About the sheet music:

Matt Springer's mallet percussion ensemble arrangement of Aram Khachaturian's music, "Four Pieces from the Gayane Ballet Suite", was originally published by C.P.P./Belwin (later Warner Bros.) in 1994, and was re-released in 2005 as a Khach-22 production.  These four movements from the Gayane Ballet Suite are very well suited for this mallet percussion ensemble instrumentation, consisting of four players using glockenspiel, xylophone, vibraphone, and two 4-octave (C-C) marimbas (the music is written such that the two marimba players can use one marimba if necessary).  It is rated for advanced high-school players and up.

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More details, including program notes, reviews, history, and streaming audio of each movement

For non-percussionists:  What are these instruments?