Echo removal in tubular acoustic systems: Passive and active techniques

Acoustic pulse reflectometry (APR) has been shown to be a very capable means of identifying features in tubular objects. APR systems excite a test object with a sound wave and listen for reflections, indicating the presence of features in the test object. An undesirable effect of this process is that the returning sound wave is re-reflected by the loudspeaker and re-enters the system. This paper presents two complimentary techniques that may be used to remove unwanted echoes in APR systems. The first approach uses two axially separated microphones to separate forward and backward propagating waves. This passive technique is shown to be highly capable of cancelling undesired echoes in the system. The second approach actively cancels unwanted echoes by introducing a phase inverted version of the wave that is incident on the loudspeaker. The active cancellation operates in real-time using the measured backwards propagating wave. As a consequence of the proposed techniques, the effectiveness of APR when applied to detecting features within tubular systems is improved considerably. The empirical results presented at the conference will demonstrate that corrosion effects, such as holes and pits, located in short lengths of pipes, can be detected clearly within seconds.