So, I made this observation years ago and thought that I really had it solved or rather I perhaps just left it alone. But, me being the obsessive learner and having to know it all has really bought me to a new level of crazy. I am definitely over analyzing but for the years I have been learning Japanese I always have heard あい (and some other 二連母音） as somewhat of a diphthong at times (in normal conversation). At times I hear the distinction between the vowels and other times I don't. I've done some research and came across a few interesting articles. Who knows their credibility but I'll present them anyway.

First:

"日本語では通常の会話においてアエ、アイ、アウ、アオ、ウエ、ウイ、オエ、オイ、オウなどが二重母音として現れやすい。しかし、ゆっくり丁寧に発音されると母音接続となり、日本語話者の意識では2つの母音として扱われる。"

This was taken from Wikipedia. Now I have presented this information to natives and only got one response but the response was that even after thinking about it they still didn't understand the difference. This is essentially what it says here. That diphthongs appear in Japanese conversation easily but to Japanese natives it is handled as two vowels. I really depends on the conversation, but generally (especially if a phrase ends in あい as in　行きなさい）the い at the end can sound like the い when it is unvoiced between two voiceless consonants (聞く、好き、素敵、騎士).

Now, periodically い　or う at the end of a phrase may become voiceless which is most commonly seen in です　or ます but I wonder if this same concept applies to い （う）　when it is by itself at the end of phrase as in 行きなさい. In addition, I wonder if it applies even in the middle of other mora (i.e.大体). EDIT: I think I'm using the terminology incorrectly but what I am trying to say is the い at times can sound very "down-played" for lack of a better term.

A second interesting find I found was this:

「二連母音」：読んで字のごとく、母音２つの連続
①
■ああ／あい／あう／あえ／あお
■いあ／いい／いう／いえ／いお
■うあ／うい／うう／うえ／うお
■えあ／えい／えう／ええ／えお
■おあ／おい／おう／おえ／おお

②あ行以外＋あ行による２音も 母音が連続すると言える。
例
【おかあさん】⇒＜ka＋a＞⇒ aa
【貝】⇒＜ka＋i＞⇒ ai
------
「二重母音」：始めと終わりで音色が２つ現れる１音の母音。
※二連母音とは別物
※日本語には二重母音は 文字として現れないが存在する。
※※「や・ゆ・よ」「ゃ・ゅ・ょ」は二重母音と言えなくもないが、二重母音とは言わない。

二連母音のうち、下記のものは、二重母音で発音されることがある。

■あい／XX／うい／XX／おい
■あう／いう／XX／XX／おう

例
愛（ai） 悔い(ui） 恋（oi）
⇒(ii)(ei)は稀
会う(au) 言う(iu) 追うしかない（ou）
⇒(uu)(eu)は稀


※ただし、上記例も １音１音ハッキリ
/あ・い/く・い/こ・い/
/あ・う/い・う/お・う/ 等と い・う、ば・あ・い、は
二重母音ではなく、二連母音である。

I personally don't know if a subscribe to diphthongs completely existing in Japanese. It's not a completely different sound being produced. I think they are similar to a diphthong. I feel what I have concluded is that this phenomenon is a direct result of the weak vowels い and う SORT OF blending with the previous vowel but not completely. I definitely babbled on about this and probably did not use some linguistic terms correctly so forgive me. I would like to hear your thoughts on this.

I'm not a linguist but It seems to me that the diphthong and and the not-diphthong version are allophonic in Japanese. い　is sometimes devoiced but only when it comes between two voiceless consonants. I've head both the prefectures 愛知 and 高知 devoiced. My guess is that because in something like 行きなさい, the あ　part is voiced, so the い won't devoice.

ZeRinku Wrote:Now, periodically い　or う at the end of a phrase may become voiceless which is most commonly seen in です　or ます but I wonder if this same concept applies to い （う）　when it is by itself at the end of phrase as in 行きなさい. In addition, I wonder if it applies even in the middle of other mora (i.e.大体). EDIT: I think I'm using the terminology incorrectly but what I am trying to say is the い at times can sound very "down-played" for lack of a better term.

I can't really comment on the rest of your post, but the rules about devoicing are really simple and very regular, even across most dialects. If you have an unvoiced consonent (k, t, p, h, s), followed by u or i, and then followed by an unvoiced consonent or nothing (k, t, p, h, s, nothing), the u or i will be devoiced. The exception is certain words that are accented on a vowel that should be devoiced; because they are accented they become voiced (there are very few of these words). What's more, things are never devoiced outside of that situation. So, no, i and u are never devoiced in a word like 行きなさい.

