User:Ragszwiki/Sandbox

Non-Contact Ultrasound (NCU) is a method of non-destructive testing where ultrasound is generated and used to test materials without the generating sensor making direct or indirect contact with the test material or test subject.

Most conventional ultrasound methods require the use of some type of acoustic coupling medium in order to efficiently transmit the energy from the sensor to the test material. Couplant materials can range from gels or jets of water to direct solder bonds. In Non-Contact Ultrasound, basic ambient air is the only acoustic coupling medium. NCU allows some materials to be inspected which otherwise can’t be inspected due to fear of contamination from couplants or water.

In general non-contact ultrasound would facilitate: :

1. Analysis of early-stage materials formation – powders, green, consolidated, un-polymerized, liquid-sensitive, porous, hygroscopic and other materials, or when contact is simply a nuisance.

2. Testing of materials, components, containers, etc., that are continuously rolled on a production line.

3. Evaluation of food, pharmaceutical, and bio-medical products.

4. Analysis of materials where liquids are scarce or impractical to use, such as in zero gravity environment.

5. Non-invasive diagnostics where contact with a patient is harmful or painful.

Non-Contact Ultrasound (NCU) mode is considered difficult because of the following:

a) As sound waves have to travel from the sensor emitting ultrasound (a solid medium) directly into air, they experience an acoustic impedance mismatch. Since the acoustic impedance of the sensor (a solid) and air are very different, then most of the sound energy will be reflected or absorbed, rather than transferred across the border.

b) Ultrasound waves are also attenuated in air, due to absorption and scattering effects.

c) When the emitted ultrasound reaches the air/test material interface, another air-solid acoustic impedance mismatch occurs. It is estimated that up to 99% of the ultrasonic energy at the air-test material interface is reflected back to air and only 1% of the energy actually passes through the test material.

Since b) and c) above are natural phenomena of nature, most of the improvements in propogation and usage of non-contact ultrasound has come from improvements in sensor design to provide better acoustic impedance match between the  sensor and the air column and increase the effective ultrasound energy transmitted to air.

Currently, non-contact ultrasound is used in the inspection of composites, which can degrade if immersed in water, green ceramics, plastics, wood, asphalt, concrete and food materials.