Different objects, when hit, plucked, strummed, or in some other way set in motion, will vibrate. This vibration casts out a sound wave at the object's own natural frequency. The human ear can interpret those sound waves in the frequency range between 20Hz to 20kHz. These differences in natural frequency are why, say, a piano and a wind chime both sounding an "F" note, will sound different even though they are creating the same note.
Often two similar-looking acoustic guitars can sound very different because of the materials from which they are constructed. This holds true for all instruments, but in this section the guitar will be used as an example. The reason two similar-looking guitars can produce different sounding tones is because of the wood, and the respective densities of those woods, from which they are constructed. Denser woods like ebony influence sounds by absorbing less of the sound wave, leaving more of the string sound audible. Less dense woods, like basswood, absorb certain higher-level frequencies to leave warmer, more bass-responsive tones.
Recording studios have sound rooms with baffling in them to deaden sound and prevent the influence of the size of the room and the wall materials from influencing the sound recording. This is because the reverberation from an environment can alter the natural frequency of sound waves. One can easily hear the difference between a sound made in a large, high-ceiling room, like a church, where the reverberation of the frequency combined with the natural tone alters the tone, and a sound made outside, where no reverberation occurs, leaving more of the natural note to be heard.
Some composers, recognizing the way different elements can alter normal sound vibrations and affect tone, experiment with ways to create new and different compositions. Perhaps the most well-known composer of this movement was John Cage, who used to "prepare" his piano. He would do this by putting an assortment of objects on the strings of his piano. These objects included nuts, bolts and pieces of rubber, among other things. When the hammer of the piano hit the strings, these items altered the tones listeners expected to hear from pianos. Employment of this technique meant that the same songs would render different tonal results with each performance.