Automotive horns are designed to produce sound which is measured in decibles. This sound is produced from basic energy sources such as electricity,air pressure, vacuum, and mechanical sources. Electronic sound devices also can produce high decible noise. On the internet you might try any one of the various search engines and look for manufactures of automotive horns.

car horns are made with a coil operated hammer which strikes a diaphragm made of thin steel, in effect a buzzer uses the same circuit. Then then couple the high acoustic impedance of the diaphragm to the low acoustic impedance of the air via a trumpet(which is usually folded to make it smaller). high frequencies = small trumpet. Large diameter and long trumpet = lower frequencies, like truck horns, who need to be loud to make idiots hear them over their rock music