He made that poor piano moan with melody.

O Blues!

Swaying to and fro on his rickety stool

He played that sad raggy tune like a musical fool.

Sweet Blues!

Coming from a black man's soul.

O Blues!

In a deep song voice with a melancholy tone

I heard that Negro sing, that old piano moan—

"Ain't got nobody in all this world,

Ain't got nobody but ma self.

I’s gwine to quit ma frownin'

And put ma troubles on the shelf."

What is the effect of repeating the phrase “piano moan” in the poem?