An easy way to initially visualize what's happening ultrasonically and why flaws are detectable is to take a "trek" along with an ultrasonic tone burst (longitudinal wave mode) as it travels from the transmitter probe, through air to the material being inspected, through the material and eventually through air again to the receiver probe.
The Ultrasonic Trek
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Transmission Into Air: Initially, a portion of ultrasonic energy is lost (due to the transmitter probe/air impedance mismatch) as the tone burst is intially launched into the air.
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Airpath To Material: Once launched, the tone burst travels through the air and loses further energy due to attenuation (absorption) and beamspread (diffraction). For example, about 3dB of tone burst amplitude is lost by traveling across an airpath of 6". (There is also a "complex" radiation field, called the Near Field, which extends about 1.9" in front of the Model ATI transmitter probe).
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Entry Into Material: A large amount (over 99%) of the incident tone burst energy is lost due to reflection at the material surface, with only a small amount of the ultrasound actually entering the material. This reflection loss is caused by the extremely large acoustic impedance mismatch at the air-material interface. The acoustic impedance mismatch can be thought of as a "valve or shutters" which determine how much ultrasound is permitted to cross the interface (a perfect impedance match allows all the ultrasound to pass-no reflection).
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Travel Through Material: As this weakened tone burst travels through the flawless solid material, it experiences further energy loss caused by attenuation (both absorption and scattering, in the case of a solid), as well as by additional beamspreading.
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Exit From Material: The remaining burst now experiences another huge energy reflection loss (again over 99%) due to the same excessive material-air acoustic impedance mismatch as was experienced at the above-mentioned entry surface. At this point, the tone burst has lost more than 99.9% of its energy due to only the two surface reflections.
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Airpath To Receiver Probe: After exiting the material, the now extremely low energy tone burst experiences further attenuation and beamspread loss as it travels along the airpath to the receiver probe.
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Reception From Air: Finally, when this low-level tone burst impinges on the receiver probe, an additional energy loss is experienced during its ultrasonic transfer to (ultrasonic activation of) the probe.
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Impact Of Material Flaw: Ironically, while impedance mismatches at the two material surfaces can be consid-ered energy-stealing enemies, the large air-material mismatch caused by a flaw (delamination, split or cavity) becomes a great ally. Basically, the tone burst experiences another similar 99%-plus energy loss when it impinges on a flaw (assuming the flaw is large enough to intersect the complete beam). Even"pressed together" delaminations at this test frequency (50KHz) will still reflect 99%-plus of the signal energy (because there is a microscopically thin layer of air still present at the flaw site). Thus, the flaw "blocks" a huge percentage of the normal (flawless) tone burst energy - making the flaw readily detectable.
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