NEW YORK, NY.- For 41 years, Joan Lader has rented a slender studio apartment just west of Union Square in Manhattan. Through its door, a narrow entryway leads to a doll-size bathroom and an efficiency kitchen. In the main space, where a visitor might expect to find a bed, Lader has arranged the instruments of her trade — a piano, a keyboard, balance balls, straws, a box of tissues, a skeleton in a jaunty hat.

Lader has never advertised, never solicited clients. But for two generations of Broadway stars, as well as dozens of opera singers and pop and rock luminaries, she remains an indispensable vocal therapist and vocal coach. She even received a Tony Award in 2016 for excellence in theater.

And while proper breathing is fundamental to her practice, she has scarcely paused for breath since that award. She continues to work seven hours each day, seven days a week. (“I wish she would take a break,” Patti LuPone, a longtime student, told me.) For Lader, 77, the work is her calling, a synthesis of artistry, science and according to her clients and fans, something akin to magic.

“I’ve called her a witch in front of people, many times,” music director Rob Fisher said. “I’ve never seen anybody else do the hocus-pocus that she sometimes does.”

Composer Tom Kitt can nearly always tell when a singer has been working with Lader. “They have opened up in the beautiful way,” he said. “They are empowered, and they feel confident.”

I met Lader on a wintry afternoon last month. She had struggled to find time to see me, but a cancellation had opened a narrow window in her schedule. She showed me into her space, noting the 8-inch soundproofing along the wall that borders the apartment next door. “Cats” paid for that, she said.

Lader has worked with the likes of Madonna, Paul Simon and Sting, as well as throngs of Tony winners. But like a lawyer or a priest, she keeps that confidential. She won’t disclose her clients unless they have disclosed their affiliation first. What happens in the studio stays in the studio.

Lader is slight, birdlike almost, with a focused, energetic, no-nonsense manner that made me correct my posture. Although they are roughly the same age, Betty Buckley, another client, will only address her with the formal Miss Lader.

“Because she is really such a master,” said Buckley, who has worked with her since fraying her vocal cords in the mid-1990s while performing in “Sunset Boulevard.” “I love her completely. I’m a devotee.”

As a New York City child, Lader hoped for a career in modern dance. Martha Graham, who visited the dance studio she attended, dissuaded her. (“She said something about cheekbones,” Lader said. “She said, ‘All dancers have to have cheekbones and you don’t have any.’”) But a talent for music won her a spot at the High School of Music and Art, which later became the Fiorello H. LaGuardia High School of Music & Art and Performing Arts. On a whim she chose voice over an instrumental track, and continued those studies at Penn State, where she branched out into theater. After graduation, she returned to the city, picking up voice-over work, often for radio.

Radio commercials didn’t pay enough for her to move out of her parents’ place, so she took an administrative position at the Hospital for Special Surgery. That job offered a stipend for graduate school, so in the 1970s, Lader enrolled in the speech program at Hunter College, assuming it was theater related. In fact, the course emphasized speech pathology and audiology.

“I had no idea that there was science involved,” Lader said.

Fortunately, the science fascinated her. She knew how to sing, but she had never understood how she sang — the precise mechanics of breath, mouth and vocal folds. That might have been the end of it, as she was soon hired by the Light Opera of Manhattan as a mezzo soprano. But even as she pursued a performing career, Lader could not let go of vocal therapy. Knowing that she was a singer, she said that Mount Sinai Hospital, where she had completed an externship, began to refer injured singers to her, trusting her with their rehabilitation. A few years later, she rented the Union Square studio and relinquished performance. She has been booked solid ever since.

Much of her work still deals with rehabilitation. She helps singers dealing with polyps, nodules, hemorrhages, cysts, acid reflux, vocal fold swelling. (She showed me a terrifying slideshow depicting these various ailments and disorders.)

LuPone first met with Lader in the mid-’90s, after a vocal cord operation.

“I couldn’t talk, I had nothing,” LuPone said. Lader began with Lupone’s speaking voice, then rehabbed her singing voice. “She saved my career,” LuPone said. “She gave me a technique to allow me to continue to sing with the strength and the clarity that I’ve had ever since.”

Billy Porter contacted Lader in the early 2000s, having lost his voice to acid reflux. Porter’s voice had always had a gospel sound, a rock sound, which was atypical of Broadway performers back then. During their sessions, Lader never tried to change that. “She was one of the first teachers that I met who was not about squashing that,” Porter said.

The other side of her practice involves healthy singers who are new to a role, who are struggling in a role, or who want to expand and improve their voices. Often performers come to her in a kind of existential panic, because they no longer understand how they do what they do. She encourages singers to relax and to open their whole throats, which isn’t always comfortable.

“When you’re totally free, you’re really open and some people don’t want to be open,” she said. “So you have to be very, very careful and supportive and not critical.”

If Lader is careful and supportive, she is also wholly professional. “She doesn’t lie to you,” Broadway performer Charlotte d’Amboise said. “She doesn’t say that sounds great, if it doesn’t sound great. She’s honest, straightforward. There’s hand-holding to a point, but she’s there to make it right.”

Lader is typically on retainer for a handful of Broadway shows — at the moment those include “Chicago,” in which D’Amboise has long performed; “Moulin Rouge”; “Spamalot”; and “Sweeney Todd” — seeing any performer who requires coaching.

Casey Cott (“Riverdale”) has been working with her since he was cast in “Moulin Rouge” as a replacement Christian. “She’s incredibly intuitive, which is so important because singing is such a vulnerable thing,” Cott said. He also marveled at the variety of the exercises she assigned him — balance challenges, chair poses, pretending that your ears are being tugged upward by balloons.

Those exercises — she’ll admit to dozens, possibly hundreds — are drawn from a multiplicity of methods, techniques and the work of past masters: Alexander, Feldenkrais, Jo Estill, Arthur Lessac.

“We do lots of poses in here,” Lader said. “I’m on the ground with them.” Beyond yoga, Lader relies on a host of gadgets (straws, sing rings, sing necklaces) and a network of osteopaths, chiropractors, massage therapists and psychotherapists to whom she can refer clients.

Lader can’t do everything. She can turn a bad singer into a good one and a good singer into a better one, but she can’t supply that ineffable quality that makes a voice distinct.

“You’re born with that,” said Lader, who rates her own voice as good, though not exceptional. She has some qualms about never having made it to Broadway (she and director Hal Prince used to talk about a walk-on), but she does not regret her career. And the hulking file cabinets that dominate one wall — filled with scores and folders for each client — testify to her influence and care.

Singers who spoke with me called her a virtuoso, a miracle worker, a life saver. Lader’s own assessment was more pragmatic. “I guess I’m in the right field, and I’m doing what I should be doing,” she said.

This article originally appeared in The New York Times.