One might find it difficult to believe, but Iceland's incomparable pop diva Björk has gotten even weirder.

For the past three years, she has been flooding the market with greatest hits collections, box sets, unnecessary live versions of albums and DVDs, and it would be reasonable for one to think that Björk was getting complacent, satisfied to sit back and watch the cash flow in.

Instead, Medulla, her first studio album since 2001's Vespertine, marks her boldest and strangest move since she wore a dead swan to the Oscars. Why? The entire album is made up of human vocals. Instruments not allowed.

It's the type of idea that countless musicians must have come up with over the years but few other than Björk would be brave and crazy enough to try. She enlists several choirs, beatboxers Rahzel of Roots fame and Dokaka, Icelandic throat singer Tagaq and more to help her achieve an a cappella album. She succeeds in making the most interesting music of her career.

However, those who take great risks sometimes fall. Medulla is undoubtedly a fascinating experiment. Whether it is good music is another matter entirely.

Björk shows the potential of her experiment on several complex pop songs which resemble her previous work, except in the fact that they have no electronic instruments. "Pleasure Is All Mine" is a slow, building, slightly dark song that makes a fine opening for the album. "Triumph of A Heart" is catchy pop with hard-to-believe-it's-human beats and "human trombone."

"Oceania," the album's first single, which Björk performed at the Olympics, is the peak of the Medulla method. The human beat fits this tune perfectly, but it is the choral flourishes of rising and falling notes which make the song a thing of beauty. Björk herself sings a fine lead vocal from the perspective of "mother ocean" with some of her better lyrics: "Your sweat is salty / I am why."

Several songs, however, like "Who Is It," would work just fine in a more typical arrangement, which is convenient for Björk's live performances, but not terribly impressive on record.

Much of Medulla, on the other hand, is hardly pop, but strange experimental stuff. Some tracks are vaguely pretty, such as the brief, a cappella solo "Show Me Forgiveness" and "V?kuro," in which she pulls out the Icelandic language on an album for the first time since the 80s. Most is less successful. On "Sonnets / Unrealities XI," Björk puts e.e. cummings's lyrics to music. This wasn't particularly interesting when she did it on Vespertine, and it isn't here either.

The three tracks without lyrics are annoying, particularly "Ancestors," a long duet with Tagaq about as listenable as the 12-minute drone on the new Wilco album. They try to one-up each other with weird sounds. Björk wins for greater variety, but the listener does not.

Actually, I lied about that no instruments thing?there is some piano on two songs and Björk's voice is accompanied by programmed electronic stuff on the spare "Desired Constellation." Departing the album's rule, "Desired Constellation" is probably the album's best song...which has to prove something about this crazy experiment.

Honestly, Medulla, though it might as well be stamped with "Fans Only," sounds like a Björk album. Her fans aren't too likely to run from the bizarre. Medulla could alienate casual fans, but "casual" hardly describes the average Björk fan's fan-dom.

The bottom line is that Medulla is the weakest of Björk's five major studio albums, and only features two great tracks ("Oceania" and "Desired Constellation"). It's worth a listen, but probably not a buy.