What is the name of the phoneme produced in an upper-class Briton's pronunciation of the word "Duke"? What's different in the articulation?

When someone with a Received Pronunciation accent pronounces the word duke, as in The Duke of York, he doesn't pronounce it with a "hard" 'd', as one might pronounce the word duh, but a softer type 'd', which I can only spell phonetically as "dj", so as to pronounce duke somewhat like "djuke."

My question is composed of two parts:

• What would a linguist call this phoneme?
• What is the difference in articulation between it and a "regular" /d/?

This is called palatalization of the /d/ sound: under influence of a /u/ sound ("oo" as in moon), the preceding consonant sometimes gets the palatal glide /ʲ/, pronounced like "y" in yoke. This results in /dʲuːk/, also rendered /djuːk/.

I believe this palatalization can occur with any /u/ sound, as in news, /nʲuːz/, but it is much less common with short /ʊ/ as in book or put, that is, I can't think of any example. It generally depends on the word and the dialect whether the consonant is always, sometimes, or never palatalized. There appears to be less palatalization in America and in Estuary English (middle-class South-East England) than in Received Pronunciation. This pronunciation /duːk/ is sometimes called yod dropping, from the name of the semi-vowel y in linguistics. There may be rules behind this, but I don't have the full picture, especially not on the elusive pronunciation of lu-.

I'm a speaker of Australian English (AusEng has the "y" sound in Duke that AmEng lacks) and an armchair linguist (read extensively but no formal study).

Phonology is a complex field with many approaches, analyses, and competing theories and though I know IPA I'm not an expert at phonology.

Having said that, in British and Australian dictionaries the sound in "duke" is simply regarded as /j/, the very same sound made by the letter "y" in regularly spelled words. In IPA it is called a Palatal approximant. As such it is distinct from the processes known as palatization and iotation (which in turn are not the same as each other). This means the sound in "duke" is not the same as the Hungarian "gy".

Now the complexity of phonology comes into play because sound systems are subject to many changes so the actual realization of such words in some varieties of English may be affected by these processes after all, and I don't know enough about RP specifically.

I think we need the help of a real linguist or at least some good texts which address this specific point.

I think it's simply a d with a y-glide, which can make it sound like dj when spoken quickly: Dyook rather than dook.

I'd point out that, in England in particular, there is a continuum between the 'precise' pronunciation, 'dyook' and the perhaps more common 'juke' (as in juke-box). In Britain, I don't think the American 'dook' is common - maybe some people in East Anglia say it that way (?)

I would say, for the most part, it is older members of the aristocracy that tend to use 'dyook' in its purest form. I think the gradient between 'dyook' and 'juke' may have some relation with perceived social class.

^{I assume for the purposes of the question that the Original Poster's J in "djuke" has a similar sound to the J in "jump".}

1. What would a linguist call this phoneme?

The phoneme is a voiced postalveolar affricate, /dʒ/.

2. What is the difference in articulation between it and a "regular" /d/?

This sound has an onset similar to a /d/ but retracted to the edge of the alveolar ridge. This involves a complete blockage of the air leaving the vocal tract. The air builds up behind the blockage increasing the air pressure within the aural cavity. Whereas with a /d/ this air would be released rapidly causing sudden audible plosion, in the case of /dʒ/, the air is forced across the surface of the tongue and through a narrow aperture between the tongue and the alveolar ridge. Along with the continuing vocal fold vibration from the larynx, this causes voiced audible friction.

Further information

Occasionally the adjustments we make to pass smoothly from one sound to another result in a new sound replacing the original two. This is known as coalescent assimilation. In both Southern Standard British English and also in General American this can happen when we have a /d/ followed by a /j/. The resulting sound is recognised as /dʒ/, the first sound in the word jump. For example in the sequence would you, we are likely to hear wouldju /wʊdʒu/.

Southern Standard British English often has an intervening /j/ between alveolar consonants and a following /u:/ in words where most General American speakers use only the initial consonant and the /u:/. For example the word news is /nju:z/ in SSBE, but /nu:z/ in general American.

For this reason many words that begin /du:/ in General American begin with a /dʒu/ in SSBE for many speakers. This has nothing to do with class.