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Measurement Engineering Group (EMT)
Prof. Dr.-Ing. Bernd Henning
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Research Topic Basics

Acoustic waveguide

In many applications, for example in ultrasonic flow metering, the flow channel can be approximated as acoustic waveguide. Due to this fact, the characterization and calculation of acoustic waveguides are a crucial aspect of the design and optimization process of acoustic sensor systems. For this purpose analytic, semi-analytic and numeric methods are also used beside classical Finite Element Method (FEM), to evaluate the dispersion relations in different waveguide geometries. 

For easy waveguide geometries semi-analytic methods, such as the GMM (Global Matrix Method), can be used. In a geometrical sense more complex cross-section geometries, which stay constant in axial expansion, can for example be designed and evaluated with the help of the Waveguide Finite Element Method (WFE-Method).

In acoustic waveguides multiple waveguide modes are able to propagate through the structure. Every mode therefore has its individual shape in terms of sound pressure distribution within the waveguide’s cross-section. Based on simulations, these pressure distributions can be predicted and hence, sensitivity studies can be applied to the given measurement task. For example, one may immerse a solid waveguide into a fluid for analysis for its rheological properties. Within a simulation one may identify those waveguide modes that have high sensitivity to the change of certain rheological properties of the fluid, which are in general those modes with high energy flux at the waveguide’s boundaries. Furthermore, after having identified certain waveguide modes, one is also able to predict possibilities for the selective excitation of these modes.

Time reversal acoustics

The very young discipline of time reversal acoustics is concerned with generation of locally restricted ultrasonic pulses. Here the undisturbed interference of ultrasonic waves is used, which constructively results in local maxima or destructively in local minima of the sound pressure. This is done by emitting an initial sound pulse, recording it, and then chronologically reversing and emitting the received signal again back to the initial point.

Every change in the acoustic system during the time reversal process results in an audible amplification of noise in comparison to the actual signal. This effect is similar to the very sensitive “Schlieren-effect” from optics and can be used in many ultrasonic applicatio

Level measurement

In chemistry, food-processing or biotechnology liquid levels have to be measured without accessing the container physically. As a solution, the Measurement Engineering Group has extended the known ultrasonic puls-echo-method, which will also work when built-in components such as heating pipes block the direct path between sensor and the liquid's surface. Based on the time reversal principle a virtual sound source is created at the certain point in the liquid.

The method can be divided into two phases. In the first phase a calibration is done by sending a short acoustic signal from the wanted filling level, down towards the receiver at the bottom of the tank to determine the transfer function. Due to interactions with the tank’s boundaries the receiver records a series of pulses. In the second phase the receiver is used as transmitter and sends the time reversed received signal back into the vessel. This way the actual sequence is produced on or at least close to the surface. The resulting answers are then correlated with the recorded signal sequences. Extracting the correlation maximum the time of flight can be identified and the actual filling level is calculated. FEM-simulation approves the viability of the introduced process, even though the tank geometries or the exact position of built-in devices have a significant impact on the quality of the refocusing. Methods for optimizing the region of interest are being used to adjust the tank's structure, resulting in increased robustness and high measurement accuracy.

An alternative variant, which does not use the liquid, but the tank itself as an acoustic medium, can be used to get additional information about liquid-gas-phases and is currently being reviewed in simulations and experiments.

Selective acoustic irradiation

In different audio applications, the aim is to offer information only to selected areas. This selection can be reached by conventional array techniques when using the acoustically non-linear properties of air or with the use of additional time-reversed acoustics.

In doing so a pulse-like acoustic event (cracking a balloon) is emitted and received by a microphone array. Many reflections on walls and obstacles expand the initial pulse on its way to the receivers. The chronological reversion of the received signal and the consecutive transmission from the same place as the microphone-array is placed, using a loudspeaker-array, results in a spatial and temporal limited sound event at the actual point of source. Other experiments show that the focusing is enhanced with increasing amount microphone-speaker-pairs.

