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Getting Inside a Steel Trap

By Sean Gleeson on Thursday, August 5th, 2010


When we do ultrasonic testing, one of the most important factors is the issue of coupling. What is coupling and why is it important? Well, it turns out that when a wave of energy hits another object, most of that energy is going to be reflected off the surface. This is also known as an “echo”……. But amazingly, a small portion of that energy will actually be transmitted into the object being blasted.  It is this energy that makes it inside the steel trap that we take advantage of when doing ultrasound inspection.

In example 1, the reason that all the energy doesn’t transmit into the average piece of Unobtainiun Nickel is because of the mismatch in what is called acoustic impedance.

The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V).

Acoustic impedance is important in

  1. the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances.
  2. the design of ultrasonic transducers.
  3. assessing absorption of sound in a medium.

Reference http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/acousticimpedance.htm

Example 2 illustrates the relationship between sound traveling through different mediums starting with substances with vastly different acoustic impedance and moving towards substances with an acoustic impedance that is close to the object being inspected. In the case of the Unobtainium / Nickel alloy (which doesn’t exist except in my mind and possibly on moon  Pandora ) we get to assume that this alloy has the same acoustic impedance as steel. So to sum it up as simply as possible, the closer the acoustic impedance match between two objects, the more energy get transmitted and less energy gets reflected.

The biggest problem is that air has a freakishly small acoustic impedance compaired to objects we would like to inspect. We can use different substances that have more favaorable properties to help us get energy into something we want to look inside of. Most of the time, water works great.  When you go to the doctor to get an ultrasound the technician puts the clear, cold jelly on your belly before  they start the test. This is because this jelly is formulated to have a closer acoustic impedance to your skin than air. We call this fluid couplant because it is used to couple between the transducer and the skin. The exact same idea has to be applied when inspecting airplanes or pipelines and though there are different formulations, the couplant used in industry it very similiar to the stuff used in the hospital.

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