Ultrasonic measurement of the characteristic acoustic impedance using a directional coupler to analyse reference reflections from thin layers

In this contribution the application of a directional coupler for analysing reference reflections from thin layers in ultrasonic measurement of the characteristic acoustic impedance is presented. In a prior work ([2]) it was shown, that a directional coupler can be used in combination with an ultrasonic transducer to get reflections from layers that are very close to the transducers surface. Since the directional coupler provides the opportunity to send and receive signals at the same time, these layers can be very thin ({\textless} 1 mm). Reflections that arrive at the transducer during the sending process can be analysed because the directional coupler separates the receiving signal from the input signal of the transducer. When only one transducer is used to measure the acoustic impedance of a liquid the reflections from the thin layer reach the transducer after a very short time. The reflections sum up and with the concept of reflection and transmission coefficients a relation is found between the measured amplitude and a reference amplitude which was measured before. The exciting signal could be a burst of some periods of a sinusoidal alternating voltage. The advantages of using thin layers for the described task are the small geometry that allows flexible application of the sensor and the minimisation of temperature gradients within the medium because thin layers can follow thermal fluctuations faster than thick ones. In this paper the functional principle and first achievements are presented.