4.3 Ultrasonic transmission method: measurement range 3 MHz to 5 GHz
4.3.1 Ideal signal behaviour
Figure 4.6. Schematic representation of an ultrasonic transmission cell: (1) fixed transmitting transducer; (2a) receiving transducer in the retracted position; (2b) receiving transducer in the extended position; (3) exponentially decaying sound wave; (4) cell boundary.
The ultrasonic transmission method is used when the propagation distance over which the sound pressure falls significantly is only a few centimetres, owing to the quadratic frequency dependence of attenuation. This requires a sufficiently small measurement volume. The measured sound-pressure amplitude decreases exponentially:
This method permits direct measurement of the attenuation coefficient α. It is determined from the excitation voltage of the transducer or the voltage UE recorded by the receiver:
