Originally Posted by: snojones

so what I want to know is how come extended chords (like 9ths,11th and 13ths) sound so mellow when are basicaly including all the disonant scale degrees to the chord?

Hey snojones, there are a bunch of factors:

1 - Dissonance is a relative term here. Combining different fundamental tones will create different levels of "beats" and interference / dissonance because of the frequency ratios. So, some intervals are more dissonant than others. All of the intervals in the equal temperament chromatic scale (except for octaves and unisons) are theoretically dissonant to some degree. A 4th is more dissonance than a 5th. Etc. So "dissonant" doesn't automatically equal "ugly" here.

2 - The context / expectations. If you're playing strictly "amen" church music, the 4th scale degree at the end of a phrase will sound striking and unresolved. Because that's what the expectation is, for it to move back to that very safe major third interval. However, in a jazz fusion piece, those notes will sound fine. You could stack a bunch of 4ths and it could be considered normal for that style. However, where you place that stack of 4ths might sound right or wrong. Playing a consonant chord at the wrong time can make it sound like a mistake.

3 - Melodic direction. You can easily add chromatic runs to a "nice sounding" melodic phrase as long as where you end the phrase / are landing is a "nice sounding" note. During that, you'd be hitting all of the "dissonant" notes in between, totally breaking the scale, and it still works. Also note, bending especially in blues can be VERY dissonant, but we like it. Because it's part of the vocabulary and is going somewhere... it's not just a random out of tune note.

4 - Your own tastes. I happen to love augmented triads and whole tone lines and diminished scales. It's something I associate with memories of movie scores and other music I grew up listening to. That doesn't change the fundmental dissonance of those elements, but it does make them totally acceptable to me in my musical vocabulary. So I won't automatically think "ew, that sounds bad." Much to the horror of some of my previous teachers. If you happened to grow up listening to a bunch of jazz and associate it with good things, those extended chords might sound great to you. That doesn't mean they aren't dissonant according to physics.

The concepts of Consonance and Dissonance aren't strict black & white laws for you to follow, even though they're based on physics. It's more like spice or flavor. Is a food too spicy for you? Maybe not for me.

The summary is, there's not a 1:1 link between the dissonance of equal temperament intervals and our enjoyment of them / how pleasant they sound. They kinda overlap in general, but a lot of it is context, associations, personal taste.

Make sense?

-Carl.