Mario Wolf

Modelling of sound propagation in media with continuously changing properties towards a locally resolved measurement of sound velocity

A new method to measure sound velocity and distance in homogeneous media simultaneously had been developed. It works with only one transducer and needs no additional reflectors at known positions. Instead the echoes of moving scattering particles are analyzed to determine the focus position as a second piece of information. An annular array allows to move the focus along the acoustic axis of the transducer and so to measure locally resolved. To reach also high accuracy for media with continuously changing properties, the sound field of the transducer has to be calculated within these media. A first approach for simulation by using Fermats principle is presented and compared to measurement results. It can be shown, that this model is not sufficient and that the wave equation with additional terms has to be derived and solved.

Improvement of the resolution limit caused by the width of the sound beam

Determining of size and shape of reflector with ultrasound imaging is subjected to several limitations. Beside the wavelength, the sound beam width is a second known constraint. However, the resolution can also be restricted by the analysed reflector itself by its shape, alignment, surface roughness and material. Furthermore, different reflectors can be displayed identically with a simple c-scan, so that no differentiation is possible. In this work two different basic reflectors, balls and circular disks, were analysed. An easy method to distinguish between them will be introduced. In addition, several approaches for estimating the size of balls with ultrasound will be presented.

Simultane Bestimmung von Dicken und Schallgeschwindigkeiten geschichteter Strukturen

Zusammenfassung In diesem Beitrag wird ein Verfahren zur simultanen Bestimmung von Schichtdicken und Schallgeschwindigkeiten von zweischichtigen Strukturen vorgestellt. Um zusätzliche Information zu gewinnen, wird neben der Laufzeit die Amplitude ausgewertet. Die Amplitude einer reflektierten Schallwelle hängt von der Position der Grenzfläche im Schallfeld des Schallwandlers ab und ist maximal, wenn sich die Grenzfläche im Fokus befindet. Mit Hilfe eines Annular-Arrays wird die Fokusposition variiert und die Amplitude als Funktion der eingestellten Fokussierung ausgewertet. Auf diese Weise werden Schallgeschwindigkeiten und Dicken von Stahl- und Aluminiumplatten nach einem Wasservorlauf bestimmt.

Simultaneous measurement of thickness and sound velocities of each layer in multi-layered structures

In this contribution, a novel method for determining the thickness of one layer and its sound velocity simultaneously with only one probe will be presented. Furthermore, an approach for the determination of thickness and sound velocities for two or more layers is discussed.

Transducer characterization by sound field measurements

The paper discusses different methods for characterizing an ultrasonic transducer by sound field measurements and introduces a novel easy-to-implement method besides the commonly known point reflector and hydrophone measurement methods. The characterization methods that are presented are particularly suited to measuring the actual transducer element size and determining fabrication details and asymmetries, where the necessary information is derived from the position of the ultrasonic focus and the structure of the sound field. The procedure is discussed on the basis of the following practical problems: measurement of the acoustically relevant element size of a planar 3-MHz annular array made of lead zirconate titanate (PZT) using a single point reflector; visualization of inaccuracies, asymmetries, and fabrication details for different setups with transducer frequencies between 3 and 50 MHz; determination of the element sizes of the single elements of a spherically curved 9-MHz sparse annular array and examination of the transducers focusing characteristics in a fluid containing scattering particles; and determination of the focus position of a 9-MHz single-element transducer with acoustic lens and comparison between two lens materials.

Determination of size and shape of small inclusions from sound-field information

The resolution of scanning ultrasonic systems is limited to the wavelength used and the focus size. Common methods for determination of object sizes close to or even smaller than wavelength require knowledge of the object geometry and use absolute amplitude information only. We present new approaches for determining shape and size of small objects by additional evaluation of sound field effects like the directivity of reflected waves. First measurements show that determining size and shape of objects smaller than the size of the sound field is possible with geometric considerations on maximum amplitude positions of different reflection modes. Practical usable relative evaluation criteria for time changes in dependence of displacement can be found for ball sizes down to 1/5 wavelength.

Approach for Simultaneous Determination of Thickness and Sound Velocity in Layered Structures Based on Sound Field Simulations

This paper describes a non-invasive, nondestructive method for the simultaneous determination of sound velocity and thickness of the different layers of a layered structure by means of ultrasound. It will be demonstrated how further information about the reflected sound field, in addition to the time of flight, is acquired by using annular arrays. Because of this supplementary information, reflectors or other probes at known distances are not necessary and the specimen does not have to be placed in a medium with known sound velocity. Two different evaluation methods combined with a geometric model are explained. To improve the accuracy, measured signals are also evaluated by a wave propagation model.

Investigation of Embedded Structures in Media with Unknown Acoustic Properties

For the nondestructive evaluation of components with ultrasound a priori information about the specimen is necessary. So the time of flight to a defect is measured and, with known sound velocity, it is possible to determine the correct location of the defect. In general, the sound velocity is assumed as known. If it is not known, the sound velocity has to be determined additionally. This can be done, for example, by measuring the time of flight to the backwall with ultrasound and the thickness of the specimen with a caliper gauge. However, this is impossible to realize with single-sided access to the specimen. For determining the size of inclusions, several techniques like the half-value method or the DGS-method (Distance Gain Size) are established. These methods are based on the assumption of (circular) plane reflectors. Therefore, they cannot be applied on the size determination of inclusions with curved surfaces.

Noninvasive, locally resolved temperature monitoring via simultaneous measurement of sound velocity and distance

A locally resolved monitoring of temperature distribution allows to investigate and optimize many industrial processes, like mixing or chemical reactions, as well as medical therapy, like hyperthermia for cancer therapy. In this paper a new measurement technique is applied, which allows to measure sound velocity and distance simultaneously without any reflector at a known position. Instead, the echoes of moving scattering particles are used to determine distance and sound velocity simultaneously. The focal position, depending on sound velocity, can be used as additional information besides the time of flight. Averaging over a sufficient number of echo signal amplitudes, enables to suppose a uniform distribution of scattering particles. Thus. the maximum of average echo amplitude is correlated to the focal position. An annular array, whose elements are driven with calculated time lags, is used to move the focal point along the acoustic axis of the transducer. This allows to measure sound velocity locally resolved and noninvasively. As the sound velocity is well known as a function of temperature [1], a temperature gradient was generated in an experimental set-up with a heat source at the top and a cooling at the bottom. It has been shown that a reconstruction of a minor, approximately linear sound velocity gradient is possible with the measurement technique. The resulting temperature gradient was additionally monitored and confirmed by measurements with a linear array of temperature sensors.

Measurement of sound velocity profiles in fluids for process monitoring

In ultrasonic measurements, the time of flight to the object interface is often the only information that is analysed. Conventionally it is only possible to determine distances or sound velocities if the other value is known. The current paper deals with a novel method to measure the sound propagation path length and the sound velocity in media with moving scattering particles simultaneously. Since the focal position also depends on sound velocity, it can be used as a second parameter. Via calibration curves it is possible to determine the focal position and sound velocity from the measured time of flight to the focus, which is correlated to the maximum of averaged echo signal amplitude. To move focal position along the acoustic axis, an annular array is used. This allows measuring sound velocity locally resolved without any previous knowledge of the acoustic media and without a reference reflector. In previous publications the functional efficiency of this method was shown for media with constant velocities. In this work the accuracy of these measurements is improved. Furthermore first measurements and simulations are introduced for non-homogeneous media. Therefore an experimental set-up was created to generate a linear temperature gradient, which also causes a gradient of sound velocity.