  A virtual acoustic source inside a medium can be created by emitting a
time-reversed point-source response from the enclosing boundary into the
medium. However, in many practical situations the medium can be accessed from
one side only. In those cases the time-reversal approach is not exact. Here, we
demonstrate the experimental design and use of complex focusing functions to
create virtual acoustic sources and virtual receivers inside an inhomogeneous
medium with single-sided access. The retrieved virtual acoustic responses
between those sources and receivers mimic the complex propagation and multiple
scattering paths of waves that would be ignited by physical sources and
recorded by physical receivers inside the medium. The possibility to predict
complex virtual acoustic responses between any two points inside an
inhomogeneous medium, without needing a detailed model of the medium, has large
potential for holographic imaging and monitoring of objects with single-sided
access, ranging from photoacoustic medical imaging to the monitoring of
induced-earthquake waves all the way from the source to the earth's surface.
