What we notice most about the sound of this poem is how varied it is. Yeats's ear is so fine-tuned that we wouldn't be surprised if he could hear a mouse coming from a mile away. Through the sonnet's 14 lines, he uses a number of different techniques and sentences structures to create a rich tapestry of sounds.
The first stanza is characterized by percussive, almost violent, beats and pauses. The first three words mimic the surprise of the initial panic of the attack with the two heavy beats in "sudden blow," followed by a pause that is, in poetic terms, as wide as the Grand Canyon. Similarly, the word "staggering" in the second line has a harsh, sharp sound. The technical word for all those pauses in the middle of the lines is a caesura. Yeats creates parallels between the girl and the swan that suggest the mating process: great wings/staggering girl, thighs/webs, nape/bill – finally lead to their convergence in breast/breast.
The second stanza shifts to more evenly flowing lines as the speaker begins to reflect more philosophically on the event. The lines are structured by the question "how," and the adjectives begin to pile up: "terrified," "vague," "feathered," "loosening," "white," "strange."
The adjectives continue to accumulate as the poem builds to the solemn declaration, "And Agamemnon dead," which blindsides us with its simple, direct power. We don't even know that much about who Agamemnon was, but we want to cry, "Not Agamemnon too!" The rhythm comes to a screeching halt as line 11 is fractured over two lines.
Having reached this emotional height, Yeats returns to his adjectives and percussive beats, throwing in some alliteration for good measure: "brute blood." Then the speaker asks another rhetorical question but doesn't answer it. The reader is "dropped" at the end of the poem, just like Leda is dropped to the ground by the swan.