This is what it says:
Initially unable to get either duck to quack, they began chattering when paired. Initially, no echo could be found, so the team moved to an anechoic chamber for comparison. When examined by an audio expert, it was found that the echo was "swallowed" by the original quack, due to the very similar acoustic structure between the quack and the echo. Because of this, it may be difficult to tell where the quack ends and the echo begins, both having similar waveforms on an oscilloscope and blending together in a way that makes them difficult to distinguish. In the same way, human hearing may not perceive the difference between a duck's quack and its echo.