A diphthong is a glide from one vowel sound to another within a single syllable (e.g., the phoneme /aɪ/ in the words I, my or try). This is different from a monophtong, where there is just one vowel sound (/æ/ in man or sand).
Unlike Spanish, in English diphthongs are normally considered just one phoneme, not the combination of two. Here are the eight English diphthongs:
here, near, deer, weird
endure, tour, pure, poor
make, brain, play, eight, great, they
go, low, bode, boat, toe, so, shoulder
boy, toy, noise, voice
there, air, share, rare, pear, bear
time, type, my, lie, buy, eye, high, height,
how, loud, mouse, about, cow, allow,
Diphthongs are one of the greatest sources of mispronunciation for Spanish speakers. The main difficulty is that English letters can be pronounced in several ways, so sometimes it’s difficult to know which is the correct one. An i can be said as /ɪ/ or /aɪ/, an o as /ɒ/ or /əʊ/, etc. We must also remember that vowels are often pronounced /ə/ when they are unstressed. This uncertainty leads to very common mistakes. Let’s see some examples of words which are often mispronounced by Spanish speakers.
comfortable /ˈkʌmftəbl/ /komforˈteibol/*
However, the glides included in Table 1 don’t exhaust the subject of diphthongs, not in the sense we, Spanish speakers, understand them in any case (there seem to be differences in the criteria we use to classify very similar phenomena).
There are English words such as music /ˈmjuːzɪk/, new /nju:/, one /wʌn/, win /wɪn/ or when /wen/ that we would immediately think of as diphthongs. But if they are not diphthongs in English, what are they, then? These words include sounds which are called semiconsonants or even semivowels sometimes (/j/ and /w/, explained here and here), which are very similar to vowels but not exactly the same, and that’s why they are left out of the table. However, to have the whole picture in mind, it is advisable to be aware of their existence and to know how they are considered in English.
And here are a couple of examples of diphthongs pronounced by native speakers:
Well, I thought I’d given loads away. I think I’m full of sort of heartfelt personal details about my joys and pains (David Mitchell, BBC4)
There’re jokes about the aging process (John Wilson, BBC4).
There are no absolute rules that will always allow us to know if a vowel is pronounced as a monophthong or as a diphthong, but fortunately we can use a couple of very useful guidelines. Let’s see them.
1. Double consonants. When a vowel is followed by a double consonant, it is always pronounced as a monophthong (most of the times, a short one).
apple bottle daddy buzz class dinner (but diner)
I only know one exception to this rule, the word bass (lowest tone in music), which is pronounced /beɪs/. When the word bass means a type of fish, it is pronounced /bæs/ instead.
2. The silent magic e. Have a look at the following comparison.
cloth clothe cod code not note
An old English saying goes “silent final e makes the vowel say its name”. As you can observe, the addition of a silent e to many words not only changes its meaning but the pronunciation of the previous vowel as well. This stops being a short vowel and it’s pronounced with its alphabet name: a /eɪ/ (made); e /i:/ (gene); i /aɪ/ (time); o /əʊ/ (note); u, /ju:/ (cute). The new vowel is a diphthong in all the cases but the e, which is pronounced as a long /i:/.
The silent e, which is often called the magic e, cannot be taken as a rule, because there are plenty of exceptions (some of them as common as love or have), but it does give a clue as to the pronunciation of the vowel in a great number of cases.
Now, if you want to further refine your pronunciation of diphthongs, here are five interesting points to take into account.
Five points about diphthongs
1. Length. Diphthongs last more or less the same as a long vowel (/ɑː/, /ɔː/, etc.).
2. Unequal vowels. The first element is longer and more stressed (the /a/ part of the diphthong /aɪ/ in time, for instance). The second element can be considerably weakened.
3. Not like in Spanish. When you find the sounds /a/ or /ɔ/ as part of the diphthong, you can pronounce them as in Spanish because they sound very similar. You can do this with the /a/ in time or the /ɔ/ in joy, but the other vowels are quite different. So it is advisable to check how these sounds (/e/, /ɪ/, /ə/ and /ʊ/) are pronounced in the corresponding sections.
4. Watch these two! I recommend to be particularly careful with two diphthongs, /əʊ/ and /eə/.
5. Pre-fortis clipping. Diphthongs are shortened by following voiceless consonants in the same way pure vowels are (this process is known as pre-fortis clipping). You can observe this difference in length in the recordings included in table 1. Boat is shorter than bode, make than brain and type than time. This is because /t/, /k/ and /p/ are voiceless whereas /d/, /n/ and /m/ are voiced.