A gamelan is a South-east Asian musical ensemble mostly made up of metallophones and gongs. Gamelan orchestras are particularly common on the islands of Java and Bali in Indonesia, although they also exist elsewhere in the area, albeit in slightly different forms.
Although gamelan ensembles sometimes include string and, less commonly, wind instruments, they are most notable for the large number of percussion instruments, particularly metal percussion instruments. A gamelan ensemble may include sarons and gendérs (sets of bronze bars laid out in a single row and struck like a glockenspiel), bonangs and kenongs (sets of large, drum-shaped gongs, likewise laid out horizontally on stands), gambangs (similar to sarons or genders but with wooden bars instead of metal ones) and a variety of hanging gongs and drums.
The tuning and construction of a gamelan orchestra is a fairly complicated process. Gamelans use two tuning systems: sléndro and pélog. In Javanese gamelan, sléndro is a system with five notes to the octave, fairly evenly spaced, while pélog has seven notes to the octave, with uneven intervals. Most orchestras will include instruments in each tuning, but each individual instrument will only be able to play notes in one. The exact pitches used differs from ensemble to ensemble, and give each ensemble its own particular flavour.
A peculiarity of gamelan instruments is that they do not usually have perfectly tuned octaves. Almost all other instruments tune octaves in the frequency ratio 2:1 (so that an A is 440 Hz, and the A an octave above it is 880 Hz), but gamelan instruments are usually a little sharp or a little flat. Also instruments are played in pairs which are tuned slightly apart so as to produce beats which are a consistent speed for all pairs of notes in all registers. It is thought that this contributes to the very "busy" and "shimmering" sound of gamelan ensembles.
The gamelan has been an influence on several western composers of classical music, most famously Claude Debussy who heard a Javanese ensemble play at the Paris Exposition of 1889 (World's fair). In more recent times, the American composers Lou Harrison, Evan Ziporyn, and Jody Diamond have written several works with parts for gamelan.