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A prepared piano is a piano that has had its sound altered by placing objects (preparations) between or on the strings or on the hammers or dampers.
Background information 
Richard Bunger wrote a book, The Well-Prepared Piano, in which he explains how John Cage prepared his pianos and even which pianos are suitable, because of the deviation of string lengths within different brands. Bunger also clarifies why the preparations were done in such ways; in other words, which adaptation creates which sounds (harmonics obtained, timbrel effects, etc.). The timbre of the instrument changes dramatically when preparations are introduced. Much of the technique is related to the harmonic positions of the strings. For instance a preparation on 1/2 of the string length causes a different sound than on 1/3. In other words, the preparations don't cause a random sound as often assumed.
John Cage coined the term prepared piano and was undoubtedly the composer who made the technique famous. He credited Henry Cowell and, to a lesser extent, Erik Satie, for contributing to the idea, but it is unclear if Cage was aware of many other precedents described below.
Since the later days of the harpsichord (17th–18th century), stringed keyboard instruments could have registers, for instance giving a drier or more ample sound when the instrument's stop was pulled (a stop in the meaning of a similar disposition for organs, known as organ stops).
When the first pianos were invented around the beginning of the 18th century, the only "coloring" of the sounds produced by the instrument resulted from how the individual keys were pressed (loud = forte, or softly = piano, giving the name to the instrument: fortepiano). A type of register, first implemented with a stop above the keyboard, which became a standard device for pianos in the second half of the 18th century, was engaging or disengaging the muting of the strings after the release of a key. Only by the end of the 18th century, the muting mechanism was triggered with a pedal, after an intermediate period when this register was operated by the pianist's knees.
Reed stop 
But the idea of harpsichord-like registers lived on: in the early 19th century some pianos were provided with a reed stop, which lowered a strip of paper onto the strings. This led musicologists such as Tom Beghin to believe that the technique of placing a strip of paper on piano strings would probably have originated before it was standardised as a register operated with stops, and that, for instance, Mozart's Alla Turca can safely be played with a piece of paper on some of the strings as a historical interpretation.
Turkish stop 
Around the turn of the nineteenth century, Turkish music was so popular that piano manufacturers made special pianos with a Turkish stop, also called the military or Janissary stop. The player would press a pedal that caused a bell to ring and/or a padded hammer to strike the soundboard in imitation of a bass drum. The Turkish stop was popular for playing the famous Mozart Rondo alla Turca, K. 331.
Tack piano 
The phrase prepared piano is also sometimes applied to other kinds of preparations. The tack piano is a piano that has been altered by inserting thumbtacks or small nails into the striking end of each hammer, so that the instrument will produce a more percussive sound and brighter timbre. The resulting tone often resembles the sound of a very old and derelict piano. The tack piano has been used primarily in honky-tonk-style piano playing, or to make a piano sound like an antique piano that might have been heard in a saloon or brothel around the early 20th century. The application of tacks is generally discouraged by piano technicians as the tacks can drop off the hammers and lodge in the strings or jam the mechanism, or the fact that placing tacks inside felt hammers renders the felt unvoicable and, therefore, ruins the hammers. On normal pianos, felt coverings on the hammers will harden and compress with use (though not usually for at least several decades, unless it is a heavily used concert piano), yielding a characteristic bright, tinny sound. This can be cured by softening the hammers with a device consisting of multiple needles called a "voicing needle". Where the felt is too far gone, the hammers can be replaced.
Satie's Piège de Méduse 
In the piano version of his Piège de Méduse (1913 or 1914) Erik Satie's score called for placing sheets of paper on the piano strings in order to imitate the mechanical sound of a monkey puppet that figured in the play.
Villa-Lobos's Choros no. 8 
In his 1925 work for two pianos and large orchestra, Heitor Villa-Lobos added to his score instructions to the pianist to insert pieces of paper between the strings and the hammers to attain a certain sonority.
In the 1920s, a new invention was presented, the Luthéal, which extended the register possibilities of a piano to its maximum, producing cimbalon-like sounds in some registers, exploiting harmonics of the strings when pulling other register-stops, and also some registers making other objects, which were lowered just above the strings, resound. But that instrument became obsolete before it became popular, partly due to most of the mechanics of the instrument being too sensitive, needing constant adjustment. The only pieces in the general repertoire to feature the Luthéal are L'enfant et les sortilèges (1920–5) and Tzigane (1924) by Maurice Ravel, performances of which tend to substitute an upright piano, either prepared with paper or straight.
Another precedent to the prepared piano was an experiment by the French composer Maurice Delage (1879–1961): his Ragamalika (1912–22), based on the classical music of India, calls for a piece of cardboard to be placed under the B-flat in the second line of the bass clef to dampen the sound, imitating the sound of an Indian drum.
