Sibelius' String Quartet in D minor, Op. 56 (1908-1909)—his only mature string quartet, and indeed, the only substantial chamber work he produced after the turn of the century—dates from the period between the Third and Fourth symphonies. Like the Fourth Symphony, the quartet is an introspective work; however, as the subtitle, "Intimate Voices," suggests, Sibelius creates a profoundly intimate, even mysterious, atmosphere of a sort that would be impossible to realize in a symphonic work.

The quartet's five movements are thematically interrelated, each movement beginning with a motivic alteration of material from its predecessors. Such connections, however, are primarily superficial, and each of the movements has a fundamentally distinct character. The opening Andante - Allegro molto moderato is brief and enigmatic. Changes in mood can be quite sudden, and the means of presentation are subtle enough that unwary listeners may miss a great deal. The motive of the falling fourth is of particular importance: it appears early in the exposition, assumes a crucial role in a short development section, appears at the climactic moment in the coda, and figures prominently upon its reappearance in the work's finale. The Vivace, barely two minutes in length, opens with a quiet sixteenth note transformation of a motivic idea from the second subject of the first movement. Several unexpected measures of silence precede a volatile climax in which the opening figure is reinstated as the dominant musical idea. The passionate Adagio di molto, with its broad lines and lyrical textures, is also the most economically written of the five movements. As the music slowly unfolds, one is reminded of the spacious designs of the composer's late symphonies. The almost schizophrenic Allegretto ma pesante alternates heavy plodding with a more lighthearted spirit. A subsidiary theme takes as its point of departure the sharp outburst which ended the second movement. The motion heats up towards the end of the movement, but this new force cannot be sustained, and the music resumes the pesante character of the opening. The Allegro finale opens with a frightening flourish of activity and proceeds to the main subject, stated by the viola over bounced-bow effects in the accompanying voices. A syncopated melody follows, and these two main ideas, along with the modification of the falling fourth idea from the first movement, alternate and interact, travelling through various moods and textures and building to a rollicking climax.

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