The physics behind musical instruments


One of the best museums I visited last year was the Haus der Musik – Sound Museum in Vienna. It is an amazing place where you can among other things compose your own music and try to conduct the famous Vienna Philharmonic Orchestra. But there was one part of the exhibition that I enjoyed even more, the one about instruments. By standing in a giant mouth piece for wind instruments or a giant hollow percussion body you could experience first hand how music is created. And yes, it has everything to do with physics and science.

Sound is created when an object vibrates. The vibrations cause the particles in the air around the object to vibrate too. The particles in the air then bump into their neighbors setting them into a vibrating motion as well which lets the vibration travel further. For this reason, sound is often defined as vibrations that travel through air and are detected by a human’s or animal’s ear. But it should be said here that the medium does not necessarily have to be air. Whale ears, for example, pick up vibrations that are transported through water. In physics, vibrations are commonly described as waves.

Special about music instruments is that they create so-called standing waves instead of random vibrations. In a standing wave some points, the nodes, of the vibration remain fixed while the rest vibrates with maximum amplitude which refers to the highest and the lowest points of the wave. It is these standing waves that we experience as harmonic tones when listening to music. Other irregular, random waves that are not standing, we hear as noise instead.

The properties of the vibration’s standing wave can tell us how we experience the tone. One important property of waves in physics is their frequency which describes how many waves pass one point during a specific time. The larger the frequency, the more waves pass the point and the more high-pitched we experience the tone. In contrast, the smaller the frequency, the less waves pass and the lower the tone sounds. The amplitude (remember the highest and lowest points of the wave), on the other hand, tells us how loud a tone is. The larger the amplitude, the higher the wave and the louder the tone sounds, whereas a smaller amplitude gives a softer tone.

Musical intruments can create vibrations in three different ways as explained in the infographic above. There are persussions/drums, wind instruments and string instruments. Percussions, no matter if bell or drum, have a hollow body. When being hit with sticks, hands or something else this hollow body starts vibrating and creates the tone we hear.

Wind instruments include brass instruments, such as trumpets and saxophones, as well as woodwind instruments, such as flutes. They all have a mouth piece and a hollow tube. When the player blows into the mouth piece, an air column inside the tube is set into a vibration motion and a tone can be heard.

String instruments work through the vibration their tensioned strings. The vibrating motion can be started by different methods. The strings can be plucked, like for guitars or harps, bowed, like for violins or cellos, and hit, like for pianos.



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