Ultrasound Testing
Also: Ultrasonic authenticity testing, UT testing, ultrasonic measurement
A non-destructive testing method that uses ultrasonic sound waves to determine the density, internal structure, and material composition of precious metal bars and coins.
Ultrasound testing is one of the most reliable methods for examining precious metal bars and large-format coins for forgeries – without damaging the object. The process 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 penetrates the bar or coin, is reflected off 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 calculated:
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 for pure metals. Deviations of more than 1–2% give cause for further examination.
Why Tungsten Forgeries Are Detectable
Tungsten has almost the same density as gold (19.32 g/cm³ vs. 19.25 g/cm³) and therefore passes the Archimedes density test and the magnet test without difficulty. However, the speed of sound differs significantly:
| Material | Density (g/cm³) | Speed of sound longitudinal (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 significantly higher speed of sound than expected during ultrasound testing – and is thus reliably exposed. Particularly for kilo bars and large bar formats, ultrasound testing is therefore the standard procedure used by professional testing laboratories.
Applications and Limitations
Particularly suitable for:
- Gold bars and silver bars from approx. 50 g in weight
- Kilo bars and 400-ounce large bars (LBMA Good Delivery)
- Suspect coins with unusual sound or weight
Limitations of the method:
- Very thin objects (below approx. 2–3 mm) are difficult to measure because the transmitted and received pulses overlap
- Rough or curved surfaces require coupling gel (ultrasound gel) for good sound transmission
- Mixed alloys (e.g. a thin gold shell over another metal) can produce false signals depending on layer thickness
- The method does not replace X-ray fluorescence analysis (XRF) for precise fineness determination
Procedure for a Professional Test
- Clean the measurement points (oils and dirt impede sound transmission)
- Apply coupling gel to the transducer and the test surface
- Take multiple measurements at various points on the object (edges, centre, reverse)
- Automatic comparison with a materials database or manual evaluation of the transit time
- Documentation of results; in the event of anomalies, combine with density testing or XRF
Compact handheld devices are available for private investors that display the result directly as "Gold/Silver: OK" or "Deviation". Operation is straightforward, but requires the transducer to be placed calmly and perpendicularly on the surface. Our authenticity testing overview summarises all common methods in a comparative format.
In Brief
Ultrasound testing is the only readily accessible method that reliably detects 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 complements weight and dimension testing (coin weight checker) effectively.