Acoustic emission (AE) is a widely used technology to study source mechanisms and material properties during high-pressure rock failure experiments. It is important to understand the physical quantity that acoustic emission sensors measure, as well as their response as a function of frequency. This study calibrates the newly built AE system in the MIT Rock Physics Lab using a ball-bouncing system. Full waveforms of multi-bounce events due to ball drops are used to infer the transfer function of Lead-Zirconate-Titanate (PZT) sensors in high-pressure environments. The uncertainty of sensor transfer functions is quantified using a waveform-based Bayesian algorithm. The quantification of in situ sensor transfer functions makes it possible to apply full waveform analysis for acoustic emissions at high pressures.