pronunciation - Difference between うーん and ううん

What is the difference between うーん and ううん?

In a comment on Pronunciation of ううん, paullb said that he usually sees the negative interjection ううん written as うーん. This confused me, because I thought of ううん and うーん as different words, pronounced differently.

I decided to look it up, but most of my dictionaries don't have entries for うーん. I found one that did, though--the big Kenkyusha J-E dictionary. According to that dictionary, ううん has three meanings. The first is the "negative interjection" meaning, and the other two appear to be something else:

ううん

１【軽い否定】no; uh-huh; uh-unh.

「朝ご飯はもう食べたの？」「〜まだだよ」 "Had breakfast yet?"ー"Un-unh, not yet."

2 【言いよどむとき、苦しいとき】 er; hmm; ugh; oof.

「衝撃的なニュースに、彼は〜と言ってそれきり口をつぐんでしまった。」 At the shocking news he let out a groan [moan] and said no more.

3 【感心したとき】 oh; aha; uh-uh.

「そのリポートを読んで、学生の冷静な状況分析に教授は〜どうなった。」 When he read [upon reading] the essay, the professor murmured his approval of the student's objective analysis.

Meanwhile, the entry for うーん looks like this:

うーん　＝ううん 2,3.

That doesn't include meaning 1, so I concluded that the negative interjection ううん and うーん are separate words. (It also appears that うーん can be written ううん, but then it's not "the negative interjection ううん".)

Based on paullb's comment, though, I'm not sure if this distinction holds up in practice. Can anyone tell me if I'm on the right track? If I see うーん, could it mean "no"? If I see ううん, can that mean something other than "no"? How do I tell the difference?

(I left out a couple examples from the definition for ううん 2. Hopefully that's okay.)

Answer

This is purely anecdotal and based on my own experience and confusion with this, but ううん has generally been a negative interjection, as in definition 1, and うーん has a meaning in English like "mmmm" or "ummm" or "hmmm" or something like that, conveying reluctance. I figured this out after some confusing situations wherein I would suggest going somewhere or eating something and meeting うーん as a reply (in textual correspondence) and thinking they were saying no when they really weren't outright negating what I suggested.

This dictionary entry seems to back up this idea of ううん as a mostly negative word when used outside of a generic sort of thinking "hm" or painful noise:

http://dictionary.goo.ne.jp/leaf/jn2/17463/m0u/%E3%81%86%E3%81%86%E3%82%93/

Particularly the third.

So where ううん and うーん both can be general "noises", ううん has a negative connotation while うーん has a connotation of reluctant acceptance.