In discussions with quality managers we keep meeting the same misconception: "Acoustic testing is the same as ultrasound, right?" No. Both use sound waves, but physics, expressiveness and economics differ fundamentally.
The physics compared
Ultrasonic testing (UT)
A piezoelectric probe sends high-frequency waves (typ. 1–25 MHz) into the part. Reflections at material boundaries, cracks or inclusions are detected. Localisation via time of flight.
Strength: high spatial resolution. A 0.3 mm crack is detectable when the probe is close.
Weakness: point inspection. Full inspection of a complex casting needs many probes or repositioning – costly.
Acoustic resonance analysis (ART)
The part is briefly excited and rings freely. The entire set of natural frequencies is captured – depending on geometry, mass, material and integral defect state. Comparison vs. master or ML model decides OK/NOK.
Strength: holistic assessment in < 1 s.
Weakness: no localisation.
Comparison matrix on eight criteria
| Criterion | Ultrasonic testing | Acoustic resonance analysis |
|---|---|---|
| Inspection speed | 5–60 s per part | 0.2–2 s per part |
| 100 % capability | only with effort | standard case |
| Detection capability | local, ≥ 0.3 mm crack | integral, ~1 % stiffness change |
| Localisation | millimetre-precise | not possible |
| Material variety | metals, some plastics | metal, sinter, ceramic, composite |
| Part geometry | flat/tubular preferred | nearly arbitrary |
| Investment | 50–500 k€ (std.) – 0.5–2 M€ (PA) | 30–200 k€ |
| Inline integration | high (couplant, robot) | low (sensor + excitation) |
When to pick which
Ultrasound when defect location matters, sampling-only is OK and geometries are simple.
Resonance analysis when 100 % inline matters, geometries are complex, the question is "does it work?" rather than "where exactly is the flaw?", or material mix-ups have to be detected (UT often misses them).
Hybrid strategy
Both methods complement each other. RTE often recommends: 100 % ART as inline filter, UT sample on parts flagged "suspicious" by ART. Lowest cost per defect found.
