How the alphorn works and acoustic principles
The alphorn is a traditional wind instrument made of wood, often measuring between 3 and 4 metres in length. With no pistons or keys, it relies solely on the series of natural harmonics to produce different notes. Its operation is a fine example of the acoustic laws that govern wind instruments.
When you blow into an alphorn, you cause the air in the tube to vibrate. These vibrations travel very quickly through the air (at around 343 m/s). When they reach the end of the tube, they are partially reflected back. The back-and-forth movement of these vibrations creates what is known as a standing wave.
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The natural harmonic
When a musician blows into the alphorn, they excite a column of air inside the tube. This column vibrates according to a fundamental principle of acoustic physics: the formation of standing waves. Starting from a fundamental note (the lowest note that can be obtained), the instrument then produces higher-pitched sounds that are natural harmonics: these notes correspond to multiples of the fundamental frequency (2x, 3x, 4x, etc.).
Thus, without a mechanism for changing pitch as on modern horns, the musician can play a whole series of notes simply by changing the tension of the lips (the embouchure) and the air pressure. This phenomenon is made possible by the very nature of the pipe and the way in which standing waves interact within it.
The standing wave in a sound tube
In a pipe closed at one end (as is the case with the alphorn, where the mouthpiece forms a point of maximum pressure), sound propagates in the form of longitudinal waves. When a sound wave encounters the end of the pipe (the bell), it is partially reflected, interfering with the incident wave. This interference between the incident wave and the reflected wave creates what is known as a standing wave.
The standing wave does not appear to move: certain points, called nodes, remain stationary, while others, called antinodes, undergo maximum pressure variations. The number of nodes and antinodes in the pipe depends on the frequency produced. The higher the frequency, the more nodes and antinodes there are: this is when we enter the higher harmonics.
