Like everything from Punch Brothers, their new album, “The Phosphorescent Blues,” is ambitious. The string ensemble’s fourth full-length studio effort showcases a wide range of styles and emotions. At first glance, the quintet looks like a traditional bluegrass group, designed to play the genre’s unique steel-string blend of American country, folk and jazz: MacArthur genius Chris Thile plays mandolin and sings, and he’s backed by Chris Eldridge on guitar, Gabe Witcher on fiddle, Noam Pikelny on banjo  and Paul Kowert on double bass. Within the loosely defined category of progressive bluegrass — a catch-all term for bluegrass-inspired music that departs from tradition. Punch Brothers evoke the sounds of bluegrass, alternative rock, tender ballads and even Debussy. What’s remarkable is that none of it feels out of place.

Punch Brothers has been one of Thile’s main creative vehicles for a few years now, and each project they’ve released has played an important part in advancing the sound of progressive bluegrass, playing with the expectations of what the genre can be. Their first album, “Punch,” featured an unprecedented, esoteric 40-minute suite written as a bitter eulogy to Thile’s first marriage. Their second album, “Who’s Feeling Young Now?,” was a more accessible effort featuring structurally simpler, indie-rock-inspired tunes, yet it remained faithful to their unique instrumentation and sound.

While most bands tend to stick to a single style or theme, Punch Brothers always seem to have bigger plans, taking seemingly disparate ideas and finding creative ways to tie them all together. The variety of music featured on “The Phosphorescent Blues” is perhaps best exemplified by the first track, a 10-minute, multi-movement piece called “Familiarity.” Opening with a blend of Thile’s rapid mandolin arpeggios and gentle singing, “Familiarity” jumps from soothing to jarring when heavily bowed bass accents enter. The band then uses choral harmonies to move to a raucous chorus featuring drums — a rare inclusion. All of this fades into a distorted climax, and we’re suddenly taken back to the group’s roots with a pensive bluegrass ballad.

My favorite tracks are the haunting “Julep,” a from-beyond-the-grave love song in which Thile sings, “I died happy in my sleep . . . Heaven’s a julep on the porch,” and “Boll Weevil,” arguably the only traditional bluegrass song on the record. “Little Lights” serves as a strong closer, and does a good job of bookending the album with an exquisite, building climax.

The skill displayed on the album is astounding, and it stems from each member’s years of training in the bluegrass tradition. But it seems that when they borrowed the speed and style of bluegrass, they left behind its soul — gritty fiddling and picking have been replaced by nearly flawless mechanics, the rough twang of traditional vocal harmonies have been traded for pop-like perfection. Somehow the music sounds too polished, as if it has lost the sweat and tears that makes traditional bluegrass so great.

Still, “The Phosphorescent Blues” is a remarkable feat. It’s musically engaging, well-executed, and it will keep you guessing. If you’re looking for bluegrass, though, you might have to look elsewhere: don’t be fooled by the banjo — Punch Brothers is growing well beyond tradition. They’re not afraid to break the rules, and they’re going to sound great doing it, too.

“The Phosphorescent Blues” is available now on iTunes, Spotify, and in stores.

You can contact Benjamin Sorensen at bscoren ‘at’ stanford.edu.