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Listeners are able to discriminate more effectively among a small number of
possibilities than among a large number. It is for this reason that ICAO standardise a
small vocabulary and insist upon stereotyped procedures for all air traffic
communications.
The accurate perception of a spoken word depends not only upon the acoustic
characteristics of the word, but also upon the expectations in which that word occurs.
The Articulation Index is a measure of intelligibility and is simply “the percentage of
spoken material of any particular type which is understood by a listener” (Hawkins,
1993:164). In figure 3, when restricted vocabularies were used, the listeners knew
exactly what alternatives to expect. As the size of the test vocabulary increased, it
was necessary to increase the intensity of the speech relative to the noise in order to
maintain a given level of accuracy.
0
10
20
30
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50
60
70
80
90
100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Articulation Index (AI)
Understood
correctly
Test vocabulary limited to
52PB words
Sentences (known to
listeners)
Test vocabulary limited to
256 PB words
PB words (1000 different
words)
Figure 3: The relationship between the Articulation Index (AI) and the
intelligibility of various types of speech test materials composed of phonetically
balanced (PB) words and sentences. (From Hawkins, 1993:156.)
The value of sentence structure as context for the correct perception of words is
illustrated in figures 4 and 5. When the key words in a sentence are taken out of
context they become much harder to hear. And because language decoding is
continuous, we often have to ‘backtrack’ in order to understand the meaning of a
sentence.
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0
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60
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100
-12 -6 0 6 12 18
Signal-to-noise ratio
Per cent words
correct Words in sentence
Words in isolation
Figure 4: The effect of the sentence context upon the intelligibility of words.
(From Miller, 1951:78.)
0
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100
-18 -12 -6 0 6 12 18
Signal-to-noise ratio (decibels)
Per cent items
correct Digits
Words in sentences
Nonsense syllables
Figure 5: Intelligibility of words in tests. In order to get 50 per cent of the items
correct, the nonsense syllables had to be 17db more intense than the digits. (From
Miller, 1951:75.)
Even if we miss a word or two, we can still construct what the speaker said based on
the usual context of the situation. Context often makes much of the semantic content
clear: we use our knowledge of the world to understand language.
Deciding which word to insert into a broken communication requires:
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• knowing the probability of encountering that word in speech (or print);
• for many words, knowing the type of words most likely to be associated with the
word;
• knowing the limitations imposed on the use of the word according to variations in
function and situation;
• knowing the syntax associated with the word. (Richards, 1985)
Thus, frequently used words are more readily perceived than infrequently used words
and some words are intrinsically more audible than others. Unfortunately, the vowels
have more power than the consonants but it is the consonants that are more critical to
the correct interpretation of speech. The vowels ‘a,e,i,o,u’ are distinctly different
when compared with the consonants ‘b,p,t’ and ‘m,n’, yet the communication ‘-a- -oe
-i- -a-?’ is more difficult to interpret than ‘Wh-t d-s th-s s-y?’. Miller discusses
various methods of “butchering” speech and concludes that “the ear is very facile in
patching together the interrupted fragments of speech and in reconstructing the whole
message” (p71).
We can summarise the role of the listener and say that
perceiving speech is not a passive, automatic procedure. The perceiver contributes a
selective function by responding to some aspects of the total situation and not to others.
He responds to the stimuli according to some organisation that he imposes upon them.
And he supplements the inconsistent or absent stimulation in a manner that is consistent
with his needs and his past experience. (Miller, 1951:79)
This expectation aspect of speech is important in understanding miscommunication in
the air traffic control system.
3.4 Speech and Memory
　
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