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

An acoustic device is used to evaluate internal features and defects within tubes
by 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 the
acoustic impulse response. The device comprises a tube that has a speaker at
one 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 readily
transportable equipment. Two wave separation algorithms are presented; the
frst is a known method that has been improved to provide simplified calibra-
tion and the second is a computationally inexpensive technique that has been
adapted to improve its operational bandwidth. The techniques are critically
evaluated using a custom built test rig, designed to simulate realistic tube fea-
tures and defects such as constrictions, holes and corrosion. It is demonstrated
that, although inter-microphone attenuation is not accounted for in the second
algorithm, both algorithms function well and give similar results. It is con-
cluded that the added sophistication of the frst method means that it is less
affected by low frequency interference and is capable of yielding more accurate
results. However, in practical use as an evaluation tool, the benefits of includ-
ing inter-microphone attenuation are outweighed by the additional calibration
and computational requirements. Finally the output of the wave separation
techniques is validated by showing agreement between experimental impulse re-sponse measurements and those obtained from a theoretically derived acoustic tube simulator.