^{Standing Waves}

In order to make sound, we need a vibration, and for musical notes, we want a vibration that has constant frequency, which means stable pitch. We also want a frequency that can be easily controlled by the player. In order to get stable, controllable frequencies in string instruments, the vibration is controlled by standing waves.

In order to understand the mechanics of traveling waves in strings, we will think of a slinky. Imagine holding both ends of a slinky and having someone else make a kink in the center of the slinky by pulling it away from you and then releasing. It will move or vibrate back and forth until coming back to rest again. Now imagine the slinky being a string on a double bass. The diagram below gives a visual representation of the idea:

                                                         

Plucking or bowing a string sends a series of sine-waves up and down the string. Waves are going in both directions which causes them to interfere with each other and combine, known as superposing. The composite form made from the waves superposing is known as a standing wave. A standing wave is different from a traveling wave because even thought the waves are 'traveling,' by oscillating back and forth, there is no movement of energy up and down the string. Every standing wave has nodes, locations where the two waves cancel giving minimum amplitude, and antinodes, locations where the waves add giving maximum amplitude. In the diagram below, the green and blue waves are sine-waves moving in opposite directions on the string, and the red wave is the standing wave formed from the waves superposing. The diagram shows the process of the waves superposing.

                                           

Photos from the University New South Wales