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International Electrotechnical Commission (IEC) 60849 defines intelligibility as:
“a measure of the proportion of the content of a speech message that can be correctly understood.”
Because “understanding” involves evaluation by a human, intelligibility is by definition difficult
to quantify absolutely. What is intelligible to one person may not be intelligible to another. Some
people have hearing problems that render it difficult for them to understand what others easily
understand. Some people talk more quickly than others, or with an accent, so that they may not be
understandable in environments in which other speakers are easily understood.
In an effort to create quantifiable measures of intelligibility, the test methods take the subjective
elements, the talker and the listener, out of consideration. It is assumed that the speaker has an
average tone of voice and speaks at a normal speed. It is also assumed that the listener has
average hearing and is fluent in the language being spoken. If either the speaker or listener
deviates from the “average” the intelligibility of the communications is affected.
Several methods have been developed to measure and quantify the transmission quality of speech
with respect to intelligibility. These methods are used for rating intelligibility and take into
account room acoustics, as well as the various components of the sound system. The intelligibility
ratings derived by these methods can then be used to compare speech transmission quality at
different positions within one room or for different conditions within a space.
IEC 60849 Second Edition (1998) states that a minimum intelligibility level of 0.7 on the
Common Intelligibility Scale (CIS) must be met in all areas that require an emergency voice/alarm
communications system. NFPA 72 references IEC 60849 and a CIS score of 0.70 as the preferred
method of determining intelligibility. This makes understanding the CIS important to understanding
measurements of intelligibility.
The CIS is not a method of measuring intelligibility itself, but is a standardized scale to which a
variety of measurement methods are correlated. This allows a number of different measurement
techniques to be used with a common baseline to which they can be compared. Each of the test
methods described in the following sections has been correlated to the CIS, and can be used to
determine a CIS rating.
The correlation between the CIS-STI and CIS-%ALcons measurement methods is provided in the
following figure:
Table 3-1. Correlation of CIS and with STI and %ALcons
EXCELLENT
GOOD
FAIR
CIS
1.00 0.98 0.95 0.93 0.90 0.87 0.84 0.81 0.78 0.74 0.70
RASTI
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50
STI
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50
%ALcons
0.0 1.0 1.3 1.7 2.2 2.9 5.0 5.0 6.6 8.7 11.4
POOR
BAD
CIS
0.65 0.60 0.54 0.47 0.39 0.29 0.16 0.00 0.00 0.00
RASTI
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
STI
0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
%ALcons
14.9 19.5 25.6 33.6 44.0 57.7 75.7 100 100 100
Continued on next page
Measures of Intelligibility
Introduction
The Common
Intelligibility
Scale (CIS)