About his composition Debussy wrote:
The music of this prelude is a very free illustration of Mallarmé's beautiful poem. By no means does it claim to be a synthesis of it. Rather there is a succession of scenes through which pass the desires and dreams of the faun in the heat of the afternoon. Then, tired of pursuing the timorous flight of nymphs and naiads, he succumbs to intoxicating sleep, in which he can finally realize his dreams of possession in universal Nature.
The work is scored for three flutes, two oboes, english horns, two clarinets, two bassoons, four french horns, two harps, two crotales and strings. Notable is the absence of trumpets, trombones, and timpani.
Although it is tempting to call this piece a tone poem, there is very little musical literalism in the piece; instead, the languorous melody and shimmering orchestration as a whole evoke the eroticism of Mallarmé's poem.
[This prelude] was [Debussy's] musical response to the poem of Stephane Mallarmé' (1842-1898), in which a faun playing his pan-pipes alone in the woods becomes aroused by passing nymphs and naiads, pursues them unsuccessfully, then wearily abandons himself to a sleep filled with visions. Though called a "prelude," the work is nevertheless complete – an evocation of the feelings of the poem as a whole.
The work is called a prelude because Debussy intended to write a suite of three movements – Prelude, Interlude, and Final Paraphrase – but the later two were never composed.
The Prélude at first listening seems improvisational and almost free-form; however, closer observation will demonstrate that the piece consists of a complex organization of musical cells, motifs carefully developed and traded between members of the orchestra. A close analysis of the piece yields a deep appreciation of the ultimate compositional economy of Debussy's craft.
The main musical themes are introduced by woodwinds, with delicate but harmonically advanced underpinnings of muted horns, strings and harp. Recurring tools in Debussy's compositional arsenal make appearances in this piece: Bracing whole-tone scale runs, harmonic fluidity without lengthy modulations between central keys, tritones in both melody and harmony. The development of the slow main theme moves fluidly between 9/8, 6/8 and 12/8 meters. Debussy explores voicings and shading in his orchestration brilliantly, allowing the main melodic cell to move from solo flute to oboe, back to solo flute, then two unison flutes (yielding a completely different atmosphere to the melody), then clarinet, etc. Even the accompaniment explores alternate voicings; the flute duo's soaring, exotic melodic cells ride lush rolling strings with violas carrying the soprano part over alto violins (the tone of a viola in its upper register being especially sumptuous). And, in the first minute of the piece, Debussy mischievously throws in a bar of complete silence, giving the listener the opportunity to explore the musical quality of negative space within a gentle flowing river of sound.