Acoustek® uses a principal known as Acoustic Pulse Reflectometry (APR). According to APR theory, an acoustic wave propagating along a pipe will generate reflections whenever it encounters a change in acoustic impedance. These variations in the acoustic impedance are directly related to changes in the cross-sectional area of the pipe.
The probe (or 'gas-gun') used to conduct the acoustic surveys consists of a sound source to generate a pressure pulse that propagates along the pipe and a transducer to record the reflected signal from any features within the pipe.
The figure below shows a schematic of the gas-gun attached to a generic pipe with a partial blockage downstream.
The pressure pulse leaves the probe and enters the pipeline, passing the microphone on its way and generating a positive pressure pulse (labelled “A” in the figure above). The pulse then propagates along the pipe and, when it reaches the feature, some of the energy is reflected back towards the probe. The microphone then detects this reflected pulse, labelled “B”. With knowledge of the time of flight and the amplitude of the reflection, the distance and size of the feature can be determined.
Different features in the pipe cause different types of reflection. A reduction in the cross-sectional area of the pipe causes a positive reflection coefficient, with its amplitude dependent upon the variation in the cross sectional area. A complete blockage results in a reflection coefficient close to 1, meaning that almost all of the propagating wave is reflected back towards the source. An increase in cross-section would result in a negative reflection coefficient. This results in a reflected signal with the opposite polarity to the propagating pulse. Similarly to a complete blockage, an open ended pipe termination generates a reflection coefficient close to -1. Meaning that most of the signal is reflected back towards the pipe source.
Acoustek® can not differentiate between certain features within a pipeline. For example changes in cross-sectional area caused by debris or pipe fittings will produce similar signatures. Therefore, knowledge of the pipe network can aid in the effectiveness of any pipe survey, although this is not essential.