Deviation in colloquial speech of nasal vowels in French

Question

Since coming to France I have noted a deviation of the nasal vowels pronunciation among native speakers from what IPA chart suggests. For example -en and -em are pronounced more akin to the nasal vowel /ɔ̃/ rather than /ɑ̃/. Furthermore I am convinced that -in sounds just like a nasalised schwa /ɘ̃/ and nothing at all like /ɛ̃/.
The peculiar part about this is that such pronunciation is observed (by me) even among seemingly educated Standard French speakers. I understand what nasal sounds are and how they're produced and also can reproduce schwa as it's present in my native language.
Is mine a novel finding that went unnoticed among linguists?

Answer 1

Admittedly, I am no expert on French, so take this answer with a grain of salt. This question seems quite subjective as well, so I may be completely off base in my analysis of what exactly is going on, but here goes.

The nasal vowels /ɔ̃/ and /ɑ̃/ can be distinguished in two ways. Firstly, by a change in oral tract articulation, such as the rounding of the lips. This is how the vowels are taught to most second-language speakers of French. The second is the quotient between the area of the nasal tract opening and the area of the oral tract opening. For /ɔ̃/, this quotient can be as high as 25/1, while for /ɑ̃/, it is almost always less than 5/1. Most speakers of French combine these distinct articulations (oral articulation and velum port opening), however, there are certain speakers, especially those with larger nasal tracts, which prefer one or the other. The paper I've linked here discusses this in further detail.

What I assume is going on with your perception of the back nasal vowels in French is you are focusing on the oral articulation of the vowels, which is what most non-native speakers, especially those not accustomed to nasal vowels, almost exclusively perceive. While it is certainly the case that nasal articulation can be properly produced and comprehended by a learner, this specific phonetic difference is particularly "in the weeds", and isn't really discussed outside of academic phonetics papers. However, it is still vital in the phonemic distinction between the two vowels.

The point I'm trying to make here is that the nasals are more akin to broad categories in terms of oral tract articulation, because there are so few of them. Much like how in languages with few vowels there is a lot of allophonic variation. However, there is a very clear distinction between the back nasal vowels outside of oral articulation, which is the nasalization quotient discussed earlier.

As for the front nasal vowels, the merging of /œ̃/ and /ɛ̃/ and their centralization has been attested since at least the 60's.

Here's another paper on nasal variation in French.