  The scattering of ultrasound from bubbles of micrometer-sized radius, such as
used in contrast enhancers for ultrasound diagnostics, is studied. We show that
sound scattering and ``active'' emission of sound from oscillating bubbles are
not contradictory, but are just two different aspects derived from the same
physics. Treating the bubble as a nonlinear oscillator, we arrive at general
formulas for scattering and absorption cross sections. We show that several
well-known formulas are recovered in the linear limit of this ansatz. In the
case of strongly nonlinear oscillations, however, the cross sections can be
larger than those for linear response by several orders of magnitude. The major
part of the incident sound energy is then converted into emitted sound, unlike
what happens in the linear case, where the absorption cross sections exceed the
scattering cross sections.