The information on the ambient conditions known from the time signal can be used to generate speech or music at a particular place. The aim is to set up acoustic markers or direction signs in museums, without creating noise pollution in the whole room. First trials showed that an implementation in a room and in HIFI quality would need about 30.000 speakers, which is not workable at the moment. This is why the Measurement Engineering Group is trying to develop methods and systems to be able to offer the same quality with less effort.

Determination of material properties

The determination of material properties is a core part of numeric simulations concerning ultrasonic phenomena. If the simulation is supposed to be realistic and shall give quantitatively correct results, the parameters for the simulation have to be known very well. This is especially true for virtual sensor design and sensor optimization. Furthermore, analysis of material properties of accelerated aged materials helps to predict their time behavior.

Solids materials

Most of the known methods for determining these properties are only useful for rather low frequencies. Some manufacturers determine their material properties based on static tensile tests, so the values cannot be used in ultrasonic applications. A well-established ultrasonic method to determine material characteristics is the immersion technique; this just is non-applicable for most polymers, because of their hygroscopic properties. The measurement conditions should especially be isobaric, isothermal and isochronic as a change of temperature by only 1 K can result in an error of 50% when calculating poisson's ratio. The Measurement Engineering Group has developed a method able to determine all acoustic material properties of a pre-conditioned sample using only one transmission measurement. The procedure is based on multimodal wave propagation within the sample under test. An inverse approach is used to determine all material properties under consideration. An immersion of the sample in a water tank is no more necessary. The measurement is realized by using only one single ultrasonic pulse and is taken place in a climatic chamber for defined and adjustable measurement conditions. Thus, the influence of temperature with respect to the acoustic material characteristics can be analyzed. The temperature dependence (and therefore the frequency dependence) of material characteristics is especially essential with respect to the acoustic analysis of polymers. Our goal is to evaluate the dependence on frequency as well as orientation of the sound velocities, acoustic absorption and acoustic impedances within the practical operation conditions (temperature, humidity, material exposure, aging, etc.). The knowledge about the material behavior considering its specific conditions is essential for a problem orientated simulation based design of modern ultrasonic sensors systems as well as for localization of a stable design point.

Piezoelectric materials

Besides the material properties of solids, also the piezoelectric properties of piezoceramics, as active elements, play an important role in the simulation of ultrasonic sensors and actuators. This is why the Measurement Engineering Group is intensively working on creating processes for determining the necessary parameters for numeric simulations. Main aspects are a good correlation between measurement and simulation and a high rate of reproducibility. A measuring setup has been made up, which, using different optimization strategies (genetic algorithms, gradient descent), seeks the dataset. It works automatically to a large range and is permanently upgraded. The initial dataset comes from the data published by the piezoceramic's producer, even though the data can usually not be transferred to the frequency spectrum in ultrasonics and include a high rate of uncertainty. A consistency check, in line with DIN IEC 483, has been implemented, so that the results can directly be used in the simulation.

Non-linear acoustics - parametric array

With high intensity ultrasound and by using non-linear effects in air, audible sounds can be created. This effect is already used in so called parametric speakers (Audio Spotlight, HSS, Sennheiser).

They produce a focused sound beam emitting to a limited area. This system is used for transmitting navigation information, only audible to the driver of a car. Though when implemented in small rooms, such as vehicles, the benefits are reduced due to reflections from the car glass and other bounding surfaces and the consequential scattering of sound into different directions. The reduction of reflection through properly designing the emitter, the array geometries and the signal pre-processing are crucial for an effective application.

Schlieren measurement

There are different measuring techniques for visualizing an acoustic field in water. A fast and non-invasive method is the Schlieren technique.

Using this method, the local change of density is evaluated with help of a laser. By using optical filtering the measurement quality can be increased. To extend the created picture of the integral intensity to a spatial view, the acoustic field has to be analysed from different directions. With this data and the help of tomographic processes the acoustic field can be sterically reconstructed.

For verification and quantitative comparison a hydrophone is available which also is used in combination with a multidimensional handling system to scan the acoustic field. By this two different ways of visualisation are available, and used depending on the objective.

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