For an instrument that requires the simple act of inhaling and exhaling, the harmonica can be considerably difficult to master. With its problematic learning curve accounting for its lack of prominence in popular song — that is unless you're a Bob Dylan folkie in the middle of a solo, or Stevie Wonder playing “Isn't She Lovely” — the harmonica has an attentive home with Blues Traveler and the virtuosic playing of frontman John Popper. Blowing, bending and distorting his collection of harps, Popper and his respected blues-rock ensemble have maintained a lower profile since the chart-topping success of their 1994 album “Four” and its hit singles “Run-Around” and “Hook.” Rewarding die-hard fans' loyalty with constant touring and shows overflowing with improvisational energy, the band still has some great jams in its repertoire. Blues Traveler performs at Innsbrook Pavilion on Wednesday, July 28, at 6 p.m. $15-$20. 423-1775.