Developments in Ceramic Materials Research

The Use of Ceramic Pots in Old Worship Places 155
a.10. Cavernous churches- The sound vases are situated on every part of the church,
without determinism or acoustic purpose.
a.11. Hexagonal churches- The vases are walled-in in every dome structure and are of
any shape, mainly on the crests of domes and of bucklers in general. The number of walled-in
vases does not depend on the size of the churches, or their position.
The difference between Greece and other countries – not neighbouring - that the vases are
walled-in at different positions, in the walls and domes. Perhaps this arises from the fact that
there are different architectural types of the churches.
4. QUALITY OF SOUND IN ROOMS
The propriety of the sound field that is related to certain hearing impressions is an object
of psychoacoustics. The physical parameters that describe the so-called “sound quality” in a
room can be described by a set of objective parameters called indexes. These indexes
according to the procedure from which they can be derived, are divided into categories. All of
them can be evaluated from the impulse response of the room.
In the acoustical assessment of concert halls, and other public spaces, reverberation time
has long been known to be the most important parameter to judge the acoustic quality of a
room. The reverberation time T60 is derived from the decay curve and is defined as the time
for the sound to die away to a level 60 dB below its original level. This quantity can be
measured using the Schroeder integrated impulse response technique [40].
Instead of reverberation time T10, T20 and T30 are measured. The three reverberation
times are used when the impulse is of low level and T60 cannot be measured. These
parameters show the reverberation time in a part of the response curve, which is usually more
useful. T10 is derived from the decay curve section between –5 dB and –15 dB below the
initial level. From the corresponding slope, T10 is calculated as the time to reach –60dB. T10
can be used to measure for instance vibrations or loss factors in building structures. Most
practical T10 values range from 0.005 to 10 s.
Proportionally, T20 is derived from the decay curve section between –5 dB and –25 dB
below the initial level. For T30 the same holds, but with –35 instead of –25 dB. From the
corresponding slope, T20 and T30 are calculated as the time to reach –60 dB. Most practical
T20 and T30 values range from 0.1 to 10 s. Most of the time, these three parameters don’t have
the same value for the same room and that is because of the field which is not diffuse.
Besides, for the same time, for different points in a room, according to the spot of the
microphone, the three parameters have different values due to the different types of the fields
in the room. Thus, in order to measure an unreliable reverberation time, we measure it at
many points and we calculate the mean value.
Another useful index is the EDT. The EDT is a magnitude that takes into account the
early reflections, which are the most important, and affect a lot the acoustics of a room. We
use it when the slope of the reduction rate is not smooth enough. The EDT is derived from the
decay curve section between 0 dB and –10 dB below the initial level. From the corresponding
slope, the EDT is calculated as the time to reach –60dB. It is closely related to the initial and