abcdefg wrote: the defining characteristic of the huyền tone seems to be that it's low, not that it's falling. The contour is almost identical to that of ngang there.
In the (northern) kinds of pronunciation I'm used to huyen is long and the voice often has a 'breathy' quality to it. Nang just sounds short and low with a glottal stop after which everything is unvoiced (and inadubile for me though I suppose native speakers here the rest of the syllable).
For me bạn often sounds phonetically like bạ or bạc.
(And hoi is usually pronounced as a shorter non-breathy fall without the final rise which is mostly restricted to syllables in isolation).