Pressure-sensitive paint (PSP)

As an array of optical microphones. An optical microphone responds to an acoustic signal by modulating light rather than, say, voltage. If that microphone is an oxygen-sensing molecule—a luminophore—embedded in a coat of paint and if the luminophore is in an excited state, then the paint’s luminescent intensity is reversibly changed as the oxygen’s partial pressure changes. That’s PSP, which has been used in one-dimensional and single-spot acoustic experiments. Going to 2D, scientists from Purdue University and the Georgia Institute of Technology prepared a special PSP that is porous enough to allow fast diffusion of air into it, and used it to coat a 365-cm² inner wall of a rectangular resonant cavity. With an operating loudspeaker mounted on another wall and a light source to continually excite the luminophores, the researchers demonstrated the system’s viability by using a CCD camera to record a particular mode of an acoustic standing wave at the wall. Shown here are images taken at phases of 0, π/2, and π. The data provided a complete history of the pressure at more than 137 000 pixel locations across the wall. The acousticians say that the technique can measure pressure amplitudes as low as 50 Pa (atmospheric pressure is about 10⁵ Pa) and has no theoretical upper limit. (J. W. Gregory et al. J. Acoust. Soc. Amer. 119 , 251, 2006 http://dx.doi.org/10.1121/1.2140935 .)