Early in February 1933, a young man in Munich went into lockdown. Karl Amadeus Hartmann — the middle name discloses his vocation — locked his music in his drawer, declaring that not a note of it would be played in Germany so long as Hitler was in power.

It was a courageous act for a composer of 27 years old and it could have cost him far more than his fledgling career. When Hermann Scherchen defiantly conducted his Miserae in Prague, the German ambassador issued a formal protest at its dedication: “To my friends who had to die a hundred times, who sleep for all eternity: we do not forget you (Dachau 1933-34).”

Hartmann was subjected to travel restrictions by the Nazis but his father-in-law, a wealthy industrialist, protected him from more brutal retribution and gave him shelter in a villa beside Lake Starnberg. Hartmann (right) may have given some assurance in exchange that he would do nothing to endanger his wife and son. Elisabeth Hartmann, with whom I had correspondence, described him as “a passive resistant”.

Others told me he slipped out at night to help fugitives cross the mountains to freedom. Hartmann was the only composer in Germany to stand up against Hitler and he did so by a form of inner exile that we have now come to know as self-isolation.

I played some of his music the other day on hearing of the death of Krzysztof Penderecki, a kindred self-distanced soul. Penderecki grew up in Debica (Dembitz), a Polish shtetl with many Jews, 70 per cent of the town. As a boy, he learned Yiddish phrases and songs. Then he saw Jews herded into a ghetto and put on death trains. Penderecki suppressed his Yiddish tunes, along with much else.

Hartmann was the only composer in Germany to stand up against Hitler and he did so by a form of inner exile

His grandmother was Armenian, a fugitive from Turkish genocide who took him to Mass at an Armenian church, in contradistinction to dominant Roman Catholicism. One of his grandfathers was German, another inconvenient legacy. At music conservatory, he composed Polish folk tunes in conformity with Stalinist doctrine. The self of this budding composer went into perpetual isolation.

His creative liberation came in the form of abstract modernism, a music that could mean whatever people thought it might.

He wrote a piece for 52 stringed instruments and called it 8’ 37” because that was its duration. “It existed only in my imagination, in a somewhat abstract way,” he said. Pressed by the 1961 Warsaw Autumn Festival for a catchier title, he named it Threnody for the Victims of Hiroshima. Among the swooping violin strings and woodpecker noises of tapped cello bridges, we hear a simulated siren, a trapped man’s cry for life. Threnody became an instant hit of the international avant-garde and Penderecki an overnight celebrity, with academic residencies in West Berlin and Vienna.

He responded with an austere St Luke Passion, atonal in its note relations but grounded in Bach and the Latin liturgy. Too godly to be performed in communist Poland, it was celebrated in West Germany. Penderecki spent part of the 1970s teaching at Yale. Contentedly remarried, he wrote a scabrous opera of sexual transgression, The Devils of Loudun.

In the 1980s, he embraced the Polish Pope and endorsed Solidarity. Living in a house he built himself on the outskirts of Krakow he became a national icon yet the more wreaths were laid at his door, the more he distanced himself. His favourite composing place was a hut on the beach at Jastrzebia Gora, looking out at the Baltic, his back to Poland.

In the interviews he gave to promote his works, he yielded few revelations. In person, he was charming, generous, disengaging. It was only after you left the room that you realised how little he had given. Like Hartmann, he maintained a cordon sanitaire, locking himself away for protection. In some later works we hear buried memories — a surge of klezmer in the sumptuous half-hour sextet, a tinge of eastern exoticism, perhaps Armenian, in the seventh symphony.

He named the St Luke Passion once as his most important work, but he may not have meant it. Modernism, writes his biographer Bernard Jacobson, was “a useful expedient, not a creative foundation”. Like Dmitri Shostakovich in Russia and Hartmann in Germany, he mastered the art of hiding in full public view.

Hartmann, after 12 years of silence, composed a piano sonata whose opening theme represented “an endless stream of Dachau prisoners trudging past us, unending the stream, unending the misery”. It bears the title “27 April 1945”.

He emerged to organise a Musica Viva concert series that reintroduced Bavarians to all the modern music they had missed in the Thousand Year Reich. His second and third symphonies, written with Mahlerian extravagance and some overt Jewish themes, were recorded with passion and conviction by those eminent exiles Rafael Kubelik and Ferenc Fricsay.

In 1963 Hartmann died of stomach cancer at the age of 58. Orchestras have mostly let his music fall silent, his isolation presenting too lasting an indictment of German music in times of plague. The only work of his to receive more than an occasional concert performance nowadays is the 1940 concerto funèbre for violin and string orchestra. That’s the one I reached for when Penderecki died.

Of Penderecki’s legacy, Threnody and the St Luke Passion will outlast us all. The world at large knows his music from The Exorcist, a horror film, and from Twin Peaks, a cult TV series.

In enforced seclusion I have been drawn to the great Adagio of his third symphony, a 1990s work written for Munich and reminiscent of the Adagietto in Mahler’s fifth symphony, itself based upon the song “Ich bin der Welt abhanden gekommen” — I have absented myself from the world. Never has music more concisely encapsulated our present artificial state of viral self-isolation.