Diphthongs as Moving Vowels

• A diphthong is a speech sound in which the articulatory mechanism moves continuously from an initial vowel position to a final vowel position. This dynamic quality is in contrast to the vowels which we have covered so far, which at least in theory are stable segments and can thus be termed monophthongs.
• The diphthongs are identified by the two vowels which they contain. Olive lists five diphthongs in American English, to which list we add number 6 below (Consult Figures 1 and 2):
  1. aI Example: five
  2. ei Example: save
  3. >i Example: boy
  4. oU Example: go
  5. aU Example: how
  6. iU Example: few

  Notice that three of the diphthongs move toward i or I, while the other three move toward U. The diphthong iU is a special case in that the nucleus of the phoneme is the second vowel U. In all other cases the nucleus is perceived as the first vowel, with the second vowel constituting a sort of off-glide.

• You may have been surprised that we did not include the two vowels symbolized by Worldbet o and e in the list of American English monophthongs. In contrast to most European languages such as French and Spanish, these vowels are almost never pronounced in monophthongal purity in American English, where they are classed as the diphthongs oU and ei.
• There are certain rules of thumb which you can use in zeroing in on a suspected diphthong. Remember that other broad categories such as the liquids and glides can involve continuous movement of formants, but only the diphthongs maintain the high energy levels of a vowel throughout their duration. Note, however, that even for the diphthongs the nucleus of the vowel will have higher energy levels than the remainder.
  • Look at F2, which usually is the most mobile of the formants in a diphthong. If the movement is up, it is one of the three diphthongs ending in i or I; if the movement is down, it is one of the three diphthongs ending in U. Remember that F1 moves down in all cases, so that F1 cannot be used to distinguish between the i and U flavored diphthongs.
  • The diphthongs aI and >i are easily confused. The o nucleus is always lower than the a nucleus, but the difference can be subtle (o = 700 Hz, a = 900 Hz). A second clue is the curved appearance of F3 in the case of /oU/. Another clue comes from the greater downward movement of F1 in the case of a. (Figure 1)
  • The diphthong ei has a subtle movement up from the nucleus e to the off-glide i. It may thus be confused with monophthongs such as i: and E. Notice the increasing widening between F1 which is moving down and F2 which is moving up. (Figure 1)
  • In both aU and oU the diphthong involves a sometimes slight drawing together of F1 and F2; the reduction of the distance between the two formants occurs because F2 is descending faster than F1. Again the problem is to determine the vowel nucleus, with aU starting out higher than oU. The U off-glide will be more pronounced in the case of a word or phrase terminal diphthong, but coarticulation may reduce the movement as in the case of aU in Figure 2, which occurs in the context of the word loud.
  • The diphthong iU is different from the rest in that the dominant F1, which is the signal for u, does not vary significantly. Only the drawing down of the much less weighty F2 signals the presence of the diphthong. However, the contrast of the words foo, who without the diphthong and few,hue with the diphthong show that there is a phonemic distinction between u and iU. Some speakers of English use iU in words where other speakers use u - for example, the word dew in some British dialects.
• British English also includes the three diphthongs i&, e&, and u& in words such as here, there, and poor where American English has a retroflex liquid (9r).
• Many languages have diphthongs (e.g. German mein, French ail), but others such as Italian and Spanish do not. In these ``diphthong-free" languages, when two vowels are juxtaposed, they are pronounced separately and constitute two separate syllables. The essence of a diphthong is the continuous movement of the articulators from the initial vowel position to the terminal vowel position. A diphthong thus forms one phoneme and is considered as the nucleus of a single syllable.
• The American variant of English is particularly rich in diphthongs, to the point that when speakers of other languages wish to caricature American English, they make fun of the fact that we have almost no stable vowels at all. Look back at the vowels and you will discover that there is a great deal of movement even in the monophthongs. (Spectrogram of an Italian speaker - Figure 3)
• To situate the diphthongs in the context of the other vowels, look at Figure 4 (from Ladefoged). The stable vowels are represented by points on the vowel quadrilateral, whereas the diphthongs are symbolized by trajectories of tongue and jaw movement.