Why is the 11th often sharpened?

Question

A book I'm reading suggests that there are two reasons that the 9#11 chord is more common than the 11th chord, and I don't understand either. > First, the natural 11th interferes with our ability to hear the root of the chord, because it's the same note as the subdominant, which is a lower note. Also, it clashes with the major 3rd. * I (having a very untrained ear) don't notice a difference in ability to hear the root between C9 and C11. The 11th is more than an octave above the root. Why would hearing the subdominant there make it harder? Is commonly agreed that it is? * Why is it problematic that the 11th clashes with the 3rd, but not that the #11th clashes with the 5th? Is this because the third is more characterising of a chord than the fifth? (Of course whatever sounds right is right and there are no rules, but I believe there is value in understanding what's written in theory books either way.)

Answer 1

So generally chords are constructed by using every other interval. In this case we have 1, 3, 5, 7, 9, and 11.

Or if we keep it within one octave for analysis, we have 1, 3, 5, 7, 2 and 4. We should look at it like this because seeing it as an octave away isn't really the case in how our ear hears it. There's only seven notes in a scale and it doesn't matter to our ear how far from the root the interval is. It matters what characteristics it takes on.

The 11th (4th) clashes with the 3rd because it produces a minor 2nd effect with a defining chord tone.

So from there we borrow from Lydian, which is the second most closely related scale to the major scale, which is built off of the subdominant. It's essentially a major scale with a raised 4th.

Our ears latch onto the melody line so the idea is to sharp that 4th, create tension that wants to resolve into the dominant and go from there.

I hope that answers your question.