Back wall echo

Figure 5 Back wall echo through 300 mm of material, before and after signal processing.

Given that the ultrasonic back-wall echo from the synthesized beam and from the single element output may differ due to the coherent summing, time and frequency response of synthesized beam outputs may be achieved. Therefore, flat wall must be in the far-field or at the focus point as recommended by the standard [1]. 

The system can also numerize the A-scan from the back-wall echo of the specific target, giving the central frequency, relative bandwidth and sensitivity of the 160 apertures. 

The PE data was obtained by repeating the scanning of the object, now measuring the received echo at Transducer 1. For every position, (x, y), an A-scan was obtained from which we extracted the back wall echo by means of a time gate. This back wall echo is denoted s(x, y). Note that s x, y) is a time signal that can be written s(f, x, y) where t is the time index. One example of such a back wall echo is shown in Figure 2. 

Because of the double sound path involved in PE measurements of the back wall echo, we approximate the corresponding attenuation at a certain frequency to be twice as large as the attenuation that would be obtained by an ordinary TT measurement. We propose to use the logarithm of the absolute value of the Fourier transform of the back wall echo as input data, i.e... 

Figure 2 Example of back wall echo (left) and preprocessed back wall echo (right). Results...

The back wall echoes were sampled at 100 MHz and the length of these were 70 samples, yielding a size of the input data vectors, ) = 35. An example of such an echo is shown in Figure 2 together with its log spectral amplitude. 

In this preliminary work we have investigated composite objects with a simple geometry. In future work the proposed approach will be applied to more complicated objects, in particular glued structures. Since we for such objects expect to have a less distinct back wall echo, we have reason to believe that the preprocessing method that was used in this work has to be somewhat modified. 

MHz to 40 MHz. This was highly focused so that the beam width was 0.001 in. (0.025 mm). The return echoes were digitized at 500 MHz using a Tektronix 7912 digitizing unit. This unit can record 512 data points, or 1.02 xsec of data, which means that some of the BWE (Back Wall Echo) would be eliminated for thicker bonds this was not a problem since only EWE (Front Wall Echo), BLEl (1st Bondline Echo), and BLE2 (2nd Bondline Echo) was required forevaluation purposes. Because of disk storage restrictions, data was taken in 2.5 mm increments. This time, differences between the two surface preparation types were observed in the frequency area ratio features. The feature used was... 

A problem obviously exists in trying to characterise anomalies in concrete due to the limitations of the individual techniques. Even a simple problem such as measurement of concrete thickness can result in misleading data if complementary measurements are not made In Fig. 7 and 8 the results of Impact Echo and SASW on concrete slabs are shown. The lE-result indicates a reflecting boundary at a depth corresponding to a frequency of transient stress wave reflection of 5.2 KHz. This is equivalent to a depth of 530 mm for a compression wave speed (Cp) of 3000 m/s, or 706 mm if Cp = 4000 m/s. Does the reflection come from a crack, void or back-side of a wall, and what is the true Cp ... 

ECHO is based on the principle of differential acoustic impedance (or tissue density) and the laws of reflection and refraction. Sound waves directed across tissues from a transducer will reflect back sound waves of different frequencies. The ability of the ultrasonic beam to penetrate chest wall structures is inversely proportional to the frequency of the signal. With transthoracic ECHO, frequencies of 2.0 to 5.0 MHz are used commonly in adults, and frequencies of 3.5 to 10.0 MHz are used in children. Serial determinations in a given patient using the same conditions and ECHO images (windows) provide the best form of internal control to allow comparisons of test results. In clinical trials, echocardiograms are read and interpreted independently by two or three clinicians to provide a means of control. 

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