Aftermarket horns provide a louder, and often more pleasant sound than stock horns.   Stock horns typically have an output of 97 dB.   The decibel scale is logarithmic and therefore the sound pressure doubles for every increase of 3.3 points, i.e. a horn with 118 dB is approximately seven times as loud as a stock horn.  

Diaphragm horns vibrate the diaphragm to make a more brash sound.   Most Original Equipment horns are this type.   Because of their smaller size, they are easier to mount.

Air horns use a reed and have a trumpet like sound.    The components are larger and more difficult to mount in today tightly packed engine compartments.   Auto Air Horns include a small electric compressor, while Truck Air Horns rely on the air brake system for a source of compressed air.

Air horns should be mounted pointed forward and slightly down so that no water builds up in them.