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Beethoven's rarest works re-created online

To do this, they and their collaborators transferred each note by hand from the original scores to a computerised version, manually plugging in values such as pitch and how each note should be played.

At first, they shared the results only with the scattered handful of people in their chat room. But they realised over time that others might be interested and launched the Unheard Beethoven Web site. That's drawn growing interest over the years, with their 500,000th visitor just a few weeks ago, they said.

"Things have mushroomed, with more archives and libraries sharing materials, and with actual musicians performing and recording some of these unheard works," Zimmer said. "So we're pleased to have made a bit of an impact on the musical world."

Indeed, they've approached the top of the professional world at least once. One of their standout pieces has been the reconstruction of fragments of an overture originally intended to be part of an opera based on Shakespeare's Macbeth, which was ultimately performed by the National Symphony Orchestra in Washington, D.C.

Holsbergen initially worked out fragments of the Macbeth Overture in 1999 and posted them on the Web site. A visitor suggested that other elements could be found in other Beethoven sketchbooks, and the Dutch composer ultimately pieced together an eight-minute version from the incomplete works.

It's that kind of reconstruction work that has given the pair a mixed reputation in academic and professional classical music circles. Holsbergen's Macbeth was championed by National Symphony conductor Leonard Slatkin, who premiered the work in 2001.

However, in a harsh critique of the effort, The New York Times labelled it "a sham and a shame", calling instead for new works by living composers.

Patricia Stroh, curator of San Jose State University's Beethoven Center notes that Holsbergen and Zimmer's amateur efforts have historical precedent. The composer's first biographer was an American diplomat, for example.

"With Beethoven studies, there is a long tradition of people outside the musicological world contributing a lot," Stroh said.

Whether they will leave a lasting mark outside enthusiast circles is another question.

"Merely playing previously unplayed works, digitally, is not going to create a significant base for scholarly advance," said University of Manchester professor Barry Cooper, a Beethoven scholar who has reconstructed an unfinished Tenth Symphony. "Its main advantage for scholars may be in drawing attention to works they might previously have overlooked."

Scholars aren't their audience, however. They're hoping to reach the mainstream in a way academicians and virtuosos can't.

"The idea is to promote listeners getting familiar with unfamiliar music," Zimmer said. "Scholars have had access to this stuff for well over a hundred years, and haven't done anything with it."