Directional wave separation in tubular acoustic systems - the development and evaluation of two industrially applicable techniques

摘要

An acoustic device is used to evaluate internal features and defects within tubesby determination of the acoustic impulse response. This paper concerns meth-ods of separating the total pressure wave measured in the device into its for-ward and backward travelling components, which facilitates computation of theacoustic impulse response. The device comprises a tube that has a speaker atone end and is axially instrumented with microphones. Unlike similar works,the methods presented in this paper were designed to be applied in an indus-trial context, they allow simple calibration and implementation using readilytransportable equipment. Two wave separation algorithms are presented; thefrst is a known method that has been improved to provide simplified calibra-tion and the second is a computationally inexpensive technique that has beenadapted to improve its operational bandwidth. The techniques are criticallyevaluated using a custom built test rig, designed to simulate realistic tube fea-tures and defects such as constrictions, holes and corrosion. It is demonstratedthat, although inter-microphone attenuation is not accounted for in the secondalgorithm, both algorithms function well and give similar results. It is con-cluded that the added sophistication of the frst method means that it is lessaffected by low frequency interference and is capable of yielding more accurateresults. However, in practical use as an evaluation tool, the benefits of includ-ing inter-microphone attenuation are outweighed by the additional calibrationand computational requirements. Finally the output of the wave separationtechniques is validated by showing agreement between experimental impulse re-sponse measurements and those obtained from a theoretically derived acoustic tube simulator.