Ultrasonic Testing
Also: ultrasonic authenticity test, UT testing, ultrasonic measurement
A non-destructive testing method that uses sound waves in the ultrasonic range to determine the density, internal structure and material composition of precious-metal bars and coins.
Ultrasonic testing is one of the most reliable methods of examining precious-metal bars and large-format coins for counterfeits — without damaging the object. The method exploits the material-specific speed of sound: every metal transmits ultrasonic waves at a characteristic velocity. If the measured value deviates from the reference values for gold or silver, it points to a different material inside.
Working principle
A piezoelectric probe sends short ultrasonic pulses into the test object. The wave passes through the bar or coin, is reflected at the opposite surface and returns to the sensor. From the measured transit time and the known thickness of the object, the speed of sound in the material is derived:
v = 2d / t
v = speed of sound (m/s)
d = thickness of the object (mm)
t = measured echo transit time (µs)
The calculated value is compared with the reference values of pure metals. Deviations of more than 1–2% warrant further investigation.
Why tungsten counterfeits can be detected this way
Tungsten has almost the same density as gold (19.32 g/cm³ vs. 19.25 g/cm³) and therefore passes the density measurement by Archimedes and the magnet test without difficulty. The speed of sound, however, differs markedly:
| Material | Density (g/cm³) | Longitudinal speed of sound (m/s) |
|---|---|---|
| Gold (999) | 19.32 | approx. 3,240 |
| Tungsten | 19.25 | approx. 5,220 |
| Lead | 11.34 | approx. 2,160 |
| Copper | 8.96 | approx. 4,700 |
| Silver | 10.49 | approx. 3,600 |
A gold bar filled with tungsten shows a markedly higher speed of sound than expected under ultrasound — and is thereby reliably exposed. Especially with kilo bars and large bar formats, ultrasonic testing is therefore regarded as the standard method of professional testing laboratories.
Applications and limits
Particularly suitable for:
- Gold bars and silver bars from about 50 g in weight
- Kilo bars and 400-ounce large bars (LBMA Good Delivery)
- Suspicious coins with an unusual ring or weight
Limits of the method:
- Very thin objects (below about 2–3 mm) are difficult to measure, as the transmit and receive pulses overlap
- Rough or curved surfaces require coupling gel (ultrasound gel) for good sound transfer
- Mixed alloys (e.g. a thin gold jacket over another metal) can, depending on layer thickness, produce false signals
- The method does not replace X-ray fluorescence analysis (XRF) for precise fineness determination
Procedure of a professional test
- Cleaning the measurement points (oils and dirt disrupt the sound transfer)
- Applying coupling gel to the probe and the test surface
- Several measurements at different points on the object (edges, centre, reverse)
- Automatic comparison with a materials database or manual evaluation of the transit time
- Documentation of the results; where anomalies arise, combination with density measurement or XRF
Compact handheld devices are available for private investors that display the reading directly as "gold/silver: OK" or "deviation". Handling is simple but requires steadily placing the probe perpendicular to the surface. Our coin authenticity check brings together all the common methods in comparison.
In brief
Ultrasonic testing is the only easily handled method that reliably reveals tungsten fillings in gold bars, because it scans the interior of the object — not just the surface. For large-format bars it is therefore indispensable and usefully complements weight and dimension checks (coin weight checker).