CHATHAM -- Playing classical music outdoors can offer challenges, especially for period instruments. Strings made from animal gut soak up moisture and wood is sensitive to atmospheric conditions.

Amelia Roosevelt, founder and violinist of period instrument ensemble Repast Baroque, said musicians learn how to adapt.

"We're used to it," she said. "You learn to tune quickly and not to get freaked out if you go out of tune while you're playing."

Her group will prepare for whatever the summer weather throws at them in a concert Sunday, June 22, at Performance Spaces for the 21st Century (PS21). Chatham, N.Y., can't challenge them more than a concert tour that took them to Sri Lanka, when they performed in Colombo during unusually late monsoon weather.

"We heard sounds we'd never heard before," she said.

Roosevelt and fellow Repast Baroque member John Mark Rozendaal will join guest harpsichordist Dongsok Shin for a performance, under the tent in the orchard, for the Ninth Annual Paul Grunberg Memorial Bach Concert.

Repast Baroque, a period-instrument trio, has been playing together for 10 years, with Roosevelt on violin and Rozendaal on the viola da gamba -- an early version of the cello. They perform works by a range of baroque composers, from rarely performed pieces by Buxtehude and Erlebach to masterworks from Vivaldi and Bach.

They have known harpsichordist Shin for many years.

"It's a small enough world, everyone ends up playing with everyone else," she joked.

Shin said his interest in music began when he started learning the piano at 4, but he moved on to the harpsichord when he realized how much he liked the sound. Modern pianos have about 70 mechanical steps between his hand and the sound, he said, unlike the distinctive pluck of a harpsichord.

"It's much more direct and simple mechanism," he said. "You get much more tactile satisfaction."

Unlike Roosevelt, who uses a violin made in 1773, still with an original neck (it has less of an angle than a modern violin and a lower pitch) and gut strings, Shin plays an instrument with materials based on a Flemish design from the early 18th century. He built it himself out of a kit in the ‘80s, which he said was "like putting together a model airplane, but on a much, much larger scale."

His career had developed along with growing interest in early music among American audiences. "When I started, it was still a musicological adventure," he said, with a lot of attention to the historic and culture background.

Roosevelt also said the scene has changed dramatically since she began. She went for much of her training to Amsterdam and Cologne, two major centers of early music, and found on her return to the States in the 1990s that the scene here was still far behind Europe. That began to change with greater interest in historic music performance at conservatories, with more teaching and scholarship on the subject.

"That has been really great for the early music scene," she said. "I feel there is a growing audience."

She said there remains a number of different approaches to performing this music. Some artists and ensembles choose to emphasize the research and period fidelity of their work. For Roosevelt, performing means "making a beautiful interpretation" and communicating with an audience the emotion of the music.

"We don't tend to be smug about knowing more than other musicians what makes [our interpretation] better," she said. "We don't substitute that for good musicianship."

Judy Grunberg, founder and president of the board of directors at PS21, said the Bach concert series there began as an on-going tribute to her late husband, Paul, who passed away in 1997.

"Bach was his favorite composer of all, and we played him a lot at home," she said. "This is a way of honoring [my husband] and spending a few hours listening to Bach every year."

Grunberg said each year they put together the program, they look at different approaches of playing and ways of hearing Bach, from period instrument ensembles to modern adaptations.

"From my point of view there's nothing you can't do with Bach," she said.

The program for the PS21 concert includes a selection of pieces transcribed for different instruments. An organ trio rewritten for harpsichord Roosevelt describes as "an exuberant, fun, beautiful piece." They will also play an arrangement of a portion of Bach's Fifth Brandenburg Concerto. She said the idea of changing the instruments was common to Baroque composers.

"There was flexibility about instrumentalization in many cases," she said.

They will also play a sonata for viola da gamba and harpsichord, a piece that "requires a lot of improvisational work on the part of the harpsichord," Roosevelt said.

"We are interested in all kinds of baroque music," she said, "but we find Bach is central, and we love playing his music."

If you go ...

What: Repast Baroque and Dongsok Shin will perform Bach

When: 2 p.m. Sunday, June 22

Where: PS21, Chatham, N.Y.

Admission: $35 for adults, $20 for students

Information: ps21chatham.org