On a foggy night in the early 1980’s, a pilot accepted instructions from an air traffic controller while taxiing to takeoff in Tenerife. The German pilot maneuvered his aircraft into position anticipating the go ahead when the aircraft was suddenly struck by another in the process of take off.

Over 600 people perished in one of the worst aircraft accidents in history. The cause of the collision: A lack of speech intelligibility. The cockpit environment was noisy and the situation stressful, but the conditions were well within acceptable limits for proper communications between pilots and controllers.

In this case however, the German pilot misunderstood the English instructions that he received from the Spanish air traffic controller. English - a second language to both - was their only common language.

This is merely one example of the ramifications of poor speech intelligibility in our daily lives. The absolute need to comprehend instructions in life-threatening situations occurs more frequently than one might expect.

Whenever we walk into a place of public meeting, the presumption is that in the event of emergency, we will receive clear instructions. Our ability to understand these instructions directly impacts our ability to survive.

The panic and noise associated with emergencies further degrade intelligibility. Add native language differentials to this scenario and the problem intensifies.

It is clear that poor speech intelligibility on that foggy night in Tenerife was a major factor in a devastating accident. It is also apparent that this speech intelligibility obstacle was at least in part the result of a difference in native language between communicators.

But do even reasonably fluent speakers of a language experience a reduced ability to comprehend speech based merely upon a deviation from their primary language? The answer is yes.

The (Limited) Proof
Unfortunately, the native language dependence of speech intelligibility has gone relatively unnoticed, and consequently, little direct research is available regarding its existence or implications. The aviation accident in Tenerife offers some limited evidence of a problem.

And certainly our own personal struggles with non-native speech indicate the potential for reduced comprehension, particularly when actual practice of these second and third languages is infrequent.

A more scientific means of evaluation is required for an absolute determination of this phenomenon. The increased curiosity surrounding the potential for this problem has prompted some to look back at earlier research in an attempt to isolate some additional proof.

Professor Richard Campbell of Worcester Polytechnic Institute (WPI) in Massachusetts has conducted an annual experiment with his audio engineering classes, in which he attempts to determine the speech intelligibility map of a controlled environment in a lecture hall. Speech intelligibility evaluation recordings are available that place standard word lists within the phrase, “Write (word) on the line now.”

In this way the listener knows the exact position and temporal location of the desired word prior to its utterance. Professor Campbell plays this recording through a loudspeaker that he places in one corner of the room. Campbell then introduces pink noise into the room through a second loudspeaker positioned in an opposing corner.

The students are seated in rows and are distributed evenly throughout the room. After the word list has been completed, the students evaluate the percentage of words that they have heard correctly through the use of an answer key. These percentages are then plotted on a top view of the room to develop contours of intelligibility.

Invariably, the resulting contours demonstrate a higher intelligibility close to the speech loudspeaker and a lower intelligibility close to the noise loudspeaker. A gradual degradation of speech intelligibility is observed over the area between the two sources. A marked deviation from this trend has been observed when there are non-native speakers of English present in the room.

Professor Campbell indicates that he has observed holes in the intelligibility map in the locations where non-native speakers were seated. In one particular map, an oblong “hole” corresponds to the seat locations of three Asian students who speak English as a second language. The intelligibility scores of those seated behind the Asian students actually rose as the distance from the source increased.

The professor also notes that he observed a similar depression when three Argentinean students were seated together. Yet he’s quick to point out that these students are fluent English speakers, “in the sense that rapid colloquial two-way conversation was easy for them,” he explained.

Indeed more research is required before absolute conclusions can be drawn, but the repeated incidence of poor intelligibility scores particularly when viewed in comparison with the scores of native speakers of English is intriguing.

Professor Campbell intends to continue this research while introducing an additional twist. He feels that there is strong evidence supporting the notion that this language dependent intelligibility phenomenon is exacerbated through the introduction of panic.

This added stress seems to generate an environment in which the ability to focus on what is being said is significantly compromised. This inability to focus also seems to have greater impact upon non-native speakers who require this concentration for absolute comprehension. While this area is currently extremely developmental, the impact - if true - would be tremendous, particularly in emergency situations.

The Cause of Language Dependence
Due to the infancy of this exploration, hard evidence is tough to come by, and further research is required. But perhaps it is helpful to investigate why this phenomenon might exist and then arrive at a logical explanation for its cause.

The root of the problem lies in the phoneme - essentially, the smallest unit of speech. Phoneme are distinct sounds that are formed through the various combinations of letters employed in the written word.

“The phonemes of a language operate in an analogous way [to letters] and in fact alphabetic writing is derived originally from the phonemic system…”(From the book “Homo Loquens”, by Dennis Fry, page 12)

While there are only 26 letters in the English alphabet, there are over 40 phonemes, 20 of which are derived from the five vowels utilized in English word construction. This essentially proves that the uses of vowel sounds in English word enunciation are quite diverse.

“Languages differ from each other in their phoneme systems, just as they differ in grammar and vocabulary…. What the phoneme system does is to dictate for any language what particular sounds must be recognized as distinct from each other and what sound differences should be disregarded.” (“Homo Loquens”, page 15)

The phoneme differences between languages result in situations where distinct phonemes in the one language are interpreted as being the same by foreigners who are unused to the diverse pronunciations of a given lettering. This situation arises between any two languages.