Warming Note:Our air horn can used for 12V and 24V，one important thing，the vehicle must is air brake.With the air brake，The air will go into the horn，when open the switch，it can work.But if for the Oil Brake Vehivle，it will not work.If your car is oil brake，you need buy one small air pump.Then it will work.Any questions，pls kindly contact us.Thank you Product DescriptionIntroductions: NewColor:SilveryFeatures:Easy install， durableMade of metal base and chrome plated zinc trumpetsDB output:135DBVoltage: 12V/24VWeight:797gTrumpet Diameter: 9cm(3.5’’)Short Trumpet Length: 22cm(8.7")Long Trumpet Length:28cm(11.0")Fit for Car Trucks Boats All Types Automobiles，but must Air Brake.Cann't use for Oil BrakePackage includes: 1 X Truck Air HornBecause of hand measurement ，2-3cm/inch deviation in size is a common thing，please check your size carefully before you buy it