Willie Nelson, “Bluegrass.”

It’s been said the mark of a good popular song is one that can be performed with only an acoustic guitar and a voice and still resonate.

A similar theory posits that a good song can be good in a variety of styles. The latest bit of proof there is the new album from Willie Nelson—his 151st including live and compilation collections!—titled simply Bluegrass. It’s truth in advertising, as the 12 songs here, all either written or co-written by Nelson over the years, are performed using the standard bluegrass instrumentation. Mandolins, fiddles, dobros, banjos, acoustic guitars and classic harmonies give most of these songs a different feel than in their earlier country, pop or hybrid incarnations, and generally speaking it’s a breath of fresh air.

Most of the best numbers on Bluegrass are the ones that adhere to the most traditional mid-tempo bluegrass form, like the classic stuff birthed by Bill Monroe and the Bluegrass Boys from the mid-1940s into the mid-1960s, with instrumental solos over a bed of fingerpicked stringed instruments, with a lead voice supported by gentle, sympathetic backing singers.

“No Love Around” establishes the pattern here, merging that bluegrass sound with the kind of lyrics often more associated with straight country than bluegrass: “Poured my dreams, drank ‘em down, because there weren’t no love around.”

“Man With the Blues” and, especially, “Bloody Mary Morning” come from a similar place, both instrumentally and sentimentally. The veteran members of Nelson’s “Bluegrass Ensemble” are expert players, and mandolinist Dan Tyminski (of Alison Krauss and Union Station), dobro player Rob Ickes, fiddler Aubrey Haynie and guitarists Josh Martin and Bobby Terry really shine here and elsewhere.

Willie Nelson also gives some of his bluesier old songs the bluegrass treatment here. The best one of those on Bluegrass is probably “Home Motel,” on which Ickes’ dobro is the doppelganger of the electric slide guitar Eric Clapton or even Elmore James would bring to a rock or hard blues version of a song like this. It’s easy to imagine “Home Motel,” done with loud electric instruments and exquisitely sloppy drumming, on one of the later Muddy Waters albums done with Johnny Winter on the Blue Sky label. Nelson, though, infuses lyrics like “What used to be my home … has changed into just a place to stay” with the needed impact at lower volume.

Curiously, the songs on which the bluegrass approach has the least impact are two of Nelson’s best known ones. The versions of “Good Hearted Woman” and “On the Road Again” weren’t reimagined so much as simply done with bluegrass instrumentation, and these versions aren’t necessarily different enough to justify the effort.

On the other hand, “Still is Still Moving to Me” is transformed not only in the bluegrass style, but in more of a New Grass way that takes the traditional rhythms and stretches their boundaries in ways Monroe and his band only hinted at on their most progressive days.

By that yardstick, “Still is Still Moving to Me” is the best song on the album. But that assessment lessens the beauty of many of the other songs on Bluegrass. At its heart, this music is simple, but done well—as it is with these master players—and is beautiful in its simplicity.

And Nelson himself? At 90, his voice may not be quite as piercing as it once was, but it’s still clear and full of emotion. One can only hope his 152nd album is coming down the pike.

Follow journalist Sam Richards at Twitter.com/samrichardsWC.