Pressure-sensitive paint is presented and evaluated in this article as a quantitative technique for measurement of acoustic pressure fluctuations. This work is the culmination of advances in paint technology which enable unsteady measurements of fluctuations over 10 kHz at pressure levels as low as 125 dB. Pressure-sensitive paint may be thought of as a nano-scale array of optical microphones with a spatial resolution limited primarily by the resolution of the imaging device. Thus, pressure-sensitive paint is a powerful tool for making high-amplitude sound pressure measurements. In this work, the paint was used to record ensemble-averaged, time-resolved, quantitative measurements of two-dimensional mode shapes in an acoustic resonance cavity. A wall-mounted speaker generated nonlinear, standing acoustic waves in a rigid enclosure measuring 216 mm wide, 169 mm high, and 102 mm deep. The paint recorded the acoustic surface pressures of the (1,1,0) mode shape at approximately 1.3 kHz and a sound pressure level of 145.4 dB. Results from the paint are compared with data from a Kulite pressure transducer, and with linear acoustic theory. The paint may be used as a diagnostic technique for ultrasonic tests where high spatial resolution is essential, or in nonlinear acoustic applications such as shock tubes.

Further results from microgravity flight experiments on acoustic shaping are discussed. In previous experiments, the authors showed that straight and curved walls could be formed using styrofoam and cereal pellets of arbitrary shape seeded in a sound field in a resonant chamber. The wall shapes corresponded to constant-pressure-amplitude contours of the sound field; however, the walls have multiple stable locations near the nodal planes. Low acoustic energy input found to be sufficient in microgravity. Here the observations are related to predictions of particle and flowfield behavior. For the low-order modes used here, acoustic streaming is an important factor in transporting particles through the chamber. Simulations using multiple harmonics indicates possibilities for constructing and transporting shapes inside the container. Flow visualization supports the streaming predictions. The streaming velocity corresponds to the predicted values for the measured standing wave pressure amplitude. Visualizations of nodal planes show the formation of shapes of specified curvature. G-jitter is of the order of 0.03g, comparable to the peak accelerations due to acoustic radiation force, limits wall formation and control time during each parabola. Complex shapes can be formed with a variety of materials in microgravity. Tailoring the sound field to specific shapes and materials appears to be within reach.

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Previous work has shown the formation of solid particles into thin walls of specified shape using a resonant acoustic field in microgravity. Here these results are summarized, and extended to study the qualitative effects of liquid addition and melting/ solidification in the acoustic field. Pure liquid forms into sheets, even in the 1-g environment, due to the static pressure differences in the resonant chamber; however these sheets exhibit instabilities, and shatter into droplets. Stable thin walls are formed from liquid with suspended powder in 1-g. With some adjustment of the frequency, the use of processes involving heating, cooling and phase changes are seen to be feasible. The implications of these findings to non-contact manufacturing and construction in space are discussed.