> What do you mean, Mark? The contrast between [p] and [b]
> is a matter of phonation type, not of lip rounding.
>
I had second thoughts after I sent it off. What I mean is that [p], at
the most reduced in English, is essentially the sound of dry lips
coming together, then opening, with little to no air coming out
(i.e., almost no sound at all, just a brief pause). [b] can get this
way too.
By lip rounding, I think I meant the vibration you can experience in
the lips making [p] and [b], bubbabubbabubbabub, the lip spasm you get
when you say 'rubber baby buggy bumpers' too fast.