109 Players. 3 Conductors. It’s Even Harder Than It Sounds.

LONDON — Calling for no fewer than three orchestras and a total of 109 players, Karlheinz Stockhausen’s “Gruppen” is one of the 20th century’s most monumental pieces of music. Yet at a rehearsal this week for a Saturday performance at Tate Modern in London, there were only three musicians in the room: the conductors Simon Rattle, Matthias Pintscher and Duncan Ward.

“Gruppen” (1955-57) is a challenge for conductors. In a standard orchestra piece, there is one maestro on the podium, leading all the players. But in “Gruppen,” each orchestra plays at a different tempo, and they can easily fall out of sync. So three days before the Tate concert by players of the London Symphony Orchestra, the three conductors met to coordinate their approaches.

Pencils in hand, they laid out their scores on a table and mimicked their orchestras’ parts in a cacophony of hummed notes, whoops, grunts, bleats and birdlike sounds — and every once in a while, in unison, a triumphant “Bang!”

“The minute we’re in there, it will be such a shock for everybody!” said Rattle, the London Symphony Orchestra’s music director, referring to Tate Modern’s postindustrial Turbine Hall, where the three orchestras will surround a standing audience.

Rattle described “Gruppen,” which means “groups” in German, as “irritatingly complex.”

Stockhausen left detailed guidelines. In the score, he specified the exact number of rehearsal hours that players had put in before the first performance in 1958. There were six conductors-only rehearsals of two hours each.

By Stockhausen’s standards, not a lot of preparation has gone into the Tate concert: one conductors-only rehearsal, one day of separate rehearsals by each conductor and ensemble in venues across London, and one day of combined rehearsals at Tate Modern.

Not that the piece will ever be easy, Rattle said.

“Each orchestra is very complicated on its own, but when you put the three together, it’s like this dizzying tessellation of rhythms and sounds,” Rattle said on at the second rehearsal day, as he practiced with his ensemble at St. Luke’s, a music education center in a disused 18th-century church. It meant “coordinating all these desperately complicated rhythms and cues, and not losing your head, and being able to see each other from a distance.”

Rattle — who just ended a 16-year stint as artistic director of the Berlin Philharmonic — said he got the idea for the “Gruppen” performance on a visit to Tate Modern with his artist son about five years ago, when they both stood on a footbridge overlooking the Turbine Hall. He said the space, which regularly hosts large site-specific art installations, was “so glorious, it demands that you do as so many artists have done: that you do something extraordinary there.”

His hope was to lure crowds to new music the way Tate Modern had drawn them to contemporary art. “People very happily go to the most avant-garde art exhibitions and get what they can out of it,” he said. “This type of really difficult contemporary music is something that you actually have to seduce people to come to.”

Across town at a folk art center in North London, Ward rehearsed with his orchestra. The London Symphony Orchestra’s players come highly prepared and are used to rehearsals that proceed “at breakneck speed,” he said in an interview afterward. And he and his fellow conductors have all performed the piece before: “We know the problem spots, and we know how to fix them.”

Two of the three conductors said they had met Stockhausen, who died in 2007 at age 79.

The composer attended a “Gruppen” performance by Rattle and wrote him a letter — which Rattle described as “incredibly generous” — approving his decision to expand the piece’s tempos to make them more expressive. He also said it was necessary for the conductor to tune all the tom-tom drums together with the instruments’ players at least an hour before the performance, and asked if Rattle had done so.

“Of course, no, I didn’t, and he could hear that,” Rattle said. “He knew exactly what he wanted.” By contrast, Pintscher’s experience of meeting Stockhausen was “not the happiest of encounters,” he recalled after his orchestra’s rehearsal in South London. Pintscher is the music director of Ensemble Intercontemporain — a contemporary music group founded by Pierre Boulez, who conducted one of the orchestras for the first performance of “Gruppen.” He recalled meeting Stockhausen at a performance of one of his own compositions.

“In his old age, he was very maestro, he was very guru, he was very much in his own world,” Pintscher said. “I was introduced to him as a young man, and he couldn’t care less.”

He described Stockhausen as an “absolutely tremendous” figure in the history of music, and added, “I don’t need to have liked him, or need him to have liked me.”

As the three conductors prepared to rehearse the next day with all 109 players at Tate Modern, there was some anxiety in the air.

“None of us knows what it’s going to be like when we’re in the Tate and we’re far away,” Rattle said. “We won’t necessarily hear what’s coming from the other orchestras.”

While “Gruppen” might sound chaotic and free, he noted, it was “almost fascistically ordered.” Arising from the desolation of postwar Germany, it was “an attempt to put some kind of order on a musical world after this total disorder had happened.”

Somewhere in Stockhausen’s mind, Rattle concluded, “there was this idea that we will create this new music which is utterly not to do with the history that came before it.”