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Measurement and Detection Principles

Quick Links: Detection  Unique Signature  Time and Frequency Domain
Comparing Frequencies  Analyzing Data  Operation  Sample Test


The Resonant Acoustic Method is a unique form of Resonant Ultrasound Spectroscopy (RUS), also popularly known as Resonant Inspection (RI). The ASTM E-2001-08 standard addresses Resonant Inspection.  The Resonant Acoustic Method (NDT-RAM™) provides a whole body component inspection sensitive to both internal and external flaws.  NDT-RAM can detect imperfections such as variances in overall shape, weight and density of parts. It can also detect if processes such as machining or heat treating have been missed. Below are typical flaws of powdered metal, cast, forged, welded, stamped and brazed parts.


Powder Metal Cast Forged
Cracks Cracks Cracks
Chips Cold Shuts Missed or Double Strikes
Voids Porosity Porosity
Hardness/Density Hardness/Density Hardness
Inclusions Inclusions Inclusions
Heat Treatment Heat Treatment Heat Treatment
Decarb Residual Stress Quenching Problems
Brazing Nodularity Laps
Gross Dimensions Gross Dimensions Gross Dimensions
Raw Material Contaminants Raw Material Contaminants Raw Material Contaminants
Missed Processes/Operations Missed Processes/Operations Missed Processes/Operations


Welding Stamping Brazing
Weld Quality Cracks Braze Quality
Missing Weld Delimitation Missing Brazing
Partial Weld Tearing Double Brazed
Cold Weld Wall Thickness Incorrect Braze Quality
Inclusions Inclusions Inclusions
Non-Uniform Weld Heat Treatment Non-Uniform Flow
Misalignment Misalignment Misalignment
Voids Wrinkles/Laps Voids
Gross Dimensions Gross Dimensions Gross Dimensions
Raw Material Contaminants Raw Material Contaminants Raw Material Contaminants
Missed Processes/Operations Missed Processes/Operations Missed Processes/Operations

Unique Signature

Every part has a unique vibration signature (its resonant frequency). This resonance is almost exactly the same from good part to good part. The resonance will shift when there is an internal or external change or imperfection. For example, when a bell has a crack, it does not have a clear ring and loses its ability to hold its tone.  The same is true of a part that has a flaw.

NDT RAM Time Response of a Good Part

NDT RAM Time Response Decay Rate of a Bad Part
Time response of a good part (blue) and bad part (red)

Time and Frequency Domain

Using the Fast Fourier Transform (FFT) process, the response signal is separated into discrete frequencies. This allows for easy interpretation of data in the frequency domain. Peaks represent a specific component's resonant frequency.

Comparing Frequencies

The result is a Response Frequency Spectrum that is unique to a specific part. The principal mechanical properties of an object are:
· Mass
· Stiffness
· Damping

If the Mass, Stiffness or Damping in a component are varied, the dynamic response characteristics of an impact will vary. Using this spectrum, we can look for changes in parts. Below is an example of the resonant signature for both a good and defective part.

Good Signature

Defective Signature

Analyzing Data

Focusing on a Specific Peak

By examining peaks at a closer range, variances in the frequency response are easier to see. Below is an example of a typical frequency shift. This shift is typical from 50-500 Hz. The green block represents the Pass/Fail criterion (or limit). If the peak is inside the block, the part passes and is ready for the next process.

This is an example of the absence of or the significant difference in the amplitude of a peak. The defect caused the part to dampen faster and not hold its tone.


Once the Pass/Fail criteria have been set for a given part, the NDT-RAM application can be brought online. Testing can typically occur as quickly as approximately three seconds per part* and the system can be entirely automated, allowing for 100% inspection of your high volume production line parts.

A record of every tested part is automatically stored in report form. This allows the ability to perform statistical analysis on lots, shifts, percent and types of defects.

Sample Test

Sometimes it is necessary to have more than one Pass/Fail criterion. Below is an example of a part that requires four points be checked to determine a good part from a defective part. In this test, all four points must be good for the part to pass.

Blue - passed all four limits - Good
Pink - failed three of four Limits - Defective

* Part throughput rate dependent upon part size, belt size, data acquisition setup parameters, and other part handling requirements

Learn More
What is Non-Destructive Testing?
Resonant Acoustic Method
How the Equipment Works
Advantages of NDT-RAM™
Case Studies
Comparison of Non-Destructive Methods

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