A6.1 - Guided Acoustic Waves for Liquid Property Measurement

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
A6 - Ultrasonic Sensors I
J. Rautenberg, F. Bause, B. Henning - Universität of Paderborn (Germany)
135 - 140


This contribution will give a survey on acoustic waveguides that are used for the measurement of liquid properties. Apart from research findings from literature, it will focus on the modelling, simulation and measurement of multimode wave propagation in acoustic waveguides.
So first of all the acoustic liquid properties (i.e. sound velocity, acoustic impedance and absorption) have to be introduced. They interrelate to the liquids rheological properties like density and viscosity or to other measured variables of interest, especially the concentration of a solute.
As guided acoustic waves range from the mm-scale to the km-scale, it is no surprise that they are used in lots of different scientific disciplines like seismology, geophysics, non-destructive testing, biological and chemical analysis or process measurement engineering. Some applications of different size and therewith frequency of operation will be discussed in detail, always additionally considering the special characteristics like in- and out-of-plane displacements. This leads to a second and more common way to classify acoustic waveguides: One family of sensors is characterized by the solid/liquid interface with Love-, leaky Rayleigh-, Scholte- or leaky Lambwaves on it. Another group is built by the pure liquid waveguides with single mode, bulk wave and multimode operation.
Most of the waveguide sensors are driven in such a way that only one specific mode is excited. This can be done by adopting the geometry of the waveguide to the frequency of operation and geometry of the transducer or vice versa. A basis for a proper sensor design as well as simulation of wave propagation is the dispersion relation. Taking also the shape of the modes into account it is possible to expand any acoustic excitation into its modal components and to propagate them through the waveguide. This procedure will be demonstrated for a hollow cylinder. Analytical simulation and measurement as well as FEM-results are in good agreement.
Finally the measurement of waveguide properties will be discussed. There are different ways to measure the dispersion relations. The most significant is to measure the out-of-plane displacements as a function of time and space at the end of the waveguide. The 2D - Fast Fourier Transform is a good approximation for the frequency-wavenumber plot. Less expensive but also less meaningful is the sonagram analysis of a detected signal at the end of the waveguide or the model based Principal Component Analysis.