Bottom woman radical - the indexing component. Marks 婆 as a feminine-role word: an older woman, a grandmother, a mother-in-law. The whole compound says "woman-of-the-(wave)-sound," with the radical doing all the meaning work.
Top 波 supplies the sound - bō drifted to pó with initial unaspirated/aspirated shift. The "wave" meaning has no semantic role here; the choice of 波 over a simpler phonetic was driven by sound and visual balance. 波 itself is 氵 + 皮 (skin-phonetic).
