PortisheadPortishead waited seven years to complete what became their third studio album entitled, appropriately enough, “Third.” When trip-hop trio released it in late April 2008, they revealed that they had created something new and bold that challenged the genre as well as their earlier recordings.

Trip hop – the form in which electronic music meets funk, hip-hop, jazz and soul – originated in Bristol, where Portishead was based, as were other trip-hop proponents like Massive Attack and Tricky. (The term came into vogue three years after Massive Attack’s debut album, “Blue Lines,” released in ‘91.) At its most satisfying, trip-hop stimulates the body and mind with haunting ambience and complex rhythms that create a hypnotic environment that surrounds the vocalist. In Portishead, the singer is Beth Gibbons, who is extraordinary: Internalizing her energy, Gibbons brings to the band’s music an intensity that was undeniable. She and her musical partners Geoff Barrow and Adrian Utley broke through with their ’94 debut, “Dummy,” which won the Mercury Prize as the U.K.’s best album.

With “Third,” Portishead replaced the slinky setting of their two previous albums with more solid structures of urgent, confrontational percussion, taut synths and a variety of rock-like sounds from Utley’s guitars. Bassoon, clarinet, saxophone and a hurdy-gurdy were added to the new mix. Even the way the tracks were presented signaled a change: songs ended abruptly or plowed into one another. What remained constant in Portishead was Barrow’s fat, rumbling bass, the adept use of strings, the overarching sense of pathos and dark wit, and, most importantly, Gibbons’ wonderfully expressive voice.

The new approach created some of the album’s most arresting moments. “We Carry On” bulled its way forward on relentless synth pulses and organ waves that were joined by industrial clanking and snappy patterns on a snare drum; and then Utley joined in with chugging chords on electric guitar. Big drums kicked off the opening number, “Silence,” and Gibbons didn’t enter until the thunderous beats dropped out more than two minutes in. Her voice floated above harsh sounds in “Machine Gun,” which deployed static as a rhythmic element, foreshadowing Björk’s use of Tesla coil on “Biophilia,” released in 2011. “Small” was in effect a suite with Gibbons overdubbing her voice to sing amid quiet accompaniment until pounding rock with a heavy organ entered at the midpoint and then disappeared, only to return in a slightly different form. In “Magic Doors,” the first composition Barrow brought to his bandmates for “Third,” Goldfrapp’s Will Gregory wailed away on saxophone during a brief interlude until Gibbons, the cry of a hurdy gurdy and booming bass came back.

Beyond its aggressive, automated underpinning, “Third” had other unexpected moments. “The Rip” opened as a folk song with a finger-picked guitar and Gibbons’ melodic whisper, until a mechanical rhythm and bass pattern entered forcefully. “Hunter” recalled ‘70s French pop and film scores. On “Deep Water,” Gibbons sang to chords played by Utley on ukulele, creating a sort of a Jazz Age vibe albeit one created by ghosts.

Several songs nodded toward earlier Portishead tracks: in “Plastic,” what sounded like the puck of helicopter blades replaced turntable scratching as Gibbons simmered. The closing number, “Threads,” referenced the brooding atmosphere of “Dummy” and its successor “Portishead.” But by then, the trio had made its point quite well: they were off on a new adventure.

Yet that new adventure hasn’t continued. Thus far, there’s been no fourth album from Portishead. They’ve played live many times including at major festivals – earlier this month, they performed at the Latitude Festival in Suffolk, England; Radiohead’s Thom Yorke joined in on a reading of “The Rip.” And in 2009, they released a single “Chase the Tear,” an upbeat bit of funk that sounds unlike anything they’d issued before, which may suggests the trio is off somewhere exploring new modes of expression. If not, and were “Third” to be their last studio recording, they would have ended their recording career with a bold, innovative work that will extend their legacy well into the future.