in the medium and higher
frequency range whereas in the lower range one can see the
typical absorption of a panel. As a confirmation of this, the
effect of the air backing disappears when the carpet is placed
alone. Moreover, the height of the structure sets the frequency
showing the maximum sound absorption in the lower range so
that for the higher structure the resonant frequency is shifted
downwards.
