Music is never so simple as to produce sounds that can be strictly defined by one specific wavelength. When the piano tuner strikes his tuning fork, the result may be fairly close to emitting sound at a simple frequency. But when the key of the piano is hit, the resulting sound may actually contain several frequencies. Then, if the sounds of different instruments are compared, you will notice the oboe can be distinguished from the trumpet, which does not resemble the banjo even though each musician might be playing the same note. The reason for this phenomena is called timbre, whereabouts each instrument actually emits a complex wave containing more than one frequency.
The musical term timbre does not imply a changing of the pitch, just a mixture of various sound frequencies emitted when a certain sound is played. Most musical instruments are designed so when the note is played, the resulting sound is nearly identical. Moreover, there are ways the music sound can be played, where the pitch is intentionally changed during intonation. This technique is particularly noticeable in the human voice, which can be made to fluctuate rapidly during singing in a technique called vibrato.
Time is also part of music, for there are short musical notes and long musical notes. Most musicians and singers can control the amount of time they hold a note. The sound at the beginning of the musical expression will be close to the sound made at the end, regardless if the note lasts for less than a second or continues for a quarter of a minute. In general, long notes provide better opportunity to manipulate the frequency range of the musical sound. This manipulation can come in the form of wavering fluctuations or the singer might decide to increase or decrease the pitch in only one direction.
At the website, "Voice Teacher," David Jones describe vibrato as the "slight variation of pitch resulting from the free oscillation of the vocal cords." Obviously, Mr. Jones is referring to singers, but it is also possible to mechanically produce vibrato sounds by altering the way a musical sound is delivered. For example, a blues guitar player may stretch the string while the note is sounding to change the pitch or he may move the same string multiple times in a short period time to create an oscillation in pitch. This should not be confused with tremolo, which indicates a wavering in the volume rather than pitch.