  Acoustic trapping uses forces exerted by sound waves to transport small
objects along specified trajectories in three dimensions. The structure of the
acoustic force landscape is governed by the amplitude and phase profiles of the
sound's pressure wave. These profiles can be controlled through deliberate
spatial modulation of monochromatic waves, by analogy to holographic optical
trapping. Alternatively, spatial and temporal control can be achieved by
interfering a small number of sound waves at multiple frequencies to create
acoustic holograms based on spectral content. We demonstrate spectral
holographic trapping by projecting acoustic conveyor beams that move
millimeter-scale objects along prescribed paths, and control the complexity of
particle trajectories by tuning the strength of weak reflections. Illustrative
spectral superpositions of static and dynamic force landscapes enable us to
realize two variations on the theme of a wave-driven oscillator, a deceptively
simple dynamical system with surprisingly complex phenomenology.
