Sudden and dramatic changes in single-bubble sonoluminescence (SBSL) intensity and spectral profiles occur at a critical acoustic pressure for solutions of sulfuric acid containing mixtures of air and noble gas. This intense SBSL is due to a plasma. (See p. 9315, “Plasma Quenching by Air during Single-bubble Sonoluminescence”; Photo courtesy of David J. Flannigan and Kenneth S. Suslick.)
Flannigan, D. J.; Suslick, K. S. "Plasma Quenching by Air during Single-Bubble Sonoluminescence"
J. Phys. Chem. A, 2006 110, 9315-9318.