It's a truism of club music that as soon as a subgenre emerges and becomes defined, its boundaries immediately start to blur -- and the more popular that subgenre becomes, the more its defining edges get eaten away by fusions, variations, and innovations. For one example of how that happens, check out the tracks featured on Elevator Music, Vol. 1, a compilation that documents a subgenre presented in the press materials as "140 bpm bass music." Much of it (like the pleasingly bouncy "If That's How I Feel" by Hot City) comes off feeling like whimsical and moderately deep house music; elsewhere, such as on Octa Push's "Doctor Bayard," you'll hear more than a hint of dubstep. (That last track also gets the award for Weirdest Rhythmic Use of Body Noises.) In fact, despite the pervasively thumpy house beats that predominate, it's dubstep and its variants that seem always to be lurking in the corners of these tracks: you hear it in the lurching syncopations of xxxy's generally pretty tiresome "Sing with Us" and in the slippery beats of Skinnz's "Ukraine." On the other hand, the marimbas and accordion on Hackman's "Pistol in Your Pocket" evoke the pattern-based minimalism of late-'60s Steve Reich, while the multi-layered Latin percussion on "Drums in the Deep" by Doc Daneeka creates an ambience all its own. (Then there's "Encoded," by Om Unit, which makes a misguided attempt at creating house music in 6/8 time.) The album's strongest track is also the one that generates the least consistent groove: "One of the Same," by the perennially excellent Caspa & Rusko, is powerfully funky and crazily varied in tone and rhythmic pattern. Most of this music is interesting, and a good chunk of it is compelling; all of it is worth a listen.