EddySonix® Acoustic Resonant Inspection

Acoustic Resonant Inspection is a nondestructive testing technique based on modal or vibration analysis of parts. In this method, structural dynamic features of parts such as natural frequencies and damping factors are analyzed. These features are directly correlated with mechanical properties (such as Elastic Modulus or tensile strength). A structural defect changes the mechanical properties and hence reduces some of the natural frequencies and increases the damping factors. For example, a crack reduces some resonant frequencies and increases the damping ratio. By analyzing and comparing the parameters of “Good” and “Defective” parts, we can detect defective parts.


All objects vibrate. Each object has its own vibrational modes. Each mode represents a standing wave or resonance at a natural frequency. Vibrational modes of each object are unique as finger prints of the object. These modes describe the intrinsic dynamic properties of the object.


When we impact an object or apply swept sine, we can identify its dynamic behavior or frequency response function. In response to impact, all modes are excited and each mode vibrates at a specific frequency (Eigen frequency). Each mode is a degree-of-freedom of vibration, which vibrates as a damped sinusoid at its natural frequency. The overall vibration of an object is the superposition of all modal vibrations.


An object has infinite number of modes or natural frequencies, which appear as peaks in frequency response. The frequency of modes are a function of elastic properties and geometry (CAD drawing). These frequencies are independent of impact point and intensity, and sensor location.

We can model a mode (one degree-of-freedom) with the free vibration of a Mass-Spring-Damper. The natural frequency of vibration (without damping) is f=√(k/m). In structural vibration, k represents mechanical properties (Young modulus, Elastic properties), and m represents density and dimensions (geometry).

The modes can be calculated using FEM, which corroborate the experimental results.

The modes can be calculated using FEM, which corroborate the experimental results.
A defect in a part reduces the stiffness k, and hence the natural frequency f is shifted (reduced) according to f=√(k/m).
The frequency shift has high correlation with fatigue or break strength. So the natural frequencies can represent the quality of a structure. This correlation is the basis of Acoustic Resonant Testing.


According to Standard ASTM-E2001, Resonance Testing can be used to detect defects in metallic and non-metallic parts. In a single measurement, the procedure can detect numerous defects including cracks, chips, cold shots, inclusions, voids, oxides, contaminants, missed processes or operations, and variations in dimension, hardness, nodularity, porosity, density, and heat treatment.

Industry Defect Type Product (example)
Ductile Cast Iron Nodularity, Crack, Oxides, Cold Shuts, Porosity, Heat Treatment, Inclusions, Carbides, Hardness, Residual Stress Knuckle arm, Brake caliper, Trailing arm, Crossmember arm
Gray Cast Iron Crack, Natural Frequency, Perlite Camshaft Brake disk,
Steel Crack Axle shaft, Drive shaft
Forgings Crack, Bar Ends, Double Strikes, Laps, Water quench Conrod, Knuckle arm
Aluminum Cast Crack, Oxides, Elongation, Cold shuts, Shrink porosity, Blow holes Pedals, Steering wheels
Ceramics and Glass Crack Industrial ceramics, tiles, bottles


  • Accurate and Repeatable
  • High Speed Test (1 sec), Easy to Use
  • Best for high volume 100% quality control
  • Whole part test for internal and external flaws
  • Compensation of Production Trends, Temperature, and Mass
  • No part preparation required, no consumables expenses
  • Various Tools for Data and Pattern Modeling
  • Can be Integrated with Eddy Current System
  • Easily automated
  • Industrial Computer, Robust against Noise and Dust
  • Documentation and Statistical Reports