Developments in Ceramic Materials Research

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Dimitris Skarlatos, Tilemachos Zakinthinos and Ioanis Koumanoudis
large rooms, since vases were the only available way to control and adjust the acoustic
response of rooms, but their effectiveness is still unknown. If vases were effective, why did
their use during the last centuries stop? On the side, why were they found in almost all Europe
and western Asia? No text on their function has ever been discovered [3, 4, 5, 8].
An aspect on the use of pots is that some Greek amphitheatres were provided with such
resonators in order to produce a sensation of artificial reverberation [11]. Some acousticians,
for example Knudsen [12], believed that the vases used, which actually are Helmholtz
resonators, acted as sound absorbers. Some others believed that they were used for voice
reinforcement [13, 14]. On the other hand, there were others who did not accept the
possibility that sound vases can improve the acoustics of a room, and believed in the
randomness of the result. This opinion was based on experience and not on scientific
conclusions, and therefore was unconvincing.
Bruel wonders why 1000 years later Danes placed vases in their churches [15].
Desarnaulds et al. note that the principal function of the vases was to amplify and make the
voice resound, but their utility was always controversial [3, 4].
According to Mijic and Koumanoudis, the resonators in churches were ineffective for
three reasons: They were installed in very small churches where the additional absorption is
not significant, their resonant frequencies were too low in relation to the frequency content of
worship service, and lastly, in most cases there were too few resonators to achieve audible
changes in the acoustic response of the church [1, 5]. According to Junger, Helmholtz
resonators were used to increase or provide reverberation time of open air theaters in ancient
Greece and to shorten low frequency reverberation times in Swedish and Danish churches
from the thirteenth century onward [16].
2. HISTORICAL
First reference to the theory of resonators was by Aristotle (384-322 BC). In his book
“Problems” Aristotle wonders: “Why is it that if ones buries a large jar or empty pots with a
lid on, the building echoes more and also if there is a well or cistern in the house?” (Problems
XI, 7-10) [17]. Aristotle answered his own question by adopting the theory that the
contribution of the vases to the acoustics of the rooms was mainly due to the restricted
volume of air in them and the compact texture of walls of the jars or vases, since concave
shapes resound more.
Vitruvius (80-25 B.C.), in his well-known work “De Architectura,” proposed putting
bronze vases in special places of the Roman theaters. In book V Vitruvius wrote: “…On this
principle of arrangement, the voice, uttered from the stage as from a centre, and spreading
and striking against the cavities of the different vessels, as it comes in contact with them, will
be increased in clearness of sound, and will wake an harmonious note in unison with itself…”
Vitruvius book V Ch V.3 [18]. This means that although Vitruvius, in order to name metallic
or clay vases used the Greek word “echea” which simply means sounders, the purpose of
Vitruvius’ vases was probably not the amplification of sound but the improvement of sound
quality. The vases according to his descriptions were placed in such a way in specially shaped
cavities of the landing or landings of ancient theatres (see Figure 1).