Which property of solids governs the increase in the frequency of receiving probes than transmitting probes in ultrasonic testing?
Second and higher order harmonics do not normally play an essential role in conventional ultrasonic testing. I agree with AML on his first one and a half sentences, but not on the conclusion. The transmitted signal is not composed of a single frequency but of a frequency spectrum. Ultrasonic attenuation generally depends on frequency. This means that the lower frequencies of the spectrum are attenuated differently from the higher frequencies and that the mean frequency of the ultrasonic pulse changes continuously during propagation in the medium. If the receiving probe has the same frequency spectrum as the transmitting probe, the receiver will still receive a pulse of different mean frequency. Once again, this conclusion is not in contradiction with AML's correct statement that in linear media one frequency is never transformed into another.
Elango: Let me add two more points to think about:
(1) May be the transmitting and receiving probes do not have the same frequency spectrum. You can check this by using both probes individually in the transmission/reflection mode and compare with the situation you have described.
(2) Ultrasonic attenuation normally increases with increasing frequency. One would expect a decrease of the mean frequency, not an increase as you observed. However, one can only speculate without knowing further detail.
Frank
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