I noticed that on Wiktionary and on Wikipedia it says that the "U" in Urbs was pronounced as "ʊ". I thought that Latin didn't have these kinds of lax vowels? Wouldn't it be pronounced as [u] instead of as [ʊ]?
Phonemically and historically speaking, the distinction between u and ū seems to have been solely one of length. When you lengthen a u (like before
/ns/), you get a ū; when you shorten a ū (like before
/nt/), you get a u. In Sardinian, the first Romance language to split off the family tree, u and ū merged. So it must have originally been something like
In terms of Classical-era phonetics, though, it seems u was significantly lower than ū—in fact, closer in quality to ō than to u! When Latin names are transcribed into Greek, u is transcribed with ο (e.g. Mummius who sacked Corinth becomes Μόμμιος in Greek sources). And when vowel length was lost in Vulgar Latin, u and ō merged, while ū and o remained separate from the pair.
As a result,
[ʊ] is the most common way of transcribing u phonetically. We may not be sure of the exact pronunciation, but we know it was derived from
[u], and closer to
[o] than to