Few insults hit a serious composer as hard as saying a piece of work “sounds like movie music” or, worse still, like “mood music.” The background function of a film score has stigmatized the genre as something insubstantial and inclined to cliché since the days of piano players tinkling away as silent pictures flickered in early movie houses. Mood music took hold as a genre with the introduction of the long-playing record, when albums of lush strings and sonic faux-exotica by schlocksters like Les Baxter were pitched as aural lubricants for bachelor-pad sex, and the contemporary variations on the form found on Spotify’s “chill” playlists are still considered less-than-major works by nature of their functionality as ambiance.

Hildur Guðnadóttir, the composer from Iceland best known for her scores for the HBO miniseries Chernobyl and Todd Phillips’s comic-villain thriller Joker, has emerged from years of noble obscurity in the darker corners of the art-music world to help redeem the reputations of both movie music and mood music. In the past year, she’s won an Emmy and a Grammy for Chernobyl and a Golden Globe award and an Oscar nomination for her Joker score. As the headline of an Esquire piece on her newfound prominence announced, “‘Joker’ Composer Hildur Guðnadóttir Is Shaking Up the Industry.” More accurately, the mainstream entertainment industry is shaking itself up by allowing Guðnadóttir to do, in a high-profile way, the kinds of things she’s done in avant-garde circles for years.

Now 37, Guðnadóttir has been creating bold and imaginative music on her own and with a variety of collaborators since the first decade of the 21st century. In fact, she’s been performing publicly on her primary instrument, the cello, since girlhood, encouraged by her parents: her mother, the classical vocalist Ingveldur Guðrún Ólafsdóttir, and her father, Guðni Franzson, a clarinetist, composer, and conductor.

After experimenting in various art-rock ensembles throughout Scandinavia and in Berlin, where she studied at the Universität der Künste, Guðnadóttir made her first solo album, Mount A, in 2006. Though it was released under the pseudonym Lost in Hindurness, Guðnadóttir made every sound on the recording, from the nonverbal vocals to the cello, piano, zither, vibraphone, gamelan, gamba (a Renaissance-era stringed instrument), and Mongolian khuur, which I’d never heard of till I read the liner notes. Recorded partly in a New York studio and partly in a house in the village of Hólar, Iceland (because she liked the way her cello sounded there), Mount A was rereleased in 2010 under Guðnadóttir’s own name. It is an exceptionally assured and strange recording debut, a DIY collage of sonic textures and tone colors by an artist open to the moment and hungry for surprise.

“I think there shouldn’t be limits to what we’re allowed to say or express, as long as we don’t hurt anyone,” Guðnadóttir told an interviewer in an “All Access” video. “Music should be a form of free expression.” Her willingness (or eagerness) to venture into under-explored areas of emotionality was a hallmark of her music long before she gave voice to the brooding rage that festers into gleeful sadism in Joker. If her music won’t hurt anyone, she’s happy to conjure the sound of someone, like Joker, who will.

After Mount A, Guðnadóttir worked as a cellist or composer on more than a dozen albums made in collaboration with electronic, art-pop, classical, or category-defiant musicians in Scandinavia, from established figures like Nico Muhly, Ben Frost, and the Swedish group the Knife to acts little known outside the Nordic world, like the short-lived Iceland band Rúnk and the Finnish techno duo Pan Sonic. What’s readily streamable of this output today, such as Muhly’s Speaks Volumes (2006), Frost’s Theories of Machines (2006), and Tu Non Mi Perderai Mai (a collaboration jointly credited to Guðnadóttir and the late Icelandic composer Jóhann Jóhannsson, also from 2006), only hints at the imaginative force of Guðnadóttir’s solo music.

By 2009, when she released her second album, Without Sinking, Guðnadóttir was fully formed as an experimental composer and musician. As on Mount A, she played multiple instruments and sang atmospheric, nonliteral sounds but added a trio of musicians—Jóhannsson on organ, Skúli Sverrisson on bass, and her father on clarinet and bass clarinet—to bring more colors and fuller body to the music. It is, in concept, programmatic music intended to suggest an encounter with a body of water in a place like Iceland, which is to say a place like no other. The 10 selections have simple titles such as “Elevation,” “Overcast,” “Ascent,” and “Into Warmer Air,” and the tracks feel at first like conventional ambient music. But unexpected things happen as their sounds unfurl. Acoustic instruments are overtaken by electronics, twisted around, and transformed. The familiar is disrupted by the unnamable. To take this as mood music is to allow your mood to spin and sink and land somewhere you’ve never been before.

The essence of Guðnadóttir’s music as an agent of mood was recognized early as suitable to film scores. Beginning in 2011, she was commissioned to compose music, alone or with collaborators, for a series of European films, including Tobias Lindholm’s A Hijacking and Saul Dibb’s Journey’s End. When Jóhannsson died suddenly in the midst of writing music for Garth Davis’s Mary Magdalene, Guðnadóttir completed the score. Her music for that film was celebrated for its deft mingling of classical sonorities and electronics to bring timely resonance to characters from deep history, and Chernobyl and Joker followed.

For Chernobyl, the British-made miniseries about the 1986 Soviet nuclear disaster, Guðnadóttir tapped her skill with nontraditional sounds and constructed a full musical score with no music. To evoke the grim, oppressive atmosphere of the film’s time and place, she toured the decommissioned nuclear facility used in the film and had a specialist in field recording capture the sounds she encountered there, the hums and buzzes and rhythmic clanking of the machinery. Then she processed those sounds and edited them as if they were instruments and made a grimly hypnotic anti-musical kind of music from them.

For Joker, Guðnadóttir also worked unconventionally, composing the score from the shooting script rather than a rough cut of the film. She wrote the core themes on a halldorophone, an electro-acoustic instrument that is played much like a cello but facilitates the creative manipulation of feedback and other effects. Phillips, when shooting the film, would play Guðnadóttir’s music on the set, allowing the score to drive the action. With the background music foregrounded in the filmmaking, Guðnadóttir’s score is as elemental to Joker as Joaquin Phoenix’s performance. The music, a slow-acting poison formula of acoustic instruments and electronics mixed and overheated, brings to life the Joker’s degeneration into madness and bloodlust so vividly that you could watch the movie with your eyes closed and the vocals muted and still have the Joker experience.

Reflecting on the darkness of the score in an interview, Guðnadóttir connected it to her work outside film, music that deserves to be more widely heard. “My solo music started as a way to really look inwards…without any outside dialogue,” she said. “A lot of my music is kind of contemplative, and somehow that always tends to tilt on the darker side. My inner conversation is apparently quite dark.”