  The IceCube Neutrino Observatory will be upgraded with about 700 additional
optical sensor modules and new calibration devices. Particularly, improved
calibration will enhance IceCube's physics capabilities both at low and high
neutrino energies. An important ingredient for a good angular resolution of the
observatory is a precise calibration of the positions of optical sensors. We
present the concept of newly developed acoustic sensors that are mounted inside
the optical modules and additional acoustic emitter modules that are attached
to the strings. With this system we aim for the calibration of the detectors'
geometry with a precision of 10\,cm by means of trilateration of the arrival
times of acoustic signals. This new method will allow for an improved and
complementary geometry calibration with respect to previously used methods
based on optical flashers and drill logging data.
