A vibrating object creates sound. The vibrations travel through a medium such as air or water. They cause the ear drum to vibrate creating electrical signals that the nerves transmit to the brain, allowing the hearer to interpret the sound.
The sound of the human voice is created by expelling air through the vocal cords, causing them to vibrate. Musical instruments create sound waves in many ways, but all involve vibrations, whether caused by a reed, string, the player's lips or striking two objects together so both vibrate. Frequencies between 20 and 20,000 vibrations per second are within the normal human range of hearing. Vibrations lower than this range are subsonic and above it are ultrasonic.
Sound travels in waves by subsequent compression and decompression of the molecules of the substance through which it moves. Plucking a string on a guitar causes it to move in one direction to compress molecules and leave a hole in the air. As it vibrates in the opposite direction, it repeats the process creating waves. The diameter of the string, its length and tension all affect the frequency with which it vibrates when plucked. With reed instruments, the reed vibrates at the same speed, but opening and closing holes in the instrument affects the pitch created.