Methods and systems for acoustically treating material are described. Acoustic energy is emitted into a waveguide having a wall with a shape that tapers away from the acoustic source. The wall may have a substantially conical or parabolic shape. When emitted therein, the waveguide causes convergence of the acoustic energy so as to form an acoustic focal zone at a vessel. In some embodiments, a substantial portion of the acoustic focal zone is located outside of the internal volume defined by the waveguide.