The train horn is one of the most universally recognizable sounds. Even as children, we are made to go "choo choo" to imitate the sound of a train horn going off. While the sound of modern train horns isn't exactly similar to "choo choo," this particular expression or sound has become one of our consciousness very early on. Train horns continue to be important in this age of digital technology since it's the only way that a train engineer can alert car and truck drivers, pedestrians and even livestock and other animals that a train is coming. It is also useful for simple communication between trains during times when cars swerve from one track to another.
As most people remember from period films and cartoons, train horns used to be operated using a lever or a string that was attached from the top of the cabin. Nowadays, more and more trains have moved towards using models that use push buttons. Strings and levers have been proven to be less durable and can be quite hard to use, especially for the train engineer who's trying to do several things at the same time.
Whether it's an old fashioned lever-and-pulley system, or a push-button one, the basic principles are still the same. Here is how a train horn works.
- The secret of the system is compressed air. Yes, there are no speakers - it's all air. A train's horn works by blowing a high pressure gust of air continuously. Trains with this type of horn usually store and generate compressed air just for the purpose.
- This air compressor is attached to a shaft hat that consists of a narrow opening, a floppy diaphragm that covers it and an external horn shaped like a cone that amplifies the sound. When the compressed air is released, air gushes out into the other parts of the horn, generating that sound that all of us are familiar with.
- As the conductor presses the horn button, a strong gust of air goes through the narrow opening of the horn. This makes the air flow inside of the horn's body very turbulent. In turn, this makes the diaphragm inside the horn oscillate and flap against the external opening of the horn. This type of movement by the diaphragm is what creates the loud sound.
- While the sound that is produced by the diaphragm is already loud, this sound is even further enhanced by the structure of the external horn. Just like a megaphone, the sloping opening makes the sound wave more compressed as it exits out of the train horn. Similar concepts can also be observed in fire horns and truck horns.
So next time you hear a horn, don't imaging that there's a loudspeaker somewhere inside the train. It's all just compressed air and a little bit of engineering ingenuity. When the train bellows "choo choo," it's either calling "all aboard," or giving warning to motorists and pedestrians to watch out for the oncoming train.