This paper demonstrates how flexural wave propagation in a thin plate can be modeled by estimating the combined effect of the excitation source signal and the impulse response of the ultrasonic sensor. The wave propagation in the plate is modeled using the wave equation for the flexural wave mode. A theoretical model for flexural wave propagation in thin plates has been derived, and it has been compared with measurements excited by tapping gently on the surface. The combined effects of the excitation source signal and the impulse response of the low-cost piezoelectric sensor are modeled using finite-impulse response and/or infinite-impulse response filters. Thereafter, the performances of the selected filters are compared on estimating the wave propagation in a thin quartz glass plate. Results indicate that the most accurate estimation of wave propagation has been obtained using a linear phase filter which attributes all dispersions to the flexural wave.

Ultrasound generated by means of laser-based photoacoustic principles are in common use today and applications can be found both in biomedical diagnostics, non-destructive testing and materials characterisation. For certain measurement applications it could be beneficial to shape the generated ultrasound regarding spectral properties and temporal profile. To address this, we studied the generation and propagation of laser-induced ultrasound in a planar, layered structure. We derived an analytical expression for the induced pressure wave, including different physical and optical properties of each layer. A Laplace transform approach was employed in analytically solving the resulting set of photoacoustic wave equations. The results correspond to simulations and were compared to experimental results. To enable the comparison between recorded voltage from the experiments and the calculated pressure we employed a system identification procedure based on physical properties of the ultrasonic transducer to convert the calculated acoustic pressure to voltages. We found reasonable agreement between experimentally obtained voltages and the voltages determined from the calculated acoustic pressure, for the samples studied. The system identification procedure was found to be unstable, however, possibly from violations of material isotropy assumptions by film adhesives and coatings in the experiment. The presented analytical model can serve as a basis when addressing the inverse problem of shaping an acoustic pulse from absorption of a laser pulse in a planar layered structure of elastic materials.

In this paper, a bending (flexural) wave mode in a thin plate is excited by tapping gently on the surface. The wave is recorded with single low-cost piezo-electric sensor. Due to the dispersive nature of this wave mode, the shape of the recorded signal depends on the flexural stiffness parameter of the plate. The recorded signal is also affected by the transfer function of the excitation and the sensor. In this paper we show, with simulations and measurements on a thin quartz glass plate, that the effect of the sensor and excitation can be decoupled from the bending wave, and that the flexural stiffness parameter can be estimated using a single sensor.

A way of describing photoacoustic measurement techniques is listening to light. Another way is as sensor techniques for characterisation of materials by means of light and sound interacting with the material under investigation. Photoacoustic measurements are emerging techniques and examples and suggestions of applications can be found foremost in diagnostics for medical purposes but also for industrial materials.The photoacoustic measurement techniques used in present thesis is based on the thermoelastic effect. A short light pulse is sent into a material, and energy from the light being absorbed leads to a localised heating. If the duration of the heating is short enough, a local pressure wave is built up, that propagates through the material as an ultrasonic pulse which can be detected by an ultrasound transducer and subsequently analysed.In photoacoustic measurement techniques, the advantage of high spatial resolution from measuring with ultrasound is combined with the advantage of high contrast from measuring with light. Ultrasound propagating in for example human tissue scatters less than what light does, enabling more precise positioning of the origin of the information in the measured signal. High spatial resolution is thereby achieved from measuring with ultrasound. High contrast can be reached from dierent responses to light in materials with diverse light absorption coecients. However, a disadvantage of the technique is that in biological tissue, light scattering is limiting the practically usable penetration depth.Besides for photoacoustic measurements, the thermoelastic effect can be utilised to generate laser-induced ultrasound for measurement applications. The present thesis' contribution is studies of laser-induced ultrasound in a thin semitransparent light-absorbing layer, experimentally and theoretically. To draw conclusions from measurements, one wish to know more about what happens in the absorbing layer, knowledge that could also be applied on corresponding processes in a material under study. An analytical model could thereto constitute a base for ultrasound pulse shaping, for adjusted measurements for specific applications.Through experiments on dyed polymer film, frequency spectra of laser-induced ultrasound were studied, varying thickness and light absorption coecient of the film layer structure. In the outcome of the experiments, decreased thickness was related to increased centre frequency as well as increased bandwidth of the generated ultrasound, and increased absorption together with decreased thickness was related to increased ultrasound amplitude.A one-dimensional analytical photoacoustic wave model of generation and propagation of heat-induced ultrasonic pressure wave in three material layers has been developed. From an existing model of three layers, this is an expansion enabling the two materials surrounding the light absorbing layer to be dierent. The analytical solution to the photoacoustic wave equation problem was based on a Laplace transform approach. Pressures resulting from the analytical model corresponds well to results from simulations. Analytical pressures were compared to ultrasound transducer experimental voltages, through system identication of the transducer system conversion between pressure and voltage. The system identication was however unstable. For ultrasound reception, experiments were performed on piezoelectric film with electrically conductive coatings as a transducer.

This paper presents a method for predicting the ultrasound pulses generated by thin semi-transparent polymer films, excited by a short laser pulse. The acoustic pressure is first modeled based on the physical properties of the polymer. Partial Least-Squares Regression is then used to link the model pressure to the ultrasound pulses measured by an ultrasound transducer. The uncertainty of the regression is also simulated, showing that the method is robust to noise in the measurements

We investigate laser-induced ultrasound generated in a plane semi-transparent layered polymer structure. The scope is to study relations between generated ultrasound, as e.g. amplitude, and centre frequency and bandwidth of its frequency spectrum, and properties of the polymer layers, like thickness and absorption. This knowledge can then be used when designing polymer film based, semi-transparent ultrasonic devices specifically for photoacoustic applications. The experimental study is set-up as a factorial experiment with a completely randomised design. In the experiments, the light source is a pulsed Nd:YAG laser. As absorber, a semi-transparent, non-conductive polymer film in a plane layered structure of one or more layers on a glass substrate is used. The frequency spectra of the generated ultrasound spans 2 to 20 MHz, which is recorded by a broadband PVDF ultrasonic transducer. The results show that an increased thickness of the polymer layer structure relate to a lower center frequency and a lower bandwidth, and that an increased optical absorption and a decreased layer structure thickness is related to a higher ultrasound amplitude.