Does おお sound different from おう?


I know おう sounds like a long o, but does おお sound the same, or should it be
pronounced differently?




I think that with a few exceptions, `おう` is /oː/ within a single morpheme, and
/ou/ when it crosses morpheme boundaries. `おお` is always /oː/. The same thing
is true of other kana pairs like `こう・こお`, `そう・そお`, and so on.

For example, look at the following pair of words:

[code]

      追う　　おう　　/ou/
      王　　　おう　　/oː/
    
[/code]

`追う` can be divided into the root `ow` and the verb ending morpheme `u`. The
/w/ disappears before /u/, so we're left with /o/ and /u/ in separate
morphemes, pronounced together as /ou/. The same thing happens with other
verbs that end this way, so `思う` is pronounced /omou/, not /omo:/. In
contrast, `王` contains `おう` within a single morpheme, so it's pronounced as a
long vowel /oː/. You can make any number of similar comparisons:

[code]

      沿う　　そう　　/sou/
      そう　　そう　　/soː/
    
[/code]

One tricky case is forms like `行こう`. If you follow the traditional analysis
and call this the 未然形＋助動詞「う」, it _looks like_ `行こ` and `う` are separate
morphemes, but that doesn't fit with our rule so far--this form is always
pronounced /oː/, not /ou/. In fact, separating between `行こ` and `う` is an
artifact of analyzing Japanese in kana; romanizing makes things a bit clearer.
Instead of `iko.u`, we can divide it into `ik.ou`, giving us the root `ik`
plus the hortative suffix `ou`. Once we do so, we can see that `ou` doesn't
cross morpheme boundaries, so it makes sense that it's pronounced /oː/.

* * *

The above rule mostly works, but we can come up with some exceptions like `今日`
/kjoː/ or `おはよう` /ohajoː/. What do they have in common? They were historically
subject to a class of sound changes called
[ウ音便](http://ja.wikipedia.org/wiki/%E3%82%A6%E9%9F%B3%E4%BE%BF#.E3.82.A6.E9.9F.B3.E4.BE.BF).
Let's look at these one at a time:

  * `今日` was originally `今` /ke/ + `日` /pu/. The consonant /p/ turned into /Φ/ and then was lost, leaving /keu/. This was subject to the sound change /eu/ to /joː/, giving /kjoː/. Modern kana usage respelled `けふ` as `きょう` to reflect this pronunciation, obscuring the fact that it began its life as two separate morphemes.

  * `おはよう` was originally the honorific prefix `御` /o/ + the root `早` /haja/ + the adjective ending `く` /ku/. Again the consonant before the /u/ dropped out, giving /ohajau/, which was subject to the sound change /au/ to /oː/, giving /ohajoː/.

In both cases, the sound changes created long vowels that cross morpheme
boundaries, and modern kana usage respelled them with an お段 kana plus `う`. In
cases like these, the pronunciation is /oː/ rather than /ou/.




I think that with a few exceptions, `おう` is /oː/ within a single morpheme, and
/ou/ when it crosses morpheme boundaries. `おお` is always /oː/. The same thing
is true of other kana pairs like `こう・こお`, `そう・そお`, and so on.

For example, look at the following pair of words:

[code]

      追う　　おう　　/ou/
      王　　　おう　　/oː/
    
[/code]

`追う` can be divided into the root `ow` and the verb ending morpheme `u`. The
/w/ disappears before /u/, so we're left with /o/ and /u/ in separate
morphemes, pronounced together as /ou/. The same thing happens with other
verbs that end this way, so `思う` is pronounced /omou/, not /omo:/. In
contrast, `王` contains `おう` within a single morpheme, so it's pronounced as a
long vowel /oː/. You can make any number of similar comparisons:

[code]

      沿う　　そう　　/sou/
      そう　　そう　　/soː/
    
[/code]

One tricky case is forms like `行こう`. If you follow the traditional analysis
and call this the 未然形＋助動詞「う」, it _looks like_ `行こ` and `う` are separate
morphemes, but that doesn't fit with our rule so far--this form is always
pronounced /oː/, not /ou/. In fact, separating between `行こ` and `う` is an
artifact of analyzing Japanese in kana; romanizing makes things a bit clearer.
Instead of `iko.u`, we can divide it into `ik.ou`, giving us the root `ik`
plus the hortative suffix `ou`. Once we do so, we can see that `ou` doesn't
cross morpheme boundaries, so it makes sense that it's pronounced /oː/.

* * *

The above rule mostly works, but we can come up with some exceptions like `今日`
/kjoː/ or `おはよう` /ohajoː/. What do they have in common? They were historically
subject to a class of sound changes called
[ウ音便](http://ja.wikipedia.org/wiki/%E3%82%A6%E9%9F%B3%E4%BE%BF#.E3.82.A6.E9.9F.B3.E4.BE.BF).
Let's look at these one at a time:

  * `今日` was originally `今` /ke/ + `日` /pu/. The consonant /p/ turned into /Φ/ and then was lost, leaving /keu/. This was subject to the sound change /eu/ to /joː/, giving /kjoː/. Modern kana usage respelled `けふ` as `きょう` to reflect this pronunciation, obscuring the fact that it began its life as two separate morphemes.

  * `おはよう` was originally the honorific prefix `御` /o/ + the root `早` /haja/ + the adjective ending `く` /ku/. Again the consonant before the /u/ dropped out, giving /ohajau/, which was subject to the sound change /au/ to /oː/, giving /ohajoː/.

In both cases, the sound changes created long vowels that cross morpheme
boundaries, and modern kana usage respelled them with an お段 kana plus `う`. In
cases like these, the pronunciation is /oː/ rather than /ou/.

