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Measurement Cell

Transducer

Figure 2. Schematic diagram of the experimental configuration for an ultrasonic pulse-echo experiment.

A typical experimental configuration consists of a measurement cell which contains the sample, a signal generator, an ultrasonic transducer and an oscilloscope (Figure 2). The signal generator produces an electrical pulse of an appropriate frequency and amplitude. This pulse is converted into an ultrasonic pulse by the transducer. It then propagates through the sample until it reaches the far wall of the cell where it is reflected back to the transducer (Figure 3). The transducer now acts as a receiver and converts the ultrasonic pulse back into an electrical pulse which is displayed on the oscilloscope. Because each pulse is partially transmitted and partially reflected at the cell walls a series of echoes are observed on the oscilloscope (Figure 3). The velocity and attenuation coefficient are determined from these echoes.

Each echo has traveled a distance twice the cell length d further than the previous echo and so the velocity can be calculated by measuring the time delay t between successive echoes: c

= 2d/t. The cell length is determined accurately by calibration with a material of known ultrasonic velocity, e.g. distilled water: 2d = cw.tw (where the subscripts refer to water). Alternatively, if the ultrasonic velocity in the sample is known its thickness can be determined. The attenuation coefficient is determined by measuring the amplitudes of successive echoes: A = A0e 2ad, and comparing them to the values determined for a calibration material. A number of sources of errors have to be taken into account if accurate measurements are to be made, e.g., diffraction and reflection [6].

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