Separation of mode-specific signals in a fluid-filled circular waveguide

There are a couple of acoustic systems that are well suited for continuous and reliable concentration measurement of liquids, but the reliability is a great challenge: Most systems determine the amplitude ratio of deflected ultrasound bursts at different boundaries as well as the sound velocity of the liquid in order to calculate its acoustic impedance and therewith its density. The problem is the sensitivity of variations of reference material properties as well as abrasion or soiling of the boundaries. This contribution is about a new model-based method that uses the whole surface of an acoustic waveguide as reference boundary. On the one hand the integral over a large boundary decreases influence of small boundary defects. On the other hand it is possible to analyze different modes of sound in the fluid. This redundancy can be used to increase reliability. The decomposition of the received signal in its modal components by means of principle component analysis will be discussed in detail. It will be shown that it is possible to determine the amplitudes of each mode by means of one measured signal at the end of the waveguide, even if the transducer and the electronic components are of very simple kind.