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writing. Sometimes there are clear pauses between phonemes but these do not
necessarily occur at word boundaries in normal rapid speech (as one may note
listening to a foreign language). Ambiguity may occur precisely because such
acoustic cues of spacing are absent. (My daughter picked up the following words at
weekly assembly in her first year of primary school: “Australians all have ostriches
for we are young and free”). Every language uses a different subset of all possible
phonemes available and, while there is a high degree of overlap, any two languages
contain certain sounds that do not occur in the other, and thus are especially difficult
for speakers of the other language (Ericsson and Simon, 1993). We shall consider
some aspects of English later.
3.2 Noise
The information theory perspective of communication provides that a sender transmits
a signal over a channel to a receiver, speakers are message generators and addressees
are passive recipients (Miller, 1951). Mistakes may occur in encoding or decoding
the message or may occur while the signal is in transit over the channel. All of these
sources of error are called noise. The noise of most general interest is random noise
which is a “hishing” sound composed of all the frequencies of vibration in equal
amounts. It is analogous to white light and so is often called white noise.
Irrelevant background noise reduces the sharpness of our discrimination. As the noise
increases, the listeners capacity to distinguish differences decreases, which means that
the ability to receive information also decreases. The signal-to-noise ratio—the
relationship between the loudness of the signal and that of the background noise—is
an important concept in understanding communication. The effect of increasing noise
is to decrease the area available for communication signals. The human solution to
the problem of noise in communication “...has been to use fewer different speech
sounds and to rely more heavily upon the sequences in which these sounds are
arranged” (Miller, p58)—thus, language. In ordinary speech, we interpret spoken
messages by processing visual cues, such as gestures and body language, to
supplement verbal information. The potential for miscommunication between pilots
and controllers, who cannot see each other, is great because all non-verbal cues are
absent. Figure 2 illustrates the role that visual cues play in overcoming noise by
adding to the auditory information.
15
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-30 -24 -18 -12 -6 0 infinity
Speech-to-noise ratio (db)
Words correct
(%)
Auditory and visual
cues
Auditory cues only
Figure 2: Intelligibility of words when perceived with and without visual cues
from observing the speaker. (From Hawkins, 1993:162.)
Spoken speech can be followed as fast as 400 words per minute (about 30 phonemes
per second). This is much quicker than the fastest rate we can pick out individual
sounds in any other sequence of sounds; thirty separate sounds of anything else except
natural language is perceived as white noise (Ericsson and Simon, 1993).
Perhaps the most persistent noise controllers and pilots have to compete with is the
sound of another person’s voice. It seems that it is relatively easy for a listener to
distinguish between two voices, but as the number of voices increases the desired
speech is lost in the general jabber, even though the overall intensity of the masking
speech is held the same. With several voices a continuous masking signal is produced
and the babble of four or more voices will drown out the desired voice as effectively
as any kind of other noise (Miller, 1951).
Another form of noise is equipment noise. Electrical equipment has ‘line noise’ and
radio is affected by atmospheric conditions. These may be sufficient to interfere with
communications by masking words or phonemes.
3.3 Intelligibility
In conversation we are bound by syntactic rules so that verbs, nouns, adjectives, and
so on appear in certain standard and expected patterns. Often, more words are used in
speech to encode a message than are theoretically necessary—this is called
redundancy. Its advantage is that parts of the message may be lost or distorted but
the message will still be intelligible because of the extra words. The receiver supplies
any missing portions on the basis of the context of the sentence and clues derived
from the surrounding words. Once the basic pattern of a sentence is revealed, the
range of possible words that can be substituted into the pattern is greatly decreased.
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