“Now we have the possibility of a young boy sounding like Joe Cocker,” Michael Fuchs, an ear, nose and throat specialist and the voice doctor of the choir, told me. “We can have an early adult voice without an adult personality. He sounds like his father when he’s still a boy.”

Fuchs’s own voice broke in 1983, when he was a Thomaner, age 13. He liked the voice doctor of the choir then and decided to follow his career path. Now trim, goateed, in good shoes and fashionable eyeglasses, he’s among the world’s experts on the timing of voice break. Certainly nobody cares more. “We have a problem in the choir,” Fuchs said, when we met in his office at Leipzig University hospital, echoing concerns I heard time and again. “The balance is shifting. We have more men’s voices and fewer boys’ voices.” The obvious solution — starting boys in the choir at 8 instead of 9 — does not work. The choir tried, and the 8-year-olds couldn’t handle life in the alumnat, learning all the pieces and attending all the rehearsals. So the plan now is to squeeze every day out of the soprano voices. “We try to let the boys sing as long as possible without risking overloading the boys and damaging the voice,” Fuchs said. Many of the Thomaners hold Fuchs in mystical esteem, claiming he can predict to the day when their voices will break.

“Of course not the day,” Fuchs demurred, “but maybe two, three weeks.”

In the years before the boys hit puberty, Fuchs saw them every three months to record growth, hormone levels and voice. He played for me recordings of one boy speaking and singing the same passage of a choral piece every year from age 11 to 14. Even to my untrained ear, the differences over time sounded stark, the boy’s voice becoming richer and fuller until the day it shattered. Mining data from the recordings, Fuchs constructs scatter plots showing changes over time in jitter (variation in pitch), shimmer (variation in loudness), noise component (breathiness) and range. All this data provides context so that he can distinguish what is causing flaws in a boy’s voice. “Is it a cold? A problem of singing technique? Voice break can sound like laryngitis.”

Martin Ashley, founder of Boys Keep Singing, a British group dedicated to keeping boys in choirs, is the author of the monograph “How High Should Boys Sing?” In it, he quotes a vocal expert who describes the boy-chorister sound as “clear, cold, piercing and transparent . . . disembodied, ethereal and ghostly . . . enchanted, naïve and guileless.” A typical boy’s voice falls from about 500 Hz as a baby to 300 Hz as a toddler to 200 Hz before puberty, and it’s the last sound that choir directors and listeners crave. In 1927, “Hear My Prayer,” by the 15-year-old British soprano Ernest Lough, became HMV’s first classical record to sell a million copies. In 1955, Keith Richards, then 11, performed in Westminster Abbey with two other boy sopranos for the queen.

To see how a boy is maturing physically, Fuchs threads a tiny camera through the nose and into the larynx so he can watch and record the vocal anatomy while the boy sings. (When Fuchs was a Thomaner, the doctor used what amounted to a dental mirror on a long, narrow stick to see into their throats; by comparison, Fuchs said, his test wasn’t bad, once the boys learned to control their gag reflex.) The videos are intimate and alarming — the vibrating glottis, the gaping trachea: Jonah’s view from inside the whale. “That red is the leading edge of cells growing,” Fuchs said, pointing to a vocal fold on his computer screen. “Can you hear how it’s not possible to bring the vocal cords into good vibration?” he added during a particularly scratchy patch. “If this were from overstress and not voice break, you would see it on the surface. Like if you went to the forest to cut wood. You’d have nodules on your hands and edema. Stress on the vocal cords looks the same.”

While their voices are changing, Thomaners don’t rehearse or perform, but they still have bi-weekly singing lessons, during which they work on diction and breath control. Singing too much while your voice is changing can lead to bad habits and injuries, similar to an athlete playing hurt. Late Wednesday afternoon, a tall, awkward, 13-year-old named Alexander spent half an hour with a private instructor, meticulously enunciating — la-pla-lu ma-la-plu, la-pla-lo ma-la-plo — and making sounds like deflating balloons and air compressing into tires. His teacher was a lovely young concert singer in an argyle sweater and white boots. She massaged the stress out of Alexander’s jaw and neck while he did his best to stare at the wall. “Va vey vop voop veep vip.” Toward the end of the session, Alexander sang scales ranging two octaves, his teacher writing down which notes he lost at the top, dropped in the middle and gained at the bottom.

This was nobody’s fantasy of being a Thomaner, but you can’t stop the body from maturing — even though the thought has occurred at St. Thomas. “Of course, Cantor Biller asks me at times, ‘Do you have some idea to delay onset?’ ” Fuchs said. Puberty-stalling drugs do exist. They are prescribed by endocrinologists for children with extremely early sexual development. But preserving sopranos’ voices through chemistry? Not even the most passionate choir fans suggest using pharmacology to replace the bygone castrati. “That’s absolutely forbidden from the medical point of view,” Fuchs said.