Pitches and Tones of Train Horns

This time, let us discuss about the sound that are produced by the horn and the reason why it sounds that way. The primary feature of a horn that affects the sound is the orifice and its size. The SuperTyfon from Leslie comes with a variable-orifice and a fixed-orifice design.

Larger bells have larger orifices as well. Having a large inlet allows larger volume of air to be fed into the chamber, thus the louder the sound of the horn. It is my belief that this is caused by the movement of the diaphragm further each time it oscillates, creating higher amplitude sound waves.

Secondly, the horn’s tone varies vastly on the diaphragm’s way of oscillation. Having the diaphragm constantly oscillating without resting back touching the nozzle in each cycle, the output waves are more sinusoidal comparing it to the one were the clapper touches the nozzle in a cycle. A mellower sound and not much harmonics will be produced from those horns that have diaphragms that do not rest on the nozzle in one cycle, an example of which is the M horn series.

If the diaphragm rests in each cycle touching the nozzle, a brassy sound is more likely produced. An example of which is the P horn series. It is also noticeable that the kind of material used to construct the horn and the thickness of the bell largely affects the tone of the bell.

A richer tone is produced from those with thicker and heavier castings. This might be caused by the fact that the horns have a larger metal mass that helps in the resonating of sound. This will help clarify the difference of tone with Prime and Leslie horns, which is almost indistinguishable.

Lastly, the kind of nozzle and diaphragm also facilitates in determining the output sound of the horns. An example of this is the stainless disk diaphragm and rubber nozzle of the P series horns. In the 1970s, they tried changing the stock diaphragm disks made of stainless steel into phosphor bronze and created a mellower tone. The sound of the resulting horn was closer to the M series, which used phosphor bronze nozzle, clappers and disk diaphragms. It should be remembered that the other features have greater impact to the horn’s sound, even if the material used do have effects.

As a final point, the most noticeable factor that determines the sound created by the horn, which is the bell, will be discussed. It would be based on my experience though and not from scientific basis because it is beyond my knowledge. The shape and length of the bells can changed the emitted sound waves from the “power chamber”. This is the cause for the changes of the audible sound. The things farther beyond the bell’s physics are far beyond me.

The horn bells come in various shapes and sizes. The pitch of the horn is determined by the bell’s length. With all the factors being constant, having a longer bell will produce a lower pitch. The inner diameter of the throat and the diameter of the bell also have some effects. Making the diameter of the throat bigger will yield to a lower pitch. An example of this effects are the P5, with its narrow-throated and long bells, produces a chord similar to that of the M5 horn, which has a wide-throat and short bells.

The throats shape and the opening speed also helps in determining the tone as well as the volume of the horn. A flare with a large flare opening tends to create a louder sound. Perhaps this is because the flare can project sound waves much better when it has a bigger opening.