Developments in Ceramic Materials Research

The Use of Ceramic Pots in Old Worship Places 163
In order to find the effect of distance from the resonators to sound quality, the impulse
response of the church was measured at various distances from the vases.
To conclude whether the pots have a significant effect on sound amplification, from the
impulse response the noise level was calculated. For this measurement one omni-directional
microphone at a varying distance (height) from one of them was used. Figure 18 shows the
effect of distance from one of the 31 vases inside the church, on the level. As is shown in
figure 18 there is a remarkable selective amplification up to 8 dB of sound for the third octave
band that contains the resonant frequency only. This amplification is restricted to a small area
near the vase. At greater distances, there is an attenuation of sound and this is an indication
that the absorption is dominant. It is reasonable, since the resonators according to equation
(12) have only a near field component. The effect of distance to the other bands of interest is
within the statistical error.
Measurements of Desarnaulds et al [3] showed that in a church with pots in its walls, no
significant difference in sound intensity can be found at any distance. An increase of sound
intensity was highlighted locally for the resonant frequency of the pot.
Carvalho et al [6] to analyze the sound radiation of a pot, used a short duration noise and
measured its response at the opening of the pot after the disappearance of the excitation
signal. They found out that when the excitation signal is a pure tone, equal to the pot’s
resonant frequency, the re-emitted signal by the pot, contains a strong component at the
resonant frequency, as well as a weaker contribution at the frequency that corresponds to the
second mode of the pot. When the frequency of excitation signal is different from the
resonance of the pot, the temporal signal, after cutting off, shows a spectral change of it, in
addition to phase shift.
With a pink noise excitation one can also find the presence of the resonant frequency and
its odd harmonics.
Level (dB)
100
95
90
85
80
Effect of distance on Level
Frequency band
200
250
315
400
2500
75
0 20 40 60 80
Distance (cm)
100 120
Figure 18. Effect of distance on measured noise level.