All too often, when a band loses core members, it's a bad sign -- and that goes double if the departing member is a vocalist.

In Múm's case, however, paring down to just Gunnar Örn Tynes and Örvar Þóreyjarson Smárason opened an array of possibilities for Go Go Smear the Poison Ivy.

Kristín Anna Valtýsdóttir's elfin vocals came to define Múm just as much as, if not more than, the twinkling mix of electronics and indie pop that surrounded her, and by the time of Summer Make Good, that sound -- which felt so fresh circa Yesterday Was Dramatic, Today Was OK -- seemed a little predictable.

For this album, Tynes and Smárason brought in an entirely new crew of musicians, including two vocalists, Hildur Guðnadóttir and Mr.

Silla.

Adding just one new singer can alter a group's sound radically; with two new voices on Go Go Smear the Poison Ivy, the changes are dramatic -- but they're also more than just OK.

Tynes and Smárason sound liberated from any expectations of what a Múm album should be, and they take the opportunity to stretch out and try some new approaches.

The hazy, strange innocence of the band's previous work sounded like Múm was somehow able to commit the fever dreams of sickly children to tape; here, Múm's music is still sparkling and childlike, but it's also much brighter and livelier.

"Blessed Brambles"' sprightly, ping-ponging beats and chanted boy-girl vocals make it clear that this is a different Múm right from the start, and the band spends the rest of Go Go Smear the Poison Ivy moving away from their old sound.

Sometimes, they take baby steps: "Marmalade Fires"' gently rolling melody and distorted beats are quintessentially Múm, but the song is more structured and immediate than most of the band's other work.

"These Eyes Are Berries" could be from some lost, twisted children's album; its brass, glockenspiel, and singalong "la la la"s are undeniably cheery, but the sudden, ominous twists the song takes give the impression of dancing too close to the darkest part of an enchanted forest.

Other times, Múm takes steps so big, they really should be called leaps.

"Dancing Behind My Eyelids" is easily one of the band's most animated tracks, with a beat that sounds like a hyperactive typewriter and a melody as chilly and sweet as frosted snowflakes.

"Moon Pulls," however, gets the honor of being Go Go Smear the Poison Ivy's most striking and unique song: its gorgeous, contemplative melody and Mr.

Silla's plaintive vocals make it more akin to Misery Is a Butterfly-era Blonde Redhead than anything in Múm's catalog.

All of the album's experimentation takes some getting used to -- as does its asymmetric track listing, which begins with full-fledged songs and tapers down to wordless interludes like "Rhubarbidoo"'s toy instrument fanfares.

Some fans will miss Múm's wispier, bygone days, but those willing to give the band a chance to change and grow will welcome the chance to get to know them all over again.