One short, round 'oo' pushed out loud enough to travel. Think of a single call crossing a dark field: it is one sound, it is easy to count, and it is meant to reach somebody. Everything the word does can be seen from there. Change who makes the sound and the meaning moves with it: an owl in a tree, a car horn on a street, a room full of people all letting the same noise out at once.
From the word’s history
Wiktionary records hoot as Middle English houten/huten, of North Germanic origin, related to Old Swedish huta 'to cast out in contempt' and Swedish hut! 'begone!'. So the oldest recorded picture is a human shout aimed at somebody, not a bird. The owl use fits the same word because the bird's call sounds like that held cry; that is a match of sound, and this record does not claim the owl sense came out of the older one.
字典把 hoot 記在中古英語 houten／huten，屬於北日耳曼語系，跟古瑞典語 huta（輕蔑地把人趕走）、瑞典語 hut!（走開！）同一家。所以有紀錄的最早畫面是人朝著某個人喊，不是鳥叫。貓頭鷹的用法之所以用同一個字，是因為那隻鳥的叫聲聽起來就像同一種拉長的喊。這是聲音上的相合，這裡不主張貓頭鷹的意思是從那個舊字長出來的。
Wiktionary (kaikki snapshot in this packet), entries kaikki:3f6f69a190036ae4:646744005 and :646762985, etymology 1
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