<P> The radiators of modern cars no longer determine the shape of the grilles, which have become more abstract, the radiator being of different proportions from the grille and over 15 centimetres behind it . Now grilles are usually designed so the sound of a horn can readily come out through them . Those designs that echo the shape of the grille no longer have front fenders with rather large crevices that accommodate trumpet - shaped horns . Thus some cars, often British ones, have a pair of round horn grilles on either side of the radiator grille, with a horn behind each . A luxury car's horn grilles are usually chrome - plated . </P> <P> Cars with rear engines, such as the Volkswagen Beetle and the early Porsches, necessarily have no radiator grilles in front, and so have horn grilles placed below their headlights . Some motor scooters have this feature as well, placed below the handlebars . Their horn grilles may be made of cheap plastic . These vehicles and the cheaper cars have only one horn . </P> <P> Truck (lorry) horns may be electrically operated and similar to car horns, but are often air horns driven by air from an air compressor, which many trucks have in order to operate the air brakes . The compressor forces air past a diaphragm in the horn's throat, causing it to vibrate . Such air horns are often used as trim items, with chromed straight horns mounted on top of the cab . This design may also be installed on customised automobiles, using a small electrical compressor . Usually two or more are used, some drivers go so far as to install train horns . The frequencies vary to produce a variety of different chords, but in general are lower than those of automobile horns--125--180 Hz (approximately C3--G3). Sound levels are approximately 117--118 decibels . </P> <P> Locomotives have train horns, which are air horns operated by compressed air from the train's air brake system . To distinguish their sound from truck air horns, train horns in the U.S. consist of groups of two to five horns (called "chimes") which have different notes, sounded together to form a chord . Trains typically cannot stop in time to avoid hitting obstructions and depend on being seen by the driver, so they rely on their horns to warn of their approach . Therefore, train horns are louder and lower in frequency than car horns, so that they can be heard at longer distances . The sound level is 146--175 dB . In the United States, train horns are required to have a minimum sound level of 96 dB and a maximum sound level of 110 dB at 100 ft in front of the train . </P>

Where does a car horn sound come from