A steerable acoustic waveguide apparatus includes plural plates arranged in one or more linear arrays. Steering of an acoustic beam radiated from the waveguide apparatus may be achieved through differential delays of acoustic signals resulting from differences in timing, frequency, or mode or resulting from differences in physical attributes of the plates. In one aspect, where the apparatus is used with a target that is confined by a conduit such as a pipe, a longitudinal plane for the conduit intersects the linear array, an array plane for the linear array has a conduit orientation relative to the longitudinal plane, and the conduit orientation is substantially perpendicular or substantially parallel. The waveguide apparatus may serve as a thermal buffer and may simplify access to an acoustic path in a device such as an ultrasonic flowmeter.