Developments in Ceramic Materials Research

156
Dimitris Skarlatos, Tilemachos Zakinthinos and Ioanis Koumanoudis
the highest grade of the attenuation energy and its configuration. Owing to this, EDT is
attached to speech intelligibility. Most practical EDT values range from 0.05 to 10 s.
Although room acoustic characteristics have long been described in terms of
reverberation times, other measures are now thought to better indicate the effects of a room
on speech or music.
The index that is attached to EDT is the speech intelligibility which reflects on the
“quality” of speech. Thus, speech intelligibility is defined as the percentage of the numbers of
syllables that can be perceivable from a common listener to the total amount that are
delivered during a standard conversation. The less satisfactory percentage is 95%.
In order to evaluate the pattern of time fluctuation of energy density, Thiele introduced an
index named “Deutlichkeit” This index is also known as Definition ( D 50 ) and is defined by
the early-to-total arriving sound energy ratio:
D
50
=
E
E50 100%
0→∞
where E50 E0→∞ the energy that arrives during the first 50 ms and the total energy,
respectively.
The definition D50 is strongly related to speech intelligibility and describes the quality of
speech in a room.. D50 values range from 0 to 1. The larger value of D50 the more distinctness
of the speech.
The clarity C80 is the logarithmic early-to-late arriving sound energy ratio, where “early”
here means “during the first 80 ms” and “late” means “after the first 80 ms” and it is defined
as:
C
80
10log E
=
80
E80−∞ E , E −∞ the energy that arrives during the first 80 ms and after the first 80 ms,
where 80 80
respectively.
The clarity C80 is used to describe the transparency of music. Most practical C80 values
range from –10 to 50 dB.
The way that the human ear functions and comprehends the sounds is very important in
order to evaluate the room acoustics. In particular, the distance between the sound source and
each ear is not the same and this is the reason why someone understands where the sounds
come from. Besides, one ear may be in the “shadow” of the head in relation to the sound
source and the sound level of each ear is different. Even the angle the sound waves arrive at
the human ear is important.
For all the reasons above, there is an index named IAAC that measures the difference of
the sounds that arrive at each ear. The Inter-aural Cross Correlation Coefficient is defined as
the maximum absolute of the normalized inter-aural cross correlation function, over an interchannel
time shift from -1 ms to +1 ms and is given by: