Why does ㅗ *o* sometimes sound like ㅜ *u*?

Question

When I listen to 오 on Naver Dict (or 오다 or 오늘), it sounds the same as 우. 오 sounds like a [u] sound, not an [o] sound like in Spanish como [komo]. (Click the speech icon with round, not vertical, sound waves for a human-recorded sound rather than text-to-speech.) Maybe 우 sounds a bit more emphatic, but it doesn't sound very different. In contrast, 소, 요³, and 포, and the text-to-speech 오 definitely sound like [o]. 코 and Wikipedia's [o] also sounds like [u], although if I listen to it while thinking [o] in my head, it starts to sound more like [o]. But the vowels in 오, 노, and 모 sound to me unambiguously like [u].

Is ㅗ more closed and thus sounds more like ㅜ in some certain phonological environments (after certain consonants, depending on vowel length, etc.), or is it more just that it varies by speaker or dialect? Would it be wrong to say ㅗ the way the text-to-speech says it? When ㅗ sounds like [u] to me, is ㅜ somehow even more closed, or are they really indistinguishable?

Answer 1

The slipperiness of vowels!

To my ear, there is a difference between 오 and 우 on Naver, and also between the voices that are there for 오 too. They are very close though.

There are several factors at work here:

• The same vowel phoneme /o/ can have a different phonetic range between different speech varieties (so IPA may not be as phonetic as one might expect). This is well known amongst hispanohablantes learning Italian, where distinguishing /o/ and /ɔ/ is an issue. Looking at the two formant charts for Spanish and Korean, we can see that Spanish /o/ lies around both Korean /o/ and /ɔ~ʌ/.
• The length of vowels can change the "quality" of the vowel somewhat. In Standard South Korean, the long /oː/ is phonetically more closed than the short /o/. This is likely to be rather minor in actual speech, but dictionaries tend to preserve former standards.
• There has been a vowel shift happening in modern South Korean, especially in the Greater Seoul area, with /o/ becoming [u] and /u/ fronting to [ʉ]. The contrast is weakening, however, with fronting of /u/ not being universal.