Why do Dogs Howl at Sirens?

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If a firetruck, an ambulance, or a police car goes by with it's sirens on the dogs go crazy howling! Why do the dogs do that? Does the sound bother them because of their sensitive hearing? Are they trying to protect their territory? Do they think the sirens are a lost dog and are trying to help what they think is a fellow dog? What do you think? Do you have the correct answer?