  Understanding the acoustic cavitation threshold is essential for minimizing
cavitation bioeffects in diagnostic ultrasound and for controlling
cavitation--mediated tissue ablation in focused ultrasound procedures. The
homogeneous cavitation threshold is an intrinsic material property of
recognized importance to a variety of applications requiring cavitation
control. However, acoustic measurements of the cavitation threshold in water
differ from those predicted by classical nucleation theories. This persistent
discrepancy is explained by combining novel methods for acoustically nucleating
single bubbles at threshold with numerical modeling to obtain a nucleus size
distribution consistent with first--principles estimates for ion--stabilized
nucleii. We identify acoustic cavitation at threshold as a reproducible subtype
of heterogeneous cavitation with a characteristic nucleus size distribution.
Knowledge of the nucleus size distribution could inspire new approaches for
achieving cavitation control in water, tissue, and a variety of other media.
