The best way—actually, the only way—to get the full force of "Ariel" is to read it aloud. Seriously. Read it aloud to yourself right now. Don't worry. We'll wait right here while you do.
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Back? Great. Do you hear all those repeated sounds? The long I rhyme of "cry" (21) and "I" (23)? The S sound consonance of "Stasis in darkness" (1)? The beginning B alliteration in "Black sweet blood mouthfuls" (13)? Sonic repetitions like these are all over the poem.
So much repetition can be found here, in fact, that reading "Ariel" aloud is like being in an echo chamber. The sounds of the poem don't let go of you—they repeat, disappear, return with a vengeance. And this is particularly interesting because "Ariel" is written in free verse, which means that it doesn't have a regular rhyme scheme. The rhymes in "Ariel" are frequent, but not regular, which means that they are unpredictable. You never know when a sound is gonna creep up again in the poem. The repetitions in Plath's poem can catch you off-guard, and draw you deep into its dense web. In this way, the form of the poem echoes its content perfectly; as Ariel takes our speaker on a wild ride, so the poem "Ariel" takes its readers off on a gallop.
Think we're exaggerating? Listen to Plath's recording of "Ariel." You'll be lucky to make it out of this recording on solid ground.