In 2002, Bjork released a boxed set of musical studies and influences she called “Family Tree.” It was a fascinating act of self-documentation – divided into sections labeled “Roots,” “Beats,” Strings,” and “Words” – that saw her looking both backward and forward for inspiration. Her music incorporates ancient myth and language and is filtered through the geography of her native Iceland, but it pushes the boundaries of musical experimentation and technology. She is not so much an artist of the moment, but one who pushes the present from behind and pulls it from up ahead.

Her celebrity training began young. Bjork Gudmundsdottir became a celebrity in Iceland at age 11, when she released a successful album of covers. International fame came in the late 1980s when she sang the surprise hit “Birthday” with the Icelandic pop parody group the Sugarcubes. Since the dissolution of the band in 1992, she has built a reputation as one of the most inventive, respected, and ostentatious of pop composers.

Each Bjork album has its own identity and musical focus. She has been through jazz, house, and club music. For “Vespertine,” her last studio album, released in 2001, it was the organic, twilight aspects of electronic music that took center stage. The sound was lush but hushed, filled with tiny rustling, chiming, and swelling noises which set apart Bjorks swooping voice.

On “Medulla,” Bjorks seventh studio album, released today, her musical family tree continues to branch off in surprising, and in this case frustrating, directions – out on a limb is the only place she knows how to work.

The word “Medulla” means “marrow” in Latin. “Not just your bone marrow, but marrow in the kidneys and marrow in your hair, too,” explains Bjork. “It’s about getting to the essence of something.” The essential thing here is the human voice. The album is “a capella” in the sense that almost every sound is supplied by a human voice – Bjork’s own along with human beat boxers, former Faith No More frontman Mike Patton, Robert Wyatt, and two vocal choirs – but as an adjective it’s misleading.

The album is primitive in its way, preinstrumental: It delights in the compositional possibilities of the human voice. But it is also postmodern and post-instrumental, in that many of the voices are distorted and modulated to the point that they’re unrecognizable as human in origin.

This can make for interesting, even enjoyable listening. On “Mouth of Cradle,” voices slowly waft in over ping-ponging electronics: an angelic female choir, a gothic men’s choir, Bjork doing breathing exercises. Then her voice splinters into shards, as if played and rearranged on a keyboard; a male voice contributes bullfrog bass notes. Having established the themes, the parts come together at different volumes and in different combinations in a fluctuating terrain, over which Bjork’s simple melody rides and dances. It’s a song that might appeal to lay listeners, Bjork devotees, and experimentalists alike.

This is her method throughout, opening up the songs so that we can see the insides and appreciate the workings of the component parts. She wants us to keep in mind the nature of her project and judge her work accordingly – it is music with an asterisk next to it.

The places where we forget the experiment are the most pleasurable. “Who Is It” sounds like Bjork dance floor pop: saccharine and accessible, with soaring choruses reminiscent of hits like “Big Time Sensuality.” “Oceania” features classic Bjork lyricism: grand and off-beat. Inspired by Greek mythology – the song was performed at the Olympic opening ceremonies – it is told from the perspective of an ocean goddess. “You have done good for yourselves since you left my wet embrace and crawled ashore,” she sings, and adds a little later, “your sweat is salty, I am why.”

More often, the concept overwhelms the outcome. The song “Oll Birtan” is a sound installation trading as a pop song. Bjork makes noises – “haal,” “too too” – and works them into a minimalist beat but never anything more. “Ancestors” is no less alien. It begins with electronic hyperventilation and wobbly piano and becomes a cacophony of bleats, guttural noises, and what sounds like a cat coughing up a hairball. There’s no reason to listen to it more than once.

Sadly, this is the case with much of the album. Self-imposed constraints may spur creativity, but they can also disguise a lack of ideas or boredom. Bjork’s celebration of the human voice is an off note from one of pop’s most distinctive voices.