Acoustic pulse reflectometry is a method of non-destructive testing used for locating and characterising features and defects in gas-filled objects such as tubes, pipes, and ducts. A sound wave is emitted into the waveguide and travels in the axial direction until it is partially or fully reflected at changes in cross-section. The recorded reflection sequence is analysed to reveal time-of-flight, amplitude, and reflection shape. Analysis of reflection sequences is problematic due to the presence of multiple repeated echoes. The principal contribution is the introduction and demonstration of a method of actively suppressing unwanted echoes by modifying the signal emitted by the speaker in real-time. Unlike previous work, the proposed system accounts for inter-transducer attenuation and does not require re-calibration for each test object. The proposed real-time active absorbing termination is implemented using a programmable embedded controller and shown to function effectively. A quantitative evaluation of available wave separation techniques is provided using a known metric for quantifying the performance of absorbing terminations is introduced.