Sound is, at its most basic level, a vibration through a gas, a solid or a liquid. Using a system known as the Hornbostel-Sachs classification of musical instruments, all of the different musical instruments in existence can be classified under one of five different headings depending on how they cause a state of matter to vibrate.
Aerophones produce sound by vibrating columns of air within the instruments themselves. These types of instruments are often called "wind instruments," which include brass and woodwind instruments. This category includes many types of instruments including flutes, reed instruments -- like the clarinet and the saxophone -- and lip-vibrated instruments like the trumpet and the tuba.
Chordophones, also known as "stringed instruments," are musical instruments that produce sound by means of vibrating strings. Chordophones can be broken down into six basic subcategories: neckless chordophones such as zithers; keyboard chordophones like the piano; lutes such as guitars, mandolins and sitars; lyres, which are instruments that have strings attached to a crossbar supported by two arms; harps; and bowed chordophones like violin and the cello.
Idiophones produce sound when the instrument is struck, shaken or rubbed. These instruments are made out of sonorous materials like bronze, certain types of wood or special metal alloys. Cymbals, xylophones, bells and rattles are all common idiophones.
Instruments that create sound when a stretched membrane is struck are known as membranophones. The bass drum, tom-toms and snare drums that are associated with the modern drum set are all membranophones, as are kettledrums, bongos and congas. Kazoos are also technically membranophones because the air moved by the player's voice strikes a wax membrane stretched across the hole in the end of the instrument, thus creating the kazoo's characteristic buzzing tone.
Musical instruments that create sound through the use of electronic circuits are known as electrophones. This is the newest of the five major categories, as electrophones could not be produced before people harnessed the power of electricity. Instruments like the synthesizer and the theremin are electrophones; instruments that require electronic amplification are sometimes thought of as electrophones as well, but they are generally classified according to the actual sound creation method as opposed to the amplification method; an electric guitar is therefore still considered a chordophone, for example.
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