Dogs howl for a couple of reasons. It goes back to their ancestors. They don't like to be left alone and will bark or howl until the owner returns. When dogs hear a two tone sound like a siren it stimulates this wolf behavior. It does not hurt their ears. Some singing or even harmonica playing will set them off.
Probably because they have much better hearing than you do and they can hear things twice (maybe even 4 times as better) than you can. So just imagine someone taking some loud noise and putting it straight in your ear. You don't want to hear it and it's irritating and annoying. Wouldn't you try to communicate in some way for them to stop and that it's annoying?
some high pitched noise might disturb the dog, the loud encounter might scare them, or if you have a protective dog, the dog might relate sirens to danger therefore causing it to bark to protect it's owners.
it does hurt their ears..I had a shepherd mix n a German rot tie..every time firetrucks, cops, ambulances go by with the siren blaring. only the shepherd would start howling..the rot tie would look at him with her head cocking back and forth, then she would start..but only if he does..by herself not !