In an essay about The King’s Speech for the Fortnightly Review, I wrote that the very familiarity of speech means we easily overlook how amazing its mechanics are. This occurred to me often while reading J.D. O’Connor’s superb Phonetics, a Pelican Original from 1973.
The book has a lovely paragraph on how the [d] sound in do is articulated. Complete description of such a sound is impossible because it would require mentioning an infinite number of features, so in general we note only those features that “seem to contribute substantially to the sound”.
Some of the following terminology might be unfamiliar, in which case refer to this diagram of the human vocal tract. Here, then, is [d]:
the lips are somewhat rounded (ready for the following vowel); the teeth are close together; the soft palate is raised; the tongue-tip is firmly in contact with the alveolar ridge and the sides of the tongue are in continuous contact with the sides of the palate; the back of the tongue is raised to approximately the close vowel position (again ready for the vowel); air under pressure from the lungs is compressed within the completely stopped mouth cavity and pharynx; the tongue-tip (but not the sides or the back) then lowers suddenly allowing the compressed air to escape with a slight explosion; just before the explosion the vocal cords start to vibrate in normal voice and continue to do so into the vowel.
Professor O’Connor says that although this description may seem quite comprehensive, it is very far from complete. But it serves its basic and practical purpose. [Edit: Note the two references to readiness for the following vowel. The mouth assumes different shapes for [d] depending on what comes next. To see (or feel) this for yourself, prepare to speak do, da and dee but stop before the vowel.]
It’s also a very pleasing account of an act most of us perform more or less identically, yet uniquely, every day without a moment’s thought. Think of how much exquisite unconscious coordination goes into a full sentence, or a week’s worth of conversation. How fortunate we are to have this facility.
O’Connor (1919–1998), known familiarly as “Doc”, taught phonetics at University College London; John Wells’s obituary in the Guardian describes his lectures as “witty and effortlessly informative”, which I can believe, and his writing as “elegant and readable”, to which I can attest.
A list of O’Connor’s many publications may be found on Jack Windsor Lewis’s website.
Sentence first 
I suspect the voice onset timing is, mmm, overspecified here. Many Americans and essentially all Australians actually do not voice their /d/, and what distinguishes it from /t/ is the lack of aspiration. I’d guess the same is true in Ireland, given the even wider distance between Irish /t/ and /d/, but I don’t really know.
That’s fascinating.
I prepared my mouth for ‘do’, ‘da’, and ‘dee’ and was happily surprised to find it moving to different positions in anticipation of the coming vowel.
John: I thought I voiced my /d/ in do, but I wouldn’t be the best judge! O’Connor addresses other ways of distinguishing /d/ from /t/ a couple of pages later:
Ashley: The way we’re taught sounds through the alphabet leads us, I think, to imagine that a given letter is much more discrete and consistent than it is in fluid speech.
Extraordinary what we take for granted.
And also, interestingly enough, it is usually one of the first sounds an infant articulates as in “da-da” which usually infuriates the mama.
XO
WWW
WWW: True; it’s often only when we lose something, or almost lose it, that we appreciate its value. Babies saying da da (and ma ma for that matter) is probably why those words were adapted as signifiers of parents in the first place.
Don’t it always seem to go
You don’t know what you got ’til it’s gone
They paved paradise
And they put up a parking lot.
—Joni Mitchell, “Big Yellow Taxi”