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Bachelor Theses.

Design of a measurement setup for sound field mapping of ultrasonic transducers through pulse-echo measurements on a wire reflector

Daniel Eder

Piezoelectric ultrasonic transducers are used to generate and receive ultrasound. They can, therefore, be used for different measuring applications. In many cases, there is an interest in the spatial extent of the sound field. In the course of this bachelor thesis a measurement setup was built for measuring the geometry of the transducer's sound field and plot it as a two-dimensional image.
In the realized measuring setup, an ultrasonic transducer is mounted at a fixed position and is continously electrically excited. Meanwhile, a thin copper wire (wire reflector) is incrementally moved in front of the transducer. The wire reflects acoustic waves, that can be received by the transducer as an echo signal. The magnitude of this echo signal depends on the strength of the sound field at the position of the wire reflector. Evaluating the echo signal's magnitude for different positions of the wire reflector, enables the generation of an image of the sound field. The copper wire is tensioned by a specifically designed fixture and gets positioned using a 3-axis system. The entire measuring process is controlled with a computer with MATLAB.
In this thesis the sound field mapping of two different ultrasonic transducers was carried out. The first one was an unmodified transducer and the second one was a transducer with an acoustic coverging lens applied. In both cases, a measurement range of 100x80mm was investigated. With an increment size of 0,25mm this leads to approximately 130000 measurement points for one measurement process. It turned out that the measuring result for the unmodified transducer matches the idealized sound field from theory very well in terms of near-field length and emission angle. The second measurement with the lens shows the change of the sound field and a clearly pronounced focal area. Again, the measured focal distance matches the theoretically calculated value very well.

 

Figure 1: This picture shows the measurement setup for sound field mapping of ultrasonic transducers. Figure 2: This picture shows the result of the sound field mapping of the unmodified ultrasonic transducer.

Keywords: ultrasound, sound field mapping, ultrasonic transducer, pulse-echo measurement

September 9th, 2019