Second, the string shape required to match the uniformly moving bow is different.Ī sketch of the reflection of travelling kinks caused by bowingĪ string. As the high frequency components lose energy, the sharp kinks disappear and the shape gradually approaches that of the fundamental mode of vibraiton, which we discuss below.Ī bowed string behaves rather differentlyįirst, it has a continuous source of energy, and so can maintain the same motion indefinitely (or at least until one runs out of bow. )Īs mentioned above, this motion is only observed immediately after the pluck. On this page, however, we'll concentrate on the musical implications. (The motion of waves in strings is described in more detail in Travelling Waves, which has film clips and animations. But the string still has its downwards momentum, and that carries it past the position of rest, and produces a kink on the other side, which then moves back in the other direction. Kink is moving back towards the undisturbed position (down in the sketch).Īs the kink approaches the end, it becomes smaller and, when it reaches the immovable end, there is no kink at all - the string is straightįor an instant. Or tied to a fixed object, the point of reflection didn't actually move.īut look at the motion of the string by comparing the different times Why is the reflection inverted? Well, if we assume that it is clamped Note that, at the reflections, the phase of the kink is changed by 180°:įrom up to down or vice versa. At the instants represented by (e)Īnd (m), the string is straight so it has lost the potential energyĪssociated with pulling it sideways, but it has a maximum kinetic energy. However, the high frequency components of the motion (the sharp bends in the string) quickly disappear – which is why the sound of a guitar note becomes more mellow a second or more after you pluck it.Ī sketch of the reflection of travelling kinks caused by pluckingĪ string. The motion that follows is interesting, but complicated. You pull the string out at one point, then release If you pluck one of the string on a guitar or bass, you areĭoing something similar, although here the string is fixedĪt both ends. However, reflection with any phase change will give a standing wave.) ( The fact that it is inverted gives zero displacement at the end. When a wave encountersĪ boundary with something that won't move or change (or thatĭoesn't change easily), the reflection is inverted. This effect is important not only in string instruments,īut in winds and percussion as well. The left, the kink that travels away from you is to the left,īut that it comes back as a kink to the right - the reflection You'll notice that if you initially pull the string to Next let's have a close look at the reflection at the fixedĮnd. ( Strictly, it is the ratio of tension to mass per unit length that determines speed, as we'll see below.) Light string of the same length under the same tension. It travels more slowly in a thick, heavy string than in a It also depends on the "weight" of the string. Tuning instruments - but we're getting ahead of ourselves. Increases if you stretch it more tightly. It will suddenly tug your hand sidewaysīut, if you are holding it firmly, it will reflect again.įirst you will notice that the speed of the wave in the string That the kink travels down the "string", and then What happens when you pluck a string.) You will probably see Still in one hand, stretch it a little (not too much, a little (Soft rubber is goodįor this, garden hoses are not really flexible enough.)įirst hold or clamp one end and then, holding the other end
Hose you can try a few fun experiments which will make itĮasy to understand how strings work. If you can find a long spring (a toy known as a 'slinky' works well) or several metres of flexible rubber Tightly and vibrate so fast that it is impossible to see what The strings in the violin, piano and so on are stretched For the basic physics of standing waves, see our multimedia tutorial.
The bars and skins of the percussion family. Geometrically, both are less complicated than the vibrations of This is also a useful introduction for studying wind instruments, because vibrating strings are easier to visualise than the vibration of the air in wind instruments. The stable, controlled vibration is usually produced by a standing In electronic instruments this is done with electricĬircuits or with clocks and memories. We also want a frequency that can be easily controlled by the To have a nearly constant frequency: that means stable pitch. If we want to make musical notes, you usually need the vibration How do we make musical sounds? To make a sound, we need something that Introduction: vibrations, strings, pipes, percussion.