Nobody says J.S. Bach cures post-traumatic stress disorder. But few would be surprised if his music helped, whether it is the Goldberg Variations, heard after the Paris terror attacks last November, or the St. Matthew Passion on the day after the Brussels suicide bombings last month.

Not every composer writes music with medicinal effects. Mahler's darker symphonies could make a bad day even worse. Yet Bach (despite the furrowed brow and 18th-century wig seen in formal portraits) is described by passionate advocates as music for times of modern-day crisis, music that can help rebuild order in shattered psyches - one reason the Matthew Passion went on as scheduled in Brussels.

"Bach's music has the capacity to reach out to everyone," said conductor John Eliot Gardiner in his pre-performance remarks the day after the March 22 Brussels attacks. "His St. Matthew Passion expresses better than any other music a deep sense of human suffering." Then he requested a moment of silence.

Pianist Alexandre Tharaud, who was at home only blocks away from the Paris Bataclan theater during the November attacks, speaks less guardedly: "I think Bach is the only composer to talk directly to people." Tharaud performs Ravel's Piano Concerto for the Left Hand Friday through Sunday with the Philadelphia Orchestra.

Some composers speak only as clearly as their performers do; Bach communicates under all circumstances. "You can play Bach on every medium - marimba, accordion - and it's beautiful," said the 47-year-old French pianist. "When I play Bach, my hands feel better, my body feels better, and my heart feels better."

Like Gardiner, Tharaud had the Goldberg Variations already scheduled when the attacks happened, and he opted to play with some 200 listeners sitting with him on the stage and with candles lighted in the 2,400-seat Philharmonie de Paris on Nov. 23.

"Some friends said, 'We don't want to come. We're scared to be in the subway. We don't want to leave our children,' " he recalls. "But the concert was the best response to the terrorists. And fortunately, the audience came. The concert hall was full. I wanted people onstage. I didn't want a lot of light. We were fragile."

Although classical music is often thought to be something that doesn't change over decades and centuries, historic events can dramatically change perspectives on it for years at a time, as well as how it's performed. An objective ear apprehends similarities in the post-disaster performances. Typically, Gardiner's Bach has a firm pulse and clear musical destinations. The radio broadcast of his day-after St. Matthew Passion revealed tempos that became more contemplative as the piece went on, almost as if the performers didn't want to let go of it.

In contrast to the tightly written Ravel concerto Tharaud plays with the Philadelphia Orchestra this week, Bach's expansive Goldberg Variations give the performer wide latitude: music in the 31 sections can be repeated. Running time can vary from 40 to 80 minutes; Tharaud decides in mid-performance which repeats to observe.

On Monday in New York City, Tharaud's performance was easily longer than his Warner-label recording, made before the Paris attacks. Monday night, he observed repeats in the more contemplative variations, often with quieter, more conversational phrase readings during the second iteration. The minor-key, tragedy-steeped 26th variation, which is eight minutes-plus on the recording, stretched well beyond that on Monday.

Post-Paris, Tharaud has been performing Bach not in new, traditional concert halls, but in places such as St. Paul's Greek Orthodox Church in Savannah, Ga., and St. John's Smith Square in London. His Monday concert took place in a recently established series called The Crypt Sessions, held underground at the Church of the Intercession at 155th Street and Broadway, far from the usual midtown concert halls. The crypt seats only 200.

As in Paris, atmosphere was a priority. "We had originally envisioned more spotlights on him and dimmed house lights," said producer Andrew Ousley of Unison Media. "But when he arrived and we got his piano spotlight setup, he looked around the room and said he didn't want any other lights. . . . " Except candles along the sides.

Tharaud sees his response to the terrorist attacks as a matter of silence versus noise. He describes the chaotic aftermath of the Bataclan attacks, of ambulances and sirens. By contrast, Bach's music, Tharaud says, is for him a music of "deep inner silence. The Goldberg Variations give the audience and the pianist a chance to be quiet together. It's a communion."