Post-rock? Definitely, maybe. Psychedelic? Probably certainly. Krautrock? A shred. It’s best to avoid classifying Mogwai and instead appreciate their swelling instrumental rock, whatever it is. A caveat: Anyone stuck on music with lyrics will want to stop reading here. There are no vocals to be found on the band’s entire sixth album, “The Hawk is Howling.” (Oh, and ignore the titles of everything, the band confesses that they just tend to make up whatever sounds good. My favorite is “The Sun Smells Too Loud.”)

Additionally, the music requires that the listener invest a certain degree of time and attention. Most songs lie in the 5-8 minute range and follow a theme and variation arc. Obviously this is not electric Beethoven, but it is important not to view Mogwai as a good rock band that is missing a vocalist. The group creates full, expressive music without lyrics.

The music itself exists in a plane between progressive rock and trance. While each track does center on and return to a simple vamp, the music is hardly static, as that motif is tested out and pulled in different directions. Conversely, during the instrumentals there are no real guitar heroics à la a jam or prog band. Rather, Mogwai progresses by layering variously textured synth, piano, bass and, at the forefront, effected guitar.

Each track ebbs and flows through different dynamics, yet holds a steady, relaxed 4/4 or 3/4 time. The greatest complaint against “The Hawk is Howling” is that the tracks all tend toward the soft-heavy-briefly soft dynamic sequence and cease to surprise the listener after a few pieces. Also, the deliberate (read: kind of slow) pacing works well within each track individually, but as a collection, the album feels like it needs a kick in the pants somewhere.

“Precipice,” the last track, is perhaps the best example of Mogwai’s ability to articulate without words, and showcases the band’s talent on every instrument. “I Love You, I’m Going to Blow Up Your School” also gets gold stars for a furious and fuzzy breakdown at the end that is well worth the exposition taken to set it up.

Like many of the finer things in life, “The Hawk is Howling” might not appeal to listeners raised on the three-minute pop song and seeking instant gratification. The album necessitates a commitment from its audience to take their time to savor the nuances of the band’s performance of each layer of the music. For a listener ready to appreciate rock music with both structure and development, Mogwai has set a delicious table.