  Ultrasonic acoustic fields have recently been used to generate haptic effects
on the human skin as well as to levitate small sub-wavelength size particles.
Schlieren imaging and background-oriented schlieren techniques can be used for
acoustic wave pattern and beam shape visualization. These techniques exploit
variations in the refractive index of a propagation medium by applying
refractive optics or cross-correlation algorithms of photographs of illuminated
background patterns. Here both background-oriented and traditional schlieren
systems are used to visualize the regions of the acoustic power involved in
creating dynamic haptic sensations and dynamic levitation traps. We demonstrate
for the first time the application of back-ground-oriented schlieren for
imaging ultrasonic fields in air. We detail our imaging apparatus and present
improved algorithms used to visualize these phenomena that we have produced
using multiple phased arrays. Moreover, to improve imaging, we leverage an
electronically controlled, high-output LED which is pulsed in synchrony with
the ultrasonic carrier frequency.