String piano 
The Acoustisizer is an electro acoustic musical instrument built from a grand piano utilizing magnetic guitar pickups, speakers and prepared piano strings to generate feedback sound noise effects. Built as part of a graduate project thesis at California State University Dominguez Hills by Bob Fenger (1983) a student of Richard Bunger author of the Well Prepared Piano. Speakers are built into the bottom of the instrument redirecting its own amplified sound back onto the sounding board, strings and magnetic pickups creating an amplitude intensity loop which in turn drives and vibrates suspended kinetic oscillators (assemblages of vibration sensitive materials). Secondary control parameters allow extraction of vibration and sound phenomenon from the kinetic oscillators through a series of proximity microphones and PZMs (piezo electric contact mics). An extensive article written by the inventor was published in Experimental Musical Instruments Magazine April 1991 in Nicasio California 94946. It includes pictures of the kinetic oscillators and stages of the construction process including an underbody view of the speaker configuration. See also Public Prepared Piano at Street piano 3.0 (Public Piano Project in Joshua Tree CA).  
John Cage and later composers 
Cage first prepared a piano when he was commissioned to write music for "Bacchanale", a dance by Syvilla Fort in 1938. For some time previously, Cage had been writing exclusively for a percussion ensemble, but the hall where Fort’s dance was to be staged had no room for a percussion group. The only instrument available was a single grand piano. After some consideration, Cage said that he realized it was possible “to place in the hands of a single pianist the equivalent of an entire percussion orchestra ... With just one musician, you can really do an unlimited number of things on the inside of the piano if you have at your disposal an exploded keyboard” (Cage and Charles, 38).
More recent composers to use prepared pianos include John Wolf Brennan, Roberto Carnevale, Philip Corner, Stephen Scott, Richard D. James, Jason Moran, Erdem Helvacıoğlu, Marina Leonardi, Hiromi Uehara, and Volker Bertelmann.
In popular music 
- "All Tomorrow's Parties" from The Velvet Underground & Nico (1967), John Cale prepared his piano with a chain of paper clips.
- Denman Maroney performs on what he has dubbed hyperpiano, which "involves stopping, sliding, bowing, plucking, striking and strumming the strings with copper bars, aluminum bowls, rubber blocks, plastic boxes and other household objects."
- The Noah and the Whale song "Our Window" makes use of a piano prepared with "screws, ping pong balls and tooth picks."
- Tori Amos song "Bells for Her" has her play a prepared piano.
See also 
- Anna Stella Schic (1987). Villa-Lobos, Souvenirs de l'Indien Blanc, Actes Sud, p. 82.
- Pasler, Jann (2000). "Race, Orientalism, and Distinction in the Wake of the 'Yellow Peril'." In Western Music and Its Others: Difference, Representation, and Appropriation in Music, ed. Georgina Born and David Hesmondhalgh. Berkeley, Los Angeles, and London: University of California Press, p. 107.
- Connell, Rich. "Inventor Creates New Sound of Music" Los Angeles Times Los Angeles, 21 November 1982.
- Bronson, James. "Acoustisizer creator's world is music"  Palos Verdes CA, 2 December 1982.
- Woods, Randy. "New Instrument Offers Sounds of Star Wars"  Los Angeles, 2 December 1982.
- Takahama, Valerie. "High-tech piano creates a new sound" Long Beach Press TelegramLong Beach, 2 December 1982.
- Bronson, James. "Acoustisizer creator is scaling new heights" Daily BreezeSouth Bay LA, 17 December 1982.
- Staff writer. "But Does He Burn It After Each Performance?" Keyboard Magazine Cupertino CA, February 1983.
- Fenger, Bob. "The Acoustisizer" Experimental Musical Instruments Nicassio CA, Volume VI #6 April 1991.
- Gast, Dena M. "Music sends environmental message"  Joshua Tree CA, 26 May 1995
- Mitchell, Tim Sedition and Alchemy : A Biography of John Cale, 2003, ISBN 0-7206-1132-6
- "Philadelphia FRINGE Festival 2000 - Hyperpiano". Archived from the original on 2007-12-31. Retrieved 2005-12-23..
Further reading 
- Bunger, Richard (1973). The Well-Prepared Piano. Colorado Springs: Colorado College Music Press
- Fürst-Heidtmann, Monika (1979). Das präparierte Klavier des John Cage. Gustav Bose Verlag Regensburg. ISBN 978-3-7649-2183-5.
- Cage, John, and Daniel Charles (1981). For The Birds: John Cage in Conversation with Daniel Charles. Marion Boyers London. ISBN 0-7145-2690-8.
- Dianova, Tzenka (2008). John Cage's Prepared Piano: The Nuts & Bolts. Mutasis Books Victoria. ISBN 978-0-9809657-0-4.
- 'Are You Prepared' 17-key Online Prepared Piano by Andreas Busk-Jepsen. Site includes free downloadable prepared piano sampler for Ableton Live, NI's Kontakt and Logic's EXS Sampler
- The Sound Collector - The Prepared Piano of John Cage by Tim Ovens (PDF version).
- If you build it, they will come! essay by Kyle Gann, includes video performance of preparation by Margaret Leng Tan (here).
- Prepared Piano Sample Set - By Tom Gersic. Some free, others cheap
- Prepared Piano Max/MSP-Object - By Dr. Stefan Bilbao, ported to Max/MSP by Thomas Resch
- Jingle Bells on Prepared Piano - By George Aroshidze
- Prepared piano demonstration and performance by Richard Bunger
- Epitonic.com: John Cage performed by Margaret Leng Tan, featuring In the Name of the Holocaust