Enter the sound world of Quatuor Mosaïques and check your expectations at the door. They perform on what we call period instruments: gut strings (rather than steel), classical bows (shorter than their modern counterparts), and less tension in the instruments. The resulting sound may be familiar when it comes from a larger ensemble playing, say, Bach's Brandenburg Concertos, but in a quartet, playing later repertoire, the experience is a bit through-the-looking-glass.

The lighter touch required for period instruments enforces a transparent texture throughout. If the sound of a quartet played on modern instruments comes rushing out to you, Quatuor Mosaïques draws you in. The dynamic range is more restricted than with modern instruments, and the balance between the voices is clearer. Because the players cannot dig into the strings, they must create articulation, phrasing, and accents through subleties of timing and color. As a result, even well-known pieces become revelatory experiences in their hands. For once, a string quartet does not sound like a concerto for first violin. Pianissimos are not tentative, but clear, and cantabile passages are phrased with consummate ease rather than fussy exaggeration.

The members of Quatuor Mosaïques met while playing in Nikolaus Harnoncourt's Concentus Musicus Wien, a pioneering ensemble of the period instrument movement. But the quartet's style is informed not just by academic research, but also by links to the vital performing traditions of the last century. Lead violinist Erich Höbarth was concertmaster of the Vienna Symphony and a student of the legendary Sándor Végh, who studied with Kodály and worked with Bartók and Casals. The quartet wisely eschews an unattainable ideal of authenticity; instead, they use period instruments as a means for approaching the music afresh. This approach was beautifully evident in the diverse palette of timbres and articulations they produced for a program of Haydn, Mendelssohn, and Schubert.

The concert began with Haydn's Quartet Op. 76 no. 4, nicknamed the "Sunrise" quartet for its opening motif. Haydn usually occupies a thankless role on the typical string quartet concert, the pleasant throwaway before we get to the heavyweights. But in the hands of the Mosaïques, who have specialized in Haydn ever since their first recordings in the 1980s, all the composer's humor and spontaneity were restored. Haydn's inventive harmonies were vividly realized, and fleet lines were tossed off effortlessly, without the gritty tension of some modern-instrument performances. During the rousing finale, the quartet played with gusto, bringing out the rustic elements of Haydn's style.

Mendelssohn's String Quartet in A Minor presented an entirely different sonic landscape. The articulation and phrasing had all the sing and sweep that modern instruments can give, but the lighter texture of the sound meant that the intricate inner voices were actually audible, with constantly changing color and timbre. The opening adagio was lovely and spacious, and the allegro was poised even when agitated. During a repeated sighing motif in the first movement, the players used expressive intonation and a applied a measured dose of vibrato to linger on suspensions. Vibrato throughout the performance was used thoughtfully: it was neither a constant element to the sound nor completely forbidden, but a frequent expressive choice.

Only in the Mendelssohn was the trade-off of using early instruments apparent. One missed the heft that comes from steel strings, and the sound in louder passages was at times relentlessly bright. Still, even during dense textures, the voices were balanced and the harmonies clear.

The Mosaïques' performance of Schubert's "Rosamunde" Quartet was especially eloquent. The accompaniment motif that opens the first movement, based on Schubert's song "Gretchen am Spinnrade," was equally balanced with the sweeping tune. Light touches of portamenti and the slightest hints of rubato endowed the melodies with a quasi-vocal quality, without lapsing into mannerism. In the tuneful second movement, the ensemble used very little vibrato and simple phrasing, letting the charming melody speak for itself. The perfect, restrained Minuetto showcased the amber tones of the middle range of the inner voices, and the final movement had a triumphant swing to it. For an encore, the quartet played one of Schubert's Deutsche Tänze, D.90.

In short, if you attend a Quatuor Mosaïques concert expecting to be blown away by the immense sound that four instruments can produce, you will be disappointed. But if you seek a subtler, deeper understanding of the music, you will have an experience that makes you wish to listen over and over again.