‘Adult” is sometimes used to signify questionable content: adult films, adult books, adult situations. It’s the same for rock ‘n’ roll, only not because of prurience. Somehow, rock has retained its youthful exuberance well into its fifth decade, making the idea of “adult rock” a study in conflicting ideas. The friction between freewheeling rock and quasi-sensible adulthood is exactly what Brooklyn quintet the Hold Steady has explored over its four-year career, and never more so than on its new album “Stay Positive,” out today.

Hold Steady is not now, nor has it ever been, for the youth. Yes, the band’s 2006 breakthrough was titled “Boys and Girls in America,” and singer-songwriter Craig Finn loves to sing about the trials and tribulations of that great American wasteland between the start of high school and college graduation. But these stories and characters have always come from a man young enough to remember how those days felt, old enough to view them through a nostalgic patina, and smart enough to know that such rosy memories are the myths of remembrance. Over three albums, Mr. Finn has packed the Hold Steady’s driving, classic rock ‘n’ roll crunch with cynical tales of short-term hookups, broken hearts, booze-filled nights, days of Catholic school guilt and nights with guiltless Catholic-school girls, and other assorted rock clichés filtered through his whip-smart mind.

Now, though, it sounds as if the smells and tastes of youth are harder to come by for Mr. Finn, 38, and he’s trying the mantle of elder statesman on for size. “Stay Positive” is lyrically informed by the community-building and empowerment of 1980s American punk’s do-it-yourself ethos, an attitude harnessed by Washington, D.C.’s Positive Force activist group and the optimistic outlook of bands such as Youth of Today and 7 Seconds, both of which Mr. Finn name-checks in the title track, and the Clash’s Joe Strummer, who is canonized in the album’s lead track, “Constructive Summer.”

Such idealism isn’t new — punk bands have been paying respects to the bands that inspired them from day one — but the setting of such DIY positivism is. Thanks to guitarist Tad Kubler, the Hold Steady has honed its sound into stellar classic rock — a big, fuzzy guitar crunch laden with pop hooks — with keyboardist Franz Nicolay adding melodic piano accents and curlicues. This rock, boisterous and rollicking, squarely earns the Hold Steady its most frequent comparison — 1970s-era Bruce Springsteen and the E Street Band — and never more so than on the album’s lead single, “Sequestered in Memphis.” Mr. Nicolay’s sprightly piano line sets up a guitar- and organ-powered ode that heroically charts an ill-fated one-night stand.

Elsewhere, the group hammers out last-call laments (“Lord, I’m Discouraged”), epic country rock (“Magazines”), and, in the album-closing “Slapped Actress,” a rousing high-wire act that splits the difference between self-referential preciousness and a bona fide rock anthem. And while the album is another lyrical tour de force from Mr. Finn, the band’s sincere appropriation of classic rock is starting to show its limitations. “Boys and Girls in America” was the first Hold Steady album that actually sounded nostalgic; while “Stay Positive” is more musically rich, it’s only because the band has become so proficient at recreating ’70s classic rock.

The problem is that the band has always used that sound as a reference, not a goal. Its 2004 debut, “Almost Killed Me,” tried to create preening classic rock out of jittery punk insouciance, and in the process came up with a hybrid that was better than both. So while “Stay Positive” earns the Hold Steady its rightful designation as the Bruce Springsteen and the E Street band of its generation, it’s a bittersweet coronation knowing the band is capable of so much more.

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Such an identity crisis doesn’t afflict Brooklyn’s Nation Beat — a grand achievement given the diversity of this outfit. The sextet’s new “Legends of the Preacher,” also out today, is a dizzying cocktail of music of another culture, and it deserves credit for acknowledging that, in many parts of the globe, that other culture is America’s.

“Legends of the Preacher” is informed primarily by the many forms of rhythm coming out of Northeast Brazil, traditional music called ciranda, forró, frevo, maracatu, and repente. These dance and musical styles have been shaped by various instrumentations, and all are informed by African drumming. Nation Beat’s musical command of these forms is superb and made more impressive by what the group chooses to intertwine them with: traditional North American music such as country folk, funk, and New Orleans jazz. The result is an intoxicating fusion of ideas and beats that never ceases to thrill over the album’s 16 tracks.

What’s most impressive is how seamlessly and organically everything flows together. The band’s almost reverent treatment of Hank Williams’s “I’m So Lonesome I Could Cry” — propelled by layers of percussion, Raphael McGregor’s lap steel guitar, Skye Steele’s violin, and vocalist Liliana Araújo nailing a Southern twang with verve and wit — appears sandwiched between the jubilant, Brazilian Portuguese sung “Salve Ze” and “Coroa Imperial.” “Salve Ze” is little more than a showcase for Nation Beat’s rhythmic mastery, sculpting this short burst of joy of nothing but acoustic percussion instruments — including hand claps — and voices. And “Coroa Imperial” somehow finds a way to carry over the countrified strings of Williams’s classic song into a levitating blast of euphoric Brazilian pop.

Such gleeful genre experimentation has been the hallmark of Brazil’s storied late 1960s singer/songwriters, such as Gilberto Gil, Caetano Veloso, and Tom Zé. Those songwriters always invested their experimentations with a political edge, and there’s something almost anarchistic in the compulsive pop hybrids of Mssrs. Gil and Zé. Nation Beat is obviously the product of a different era, and their fusion isn’t as interested in the political sparks that might fly off their cultural experimentation.

But that doesn’t make what the group pulls off on “Legends of the Preacher” any less startling. The band’s choice to fuse the musical strains here is, in and of itself, a radical idea, and the sophistication and unadulterated passion the group brings to its enterprise elevates the sound into the intoxicating. And Nation Beat can get there with little more than a gorgeous voice and some background noises, as evidenced on “Banzeiro.” An assortment of rhythmic noisemakers — making sounds like old mattress sprigs uncoiling — provides the stark setting for Ms. Araújo’s seductive voice, singing at a saturnine pace, with an occasional backing voice joining in. No idea what she’s singing, but this threadbare song conveys its overwhelming emotion regardless.