There's nothing special about です or ます that make them devoiced, they simply follow the rule, and I'm not sure they're the most common devoicing in Japanese (since the distal-polite form is not used among family members, which is probably the most prevalent conversational group nationwide). You'll notice, for example, that the u in ですよ is not devoiced, because y is not an unvoiced consonent. Many learners who mistakenly devoice the u here end up pronouncing ですよ as something similar to でしょう.

(One interesting thing to note is that devoicing seems to be done conceptually. You'll hear devoiced vowels in the athletic chant いち、に、さん、し、ご、ろく、しち、はち at 1, 4, 6, 7, and 8, even when the athletes are counting very fast. Presumably because they're thinking of the numbers individually, so the に after いち does not chance the vowel in ち to being voiced, etc. You'll also notice that 7 may not be devoiced fully because there's too much devoicing going on around it, and it has a nice rhythm to it when you voice the ち. Devoicing 4 also seems optional.)

Tzadeck Wrote:I can't really comment on the rest of your post, but the rules about devoicing are really simple and very regular, even across most dialects. If you have an unvoiced consonent (k, t, p, h, s), followed by u or i, and then followed by an unvoiced consonent or nothing (k, t, p, h, s, nothing), the u or i will be devoiced. The exception is certain words that are accented on a vowel that should be devoiced; because they are accented they become voiced (there are very few of these words). What's more, things are never devoiced outside of that situation. So, no, i and u are never devoiced in a word like 行きなさい.

There's nothing special about です or ます that make them devoiced, they simply follow the rule, and I'm not sure they're the most common devoicing in Japanese (since the distal-polite form is not used among family members, which is probably the most prevalent conversational group nationwide). You'll notice, for example, that the u in ですよ is not devoiced, because y is not an unvoiced consonent. Many learners who mistakenly devoice the u here end up pronouncing ですよ as something similar to でしょう.

(One interesting thing to note is that devoicing seems to be done conceptually. You'll hear devoiced vowels in the athletic chant いち、に、さん、し、ご、ろく、しち、はち at 1, 4, 6, 7, and 8, even when the athletes are counting very fast. Presumably because they're thinking of the numbers individually, so the に after いち does not chance the vowel in ち to being voiced, etc. You'll also notice that 7 may not be devoiced fully because there's too much devoicing going on around it.)

Yes, I am aware of the rules when it comes to u and i being devoiced. I know that 行きなさい doesn't fall under these rules per say but I was stating that there is a possibility that since both i and u are the weak vowels in the language that there may be some sort of connection as to why the 二重母音 mentioned all end in i or u. To me, when an i is devoiced (between consonants) that i sound that is slightly there is the sound I hear when あい is spoken at times. It's almost like a blowing out of the breath in place of a crisp い sound.

ZeRinku Wrote:Yes, I am aware of the rules when it comes to u and i being devoiced. I know that 行きなさい doesn't fall under these rules per say but I was stating that there is a possibility that since both i and u are the weak vowels in the language that there may be some sort of connection as to why the 二重母音 mentioned all end in i or u. To me, when an i is devoiced (between consonants) that i sound that is slightly there is the sound I hear when あい is spoken at times. It's almost like a blowing out of the breath in place of a crisp い sound.

Sorry, didn't mean to be condescending if I came off that way.

Tzadeck Wrote:Many learners who mistakenly devoice the u here end up pronouncing ですよ as something similar to でしょう.

Russians who get the idea that the u in です is always devoiced also end up with a funny way of pronouncing things like ですが. In Russian most unvoiced consonants become voiced when followed by a voiced consonant, so they pronounce it as "dezga". There isn't a Japanese word to confuse it with, but it still catches people off-guard.

Tzadeck Wrote:Sorry, didn't mean to be condescending if I came off that way.

No, not at all! It's always a little difficult to accurately portray the underlying tones of a message through text. I apologize if I came off a little on edge in the answer.

vonPeterhof Wrote:

Tzadeck Wrote:Many learners who mistakenly devoice the u here end up pronouncing ですよ as something similar to でしょう.

Russians who get the idea that the u in です is always devoiced also end up with a funny way of pronouncing things like ですが. In Russian most unvoiced consonants become voiced when followed by a voiced consonant, so they pronounce it as "dezga". There isn't a Japanese word to confuse it with, but it still catches people off-guard.

It really irks when です is simply taught as being pronounced as des all the time. I honestly believe that those who are adamant about learning the language should have at least some sort of introduction to weak vowels and voiceless consonants. I know I would have benefitted way back in the beginning.