Thanks! Sorry for being stubborn: At least according to Google Translate, ハドウケン also sounds a lot like the English "oh" to me (instead of the "oo" that I roughly hear in the Japanese voice clips). Is this just a Google Translate issue (i.e. is the "oo"-like sound also an acceptable pronunciation)?

Also, I've updated my original question to change "Ryu pronounces it like English 'ou'" to "Ryu pronounces it like English 'oo'". Sorry about that, a typo on my part :-(

Is there any chance you could switch to IPA to explain what you are hearing? I’m having a hard time understanding what you are trying to express with “oo” vs “ou” vs “oh”.

Sure, sorry—I've edited the question accordingly!

@BalinKingOfMoriaReinstateCMs May I ask what your native language is? If it is English, what variety of the English language?

@EddieKal General American English, roughly.

@BalinKingOfMoriaReinstateCMs K, in that case, your phonemic transcription makes sense in that context. Let me take a stab at this.

I guess you’re trying to differentiate the long vowel [[oː]] (doesn’t really get used in any English words afaik) vs the diphthong [[oʊ]] (the usual English extended “o” like “open” “slow” “dough” etc) vs the dipthong [[ou]] (also not used in English afaik). The オー and オウ in Japanese refer to the first and third there.

@DariusJahandarie I agree with your assessment of the long vowels, but I always thought オー, オウ were practically pronounced the same. My understanding is the difference in transcription reflects a wish to more accurately represent the English long vowels including diphthongs but in reality people still pronounce the the words the same. Since the 平仮名 「う」 functions just like 「ー」 and extends a う段 or お段 sound, I was under the impression that people treat the 「ウ」in「オウ」the same way phonetically.

Nope, the purpose of writing it in katakana is to show the actual pronunciation. Generally おう gets pronounced オー, but not always. For example the verb 追う must be pronounced オウ. In addition, like I mentioned in the post, even Sino-Japanese compounds (although they are to オー 99% of the time) allow for a オウ reading if you are trying to add color for some reason.

Hmm... I took 追う to be in a different category as we are focused on extended vowels and 送り仮名 are not vowel extenders. I find your first point very reasonable (the purpose of writing it in katakana is to show the actual pronunciation), but I can't think of any examples that I have heard, and that's why I always thought it didn't really show in actualized pronunciation. I wonder if you could give some audio examples where オー and オウ, or メー and メイ, are pronounced differently in loanwords? I lean toward this from your answer: "it’s basically unnoticeable/unnotable to Japanese speakers".

For the latter, one minimal pair would be 姪 (メイ) vs the メー in 命を受ける, 盟を結ぶ, 銘を刻む etc. For the former I can’t think of any nouns which must be pronounced オウ, only verbs (追う and 負う).

@DariusJahandarie Oh, so you're saying that the voice actors are saying [ou] but I'm only noticing the final [u] (is it pronounced for much longer than the starting [o])?

(and if that's correct, is Google Translate just emphasizing the [u] way less in its pronunciation of e.g. ドウケ? 'Cause I can hear it's there, it's just pretty subtle to my ear.)

[[du]] would be like English “due”, and I definitely don’t hear that in any of your audio samples, so I was surprised you mentioned it at all, actually.

@DariusJahandarie Weird... I distinctly hear "due" in both of the samples :-P Maybe that's due to my English dialect?

Well, sorry I am not convinced. I consider it more likely a matter of individual utterance preference/style rather than a purely lexical difference. Some words may be invariably pronounced メイ or メー, but for a lot of other words it seemingly depends on the speaker. For example, as you note 姪 is rendered as メイ, as these two audios indicate, but in this actualization pitch accent aside that 銘 sounds like メイ to me rather than メー.

