DIPTHONGS IN PHONETICS

DIPTHONGS IN PHONETICS

 vowels as if they were articulated by a specific configuration
of the tongue, lips, and oral cavity, which is held constant throughout
their pronunciation. Vowels made like this are called monophthongs; others,
called diphthongs, involve a change in the configuration of the mouth.

The vowel sounds in the words boy, by, and how involve a change in
the shape of the mouth as the vowel is being produced. The vowel of boy
begins with approximately the mid back vowel [O] and finishes with approximately
the high front lax vowel [I] (or the palatal glide [j]). 

The vowel of by begins with approximately the low back vowel [a] (a low back vowel
slightly more forward than [A], but not as forward as [{]) and also finishes
with approximately [I] (or [j]). 
The vowel of how begins with approximately [a] and finishes with approximately the high lax rounded vowel [U] (or the labio-velar glide [w]). We represent these diphthongs as [OI], [aI], and [aU], respectively (though many linguists use [Oj], [aj], and [aw]).

A second set of English diphthongs is not as clearly distinguished as the
first, primarily because we tend to perceive them as simple vowels. However,
in a precise (narrow) phonetic transcription they must be represented as
diphthongs. 
The tense front vowel [e] is diphthongized. If you listen carefully
you will notice that the vowel of bate is actually pronounced [eI]. The
tense back vowel [o] is also diphthongized: if you listen carefully you will
notice that the vowel of boat is actually pronounced [oU].

 So, the front tense vowel is diphthongized by the addition of a front vowel and the back tense vowel is diphthongized by the addition of a back vowel. We can express this
pattern as a rule: Mid and high tense vowels are diphthongized by the addition
of a high lax vowel that matches the original vowel in frontness or backness.

Diphthongization of these vowels is a feature of English rather than a
universal feature of natural language. Other languages, notably Spanish and
German, do not diphthongize their corresponding vowels. The tendency to
diphthongize these vowels is one characteristic of the “foreign accent” that
betrays English speakers when they begin to learn these languages.