Fundamentals of Train Horns

Oscillating air flow is the main principle behind any horn. This oscillating air flow creates sound that varies with due the material used to construct the horn. In a train horn, the air flow oscillates through a diaphragm. This diaphragm is usually made of metal. Applying air to the horn causes the diaphragms to vibrate. Only a certain amount of air is allowed to pass through due to the diaphragm’s position. This rhythmic fluctuation of the diaphragm creates air waves, which results to audible sounds, thus the sound coming from the horns.

This principle can be easily understood by observing the color-coded illustration below (the original picture used with permission, courtesy of Glen Cleary). By following the flow of air as illustrated in the picture, you will be able to learn how the horn works.

Initially, the air flows in through an orifice (colored as yellow) from the air line pipe which is connected from the manifold. This air line combines each and every bell to the chime. The air that came in from this inlet goes to the green compartment. This compartment is primarily sealed with a diaphragm (purple). The diaphragms outer seal is permanent during the construction of the horn. However, the seal at the inner side is not permanent.

Increasing the pressure in the green area will force the diaphragm outwards. This will allow the escape of air through the nozzle (red). The air escaping then will vent into the horn’s throat (blue) which lets the air through the resonating chamber, otherwise known as bells, then continue to the atmosphere.

When the pressure at the horn’s throat goes beyond the green compartment’s pressure, the diaphragm then turns back to its “resting position”. This will seal off or will nearly seal the green compartment next to the nozzle once again. This process happens numerous times in a second, having the diaphragm constantly in motion, which causes varied quantity of air to pass through the bell continuously.

Having the diaphragm moving in a constant repetitive pattern, the air waves that passes through the bell is consistent generally. These waves also have frequencies that are low enough to be in the audible range.