When an object vibrates, its back-and-forth motion causes the molecules around it to vibrate as well, producing areas of high and low density -- called compressions and rarefactions -- that travel as longitudinal waves through the medium surrounding the object. If the molecules vibrate rapidly enough, you may hear these waves as sound.

The distance between each compression of a sound wave is called its wavelength. The shorter the wavelength, the more waves that pass a given point per second. Sound waves with short wavelengths cause our eardrums to vibrate with greater frequency -- that is, more times per second -- and we perceive these higher-frequency waves as higher-pitched sounds. Sound waves with longer wavelengths have lower frequencies and produce lower-pitched sounds.

In musical instruments such as a trombone or a clarinet, sound originates at the mouthpiece, where air blowing past either the musician's lips or a thin cane strip called a reed starts the vibrations. Blowing through the mouthpiece produces a range of frequencies. When the frequency of those vibrations matches the natural frequency at which the air inside the instrument vibrates, this produces the sound that we associate with the instrument.

The longer the instrument's body, the longer the column of vibrating air inside the instrument. Because air vibrates at a lower frequency over longer distances, longer instruments produce a lower pitch. Shortening the instrument shortens the column of vibrating air, which increases the frequency of the waves and in turn raises the pitch. You can also alter pitch by adding holes along an instrument's body. Leaving a hole uncovered can cause the air in the tube to vibrate as if the tube ended at the hole. The closer the hole is to the mouthpiece, the shorter column of vibrating air and the higher the pitch produced by the instrument.