For comparison, 命 as メー

@Eddie 姪 must be pronounced メイ. The other ones can go either way, but it’s standard to turn into メー in regular speech; enunciated speech like that which might be used in a forvo recording can result in メイ, because the word is being spelled out in some sense. Anyways you keep on moving the goal posts so I don’t think I can convince you of anything :)

@BalinKingOfMoriaReinstateCMs Maybe this will help: voca.ro/1msWa8rjHUk0 (in order: [[doː]] [[dou]] [[doʊ]] [[duː]]) The third and fourth would never be said in regular Japanese speech. The third in fact can’t even be transcribed in Japanese; the fourth can but it’s a transcription used for foreign words (ドゥー).

@DariusJahandarie Out of those four, I promise that [duː] is the closest (to me) to what I hear in youtu.be/e9IMi9iZMKo?t=33. I'm not trying to be stubborn, and I appreciate your help—so in your opinion, should I just chalk up my confusion to my ear being unfamiliar with Japanese vowels?

@EddieKal Loanwords in カタカナ語?! Now I’m just confused. That’d be stuff like ブーム or ステーキ. (Maybe you misused the term カタカナ語?)

@DariusJahandarie I retracted part of that comment as I mistakenly thought 波動拳 was a Chinese loanword rendered in カタカナ. My bad! But I hope we can both agree 追う is out of the picture.

@BalinKingOfMoriaReinstateCMs Yeah, that doesn’t sound like [[duː]] to me — I think it’s most likely your ear but feel free to get another opinion. (It could very well be my ear due to hearing it as Japanese and doing some auto-repair that I can’t consciously notice...?)

@EddieKal Sorry to interrupt your conversation with DariusJahandarie—I'm curious if you hear something like [[duː]], or if it really is just me :-)

@BalinKingOfMoriaReinstateCMs No, it is not /duː/ for sure. If we were to go the technical route, the Japanese vowel /o:/ is somewhere between 6 and 7 in this graph. The English /u:/ in General American would be pretty close to 18. Because of the BGM and the sound effects, I can see how you'd hear a /u:/ in there but I think the best way is find an audio clip where this word is pronounced normally as it'd be in everyday speech. Like this. Do you still hear /du:/?

@EddieKal Just to clarify, I am able to tell that it's not [[duː]] *per se*—I just hear it as something vaguely similar (I don't know how to describe what I actually hear). In those voice clips, the male version sounds to me like the [[duː]]-ish sound I'm asking about, whereas the female version sounds distinctly different.

Also, thank you both for your tremendous patience!

@BalinKingOfMoriaReinstateCMs So I was drafting an answer that I have discarded, in which I was going to explain why native English speakers can't tell certain sounds in other languages apart or mistake them for some other sounds. I discarded my draft because I am not sure if you are looking for a phonetic explanation and if that's not what you are looking for I think Darius' answer and follow-up comments constitute a pretty good explanation.

@EddieKal Gotcha. Just to make sure I understand: The best explanation is probably that my GenAmE-trained brain is doing some post-processing behind-the-scenes that's making the vowel sound subjectively different from what it actually is, right? (And, relating to my original question, the voice actors for English are just doing the "less interesting/cool" ō pronunciation?)

(where the vowel sound in question is the [[ou]] diphthong.)

@BalinKingOfMoriaReinstateCMs But perceiving /o:/ as /u:/ is probably not common. So I'd chalk it up to the sound effects. Btw, I wrote a totally underdeveloped and somewhat lacking answer to another question and it is kind of related to your question. Here. Hope it answers your question about English diphthongs and mora (vowel length unit) in Japanese.

(edited) Yes, I would've focused on being a GA speaker your phonetic perception as the main reason you are getting a /du:/. You probably have heard a lot of Japanese speaker can't tell /r/ (it's actually [ɹ]) and /l/ apart. The reason is they are the same sound to Japanese speakers (technospeak: they are allophones in Japanese). By the same token, a lot of native English speakers, especially AmE speakers, perceive /o:/ as /oʊ/. (This is actually reciprocal in some measure: Japanese speakers perceive /oʊ/ as /o:/, so this is where my opinion kind of differs from @DariusJahandarie)