sí in Spanish

Question

Why is if and yes the same?

Answer 1

sí: it is written with an accent, and it is pronounced using higher harmonics (in terms of frequencies), and generally with higher intensity, i.e. with stress. It means "yes".

si: it is written without an accent, and it is pronounced using lower harmonics (in terms of frequencies), and much lower intensity than any typical word with a stress. It means "if".

Source: http://www.spanishdict.com/answers/115867/differentiating-between-si-and-s-in-spoken-spanish