Musical sounds can be thought of as having three defining characteristics: pitch (or lack of it), loudness and timbre.
If a sound source vibrates in a regular manner, it produces a pressure wave which is made up of a periodically repeating pattern of compressions and rarefactions. This is interpreted by the human ear as a note of definite musical pitch. The pitch of the note - that is, how low (bass) or high (treble) the note sounds – is determined by the frequency, or number of times per second, at which the pattern of compressions and rarefactions repeats. Examples of instruments which produce pitched notes include the violin, flute and piano. These instruments tend to be used to play melodies or tunes.
Notes without a definite sense of pitch are produced when a sound source vibrates in a random manner, producing an irregular and chaotic pressure wave. Such notes tend to be produced by percussion instruments like the snare drum, the cymbals and the maracas. Unpitched notes can still be considered to be musical sounds – they tend to be used to emphasise the rhythmical structure in a piece of music.
The loudness of a sound is largely determined by the size, or amplitude, of the vibrations of the source producing it. However, it also depends on the pitch of the sound, with the human ear being less sensitive both at very low frequencies and at very high frequencies. In a musical passage, variations in loudness are used to add extra interest for the listener.
The timbre of a musical note is the quality or character of the note. It is the timbre that allows the human ear to distinguish between sounds which have the same pitch and loudness. Differing timbres ensure that we are able to distinguish between notes produced by, say, the flute and the viola.
Most sound sources vibrate at several frequencies simultaneously. The additional frequencies present in the sound wave produced by such a source are called overtones or harmonics. The relative strength of these harmonics plays a large part in determining the timbre of the sound. However, it is not the only factor. For example, time-varying aspects of the sound are also important. In particular, the nature of the attack or onset of a note plays a vital role in defining its timbre.