When you play two notes at the same time on a violin, a mysterious third note is heard. This phenomenon has been known for hundreds of years, but now we finally know why it happens.

It has long been thought that a certain unusual tone emanating from a violin can only be heard in our heads. This would be due to an abnormality in our ear canal. Now the phenomenon appears to be real. And also: the higher the quality of the violin, the better you can hear this sound.

In 1714, the Italian violinist Giuseppe Tartini discovered it: when he played two notes at the same time, he heard an unexpected third sound. This third note was later called the combination tone because its frequency is a mixture of the original tones. Often this frequency is the difference between the two tones.

For a long time it was believed that these combined tones originated entirely in our ears. Our cochlea, part of the inner ear, amplifies sound in a certain way before the sound waves reach our brain.

double notes

Physiologist giovanni cecchi from the University of Florence in Italy wanted to investigate with his colleagues whether and how different violins produce combined tones. To do this, they got a professional violinist to play a series of fixed double notes on five violins of different ages and qualities, after which the researchers analyzed the sound recordings with a computer.

The 19th century physicist Hermann von Helmholtz already showed that some instruments can produce combined tones on their own. Inspired by his work, Cecchi and his colleagues divided sound waves of different frequencies into groups. In doing so, they saw that all the violins actually produced combined tones.

the older the better

The oldest violins produced the strongest combination tones in this study. The oldest violin, made in Bologna in 1710, produced a combined pitch 75 percent higher than that of a modern factory instrument.

The researchers also wanted to know how well listeners could hear the blended tones. For this they used the three violins of the highest quality. They then invited a group of eleven professional and amateur musicians to listen to recordings of the violinist playing the double notes. Some recordings had the combined tones clipped out.

Listeners could tell the difference almost every time: the least accurate subject heard the combined tone 93 percent of the time, the most accurate heard it every time.

instrument researcher jim woodhouse from the University of Cambridge in the UK says this experiment indicates that the blended tones come from the way air vibrates and mixes within the violin. When you play a note on a violin, it’s not just the string that determines whether the sound is correct. The shape of the violin is also essential. With a high-quality instrument, it is manufactured with much more precision.