Within Received Pronunciation (RP), triphthong smoothing refers to the sound change of a triphthong to a smaller unit of sound; either a diphthong or pure vowel. In the latter case, smoothing is sometimes referred to as monophthongisation.
The opposite process, whereby a monophthong becomes a sequence of two or more vowels, is known as vowel breaking or fracture.
Triphthong smoothing takes place between a diphthong and schwa sound (/ə/), often before a lateral approximant or historical r, though neither liquid consonant is required for smoothing to occur.
Smoothing may also take place across word boundaries, when a word-final diphthong is present before a schwa; e.g. tie a knot (/taɪ ə nɒt/) or stay away (/steɪ əˈweɪ/).
Although primarily associated with RP, the pronunciations of some common words are the result of smoothing, such as our /ɑ(r)/, prayer /pɹɛə(r)/, and (outside North America) mayor (/mɛːə/).
Diphthongs and triphthongs in Received Pronunciation
Diphthongs
RP is traditionally ascribed eight diphthongs: the five closing diphthongs /eɪ/, /aɪ/, /ɔɪ/, /əʊ/, and /aʊ/, and the centring diphthongs /ɛə/, /ɪə/, and /ʊə/.
Of these eight, the three centring diphthongs are not able to undergo smoothing, due to the fact that they are never followed by another schwa.
DiphthongExample Closing bay buy boy beau bough Centring bear beer boor
Triphthongs
The five closing diphthongs followed by a schwa - /aɪə/, /ɑʊə/, /əʊə/, /eɪə/, and /ɔɪə/ - are often labelled triphthongs, even if in practice they are rarely pronounced in one syllable as true triphthongs are. This is due to their shared tendency to undergo smoothing, as compared to other three-vowel sequences as in clayey or reaudit.
Degrees of smoothing
There are two degrees of smoothing in RP. In the first, the medial element of a triphthong is elided, creating a new diphthong composed of a long vowel and schwa. In the second degree, the diphthongs are further reduced to long vowels.
+ Degrees of smoothing in the five "triphthongs" Triphthong General Advanced Example words /aɪə/ /aːə/ /aː/ buyer, choir, liable, pyre, science /ɑʊə/ /ɑːə/ /ɑː/ bower, flour, fowl, nowadays /əʊə/ /əː/ /ɜː/ blower, Goa, poem /eɪə/ /eːə/ /ɛː/ aorist, greyer, player /ɔɪə/ /ɔːə/ /ɔː/ lawyer, employer, toil
General
General smoothing creates four new centring diphthongs: /aːə/, /ɑːə/, /eːə/, and /ɔːə/. In the case of the /əʊə/ triphthong, smoothing results in a lengthened schwa sound (/əː/).
The newly created diphthong /eə/ is commonly assimilated with the /ɛə/ diphthong, creating homophones between lair and layer. For speakers who merge /a/ with /ɑ/, tire and tower will also be homophones.
Advanced
In advanced smoothing, the medial vowel and final schwa are elided, leaving behind a long vowel. The already lengthened and monophthongal /əː/ is lowered to /ɜː/.
Advanced smoothing creates numerous homophones, most prominently among speakers who already exhibit the merger between /a/ and /ɑ/, leading to the tar-tire-tower merger.
Tar-tire-tower merger homophones IPA Bauer/bower buyer bar baa/bah coward/cowered - card - cower - car ka dowel dial - dahl - fire far fah flour/flower flyer - - hour/our ire ar/are ah Howard hired hard ha-ed how're hire har ha - mire mar ma owl aisle/I'll/isle arle aal - pious parse pass power pyre par pa scour - scar ska shower shire/shyer - shah showered shired shard - sour sigher/sire Saar - - spier/spire spar spa tower tier/tire/tyre tar ta trowel trial - - vowel vial - Vaal
In Standard Southern British, the diphthong /ɛə/ is commonly reduced to long /ɛː/, producing homophones between pear and payer. Smoothing slower and mower to /ɜː/ creates homophones with slur and myrrh respectively.
The open-mid back rounded vowel (/ɔː/) is produced with the tongue in a lower position and is better transcribed as [ɔ̝ː]. This allows a distinction to be made between the smoothed vowel in coir ([kɔ̝ː]) and the unsmoothed vowel found in core (/kɔː/).
See also Diphthongisation Fusion (phonetics) Monophthongisation Syncope (phonology) Synaeresis
Notes
References
Bibliography