I wanted to ask how it's possible to create a similar siren is this of the police
But i need only the imitation of the air horn.
Like this:
**broken link removed**
A 555 as a square wave generator feeding a speaker though a smallish capacitor would sound similar. The capacitor needs to be small enough to create a differentiator with the speaker load.
Technically, it doesnt become illegal until you act like a police officer. You can dress up your car to imitate a police car, but if you turn on the lights & siren, or try to pull someone over, thats when it becomes technically illegal. Its a huge gray area.
There was some news story where a guy converted his Mustang (I think) to look like a police car from the movie Transformers, and it was allowed, as long as he didnt act like a police officer.
The law varies from country to country and even state to state. What you're saying might be perfectly true where you live but might not be the case abroad.