  A new set-up to measure acoustic wave velocities through deforming rock
samples at high pressures (up to 1000 MPa), temperatures (up to 700$^\circ$C)
and differential stress (up to 1500 MPa) has been developed in a recently
refurbished gas medium triaxial deformation apparatus. The conditions span a
wide range of geological environments, and allow us to accurately measure
differential stress and strains at conditions which are typically only
accessible in solid medium apparatus. Calibrations of our newly constructed
internal furnace up to 1000 MPa confining pressure and temperatures of up to
400$^\circ$C demonstrate that the hot zone is displaced downwards with
increasing confining pressure, resulting in temperature gradients that are
minimised by adequately adjusting the sample position. Ultrasonic velocity
measurements are conducted in the direction of compression by the
pulse-transmission method. Arrival times are corrected for delays resulting
from the geometry of the sample assembly and high-precision relative
measurements are obtained by cross-correlation. Delays for waves reflected at
the interface between the loading piston and sample are nearly linearly
dependent on differential applied load due to the load dependence of interface
stiffness. Measurements of such delays can be used to infer sample load
internally. We illustrate the working of the apparatus by conducting
experiments on limestone at 200 MPa confining pressure and room temperature and
400$^\circ$C. Ultrasonic data clearly show that deformation is dominated by
microcracking at low temperature and by intracrystalline plasticity at high
temperature.
