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 for 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 conducts ultrasonic waves at a characteristic velocity. If the measured value deviates from the target values for gold or silver, this indicates a different material inside.
Operating Principle
A piezoelectric transducer sends short ultrasonic pulses into the test object. The wave passes through the bar or coin, is reflected off the opposite surface and returns to the sensor. From the measured travel 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 travel time of the echo (microseconds)
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 Archimedes density measurement 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 considerably higher speed of sound than expected under ultrasound - and is thus reliably exposed. Especially for kilo bars and large bar formats, ultrasonic testing is therefore regarded as the standard method of professional testing laboratories.
Areas of Application and Limitations
Particularly suitable for:
- Gold and silver bars from about 50 g in weight
- Kilo bars and 400-ounce large bars (LBMA Good Delivery)
- Suspicious coins with an unusual sound or weight
Limitations of the method:
- Very thin objects (below approx. 2-3 mm) are difficult to measure, as the transmit and receive pulses overlap
- Rough or curved surfaces require coupling gel (ultrasonic gel) for good sound transmission
- Mixed alloys (e.g. a thin gold coating over another metal) can produce false signals depending on the layer thickness
- The method does not replace X-ray fluorescence analysis (XRF) for precise fineness determination
Course of a Professional Test
- Cleaning of the measurement points (oils and dirt interfere with sound transmission)
- Applying coupling gel to the transducer and test surface
- Several measurements at different points on the object (edges, centre, reverse)
- Automatic comparison with a material database or manual evaluation of the travel time
- Documentation of the results; if anomalies arise, combine with density measurement or XRF
For private investors, compact handheld devices are available that display the reading directly as "Gold/Silver: OK" or "Deviation". They are easy to operate but require steady placement of the transducer perpendicular to the surface. Our authenticity testing overview compares all common methods.
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 sensibly complements weight and dimension checks (coin weight checker).