Ultrasound Testing
Also: Ultrasonic authenticity test, UT testing, Ultrasonic measurement
A non-destructive test that uses sound waves in the ultrasonic range to determine the density, internal structure and composition of precious metal bars and coins.
Ultrasound testing is among the most dependable ways to check precious metal bars and large coins for counterfeiting - and it does so without leaving a mark on the object. The technique exploits a simple physical fact: every metal carries ultrasonic waves at its own characteristic speed. If the measured speed drifts away from the expected value for gold or silver, something other than the stated metal is hiding inside.
How it works
A piezoelectric probe fires short ultrasonic pulses into the item under test. The wave travels through the bar or coin, bounces off the far surface and returns to the sensor. From the measured travel time and the known thickness of the object, the speed of sound within the material can be calculated:
v = 2d / t
v = speed of sound (m/s)
d = object thickness (mm)
t = measured echo travel time (microseconds)
The resulting figure is compared with reference values for pure metals. A deviation of more than 1-2% is reason enough to investigate further.
Why tungsten fakes are exposed this way
Tungsten has almost exactly the same density as gold (19.32 g/cm3 versus 19.25 g/cm3), which is why it sails through the Archimedes density test and the magnet test without difficulty. Its speed of sound, however, is markedly different:
| 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 much higher speed of sound than it should - and is reliably caught out. For kilo bars and other large formats, ultrasound testing is therefore treated as standard practice in professional assay labs.
Where it works and where it doesn't
Especially suited to:
- gold and silver bars from roughly 50 g upward
- kilo bars and 400-ounce large bars (LBMA Good Delivery)
- suspect coins with an unusual ring or weight
Limits of the method:
- Very thin objects (under about 2-3 mm) are hard to measure, because the outgoing and returning pulses overlap.
- Rough or curved surfaces need a coupling gel (ultrasound gel) for good acoustic contact.
- Composite constructions (for example a thin gold shell over another metal) can produce misleading signals depending on layer thickness.
- The test is no substitute for X-ray fluorescence analysis (XRF) when precise fineness is needed.
What a professional test involves
- Cleaning the measurement points (oil and dirt disrupt acoustic contact)
- Applying coupling gel to the probe and the test surface
- Taking several readings at different points (edges, centre, reverse)
- Automatic comparison against a materials database, or manual analysis of travel time
- Documenting the results; where anomalies show up, combining with a density test or XRF
Compact handheld units are available to private investors that report the reading directly as "gold/silver: OK" or "deviation". They are easy to use, though they do require the probe to be held steadily and perpendicular to the surface. Our authenticity testing overview sets out all the common methods side by side.
In a nutshell
Ultrasound testing is the only easily handled method that reliably reveals tungsten fills inside gold bars, because it probes the interior of the object rather than just the surface. That makes it indispensable for large-format bars, complementing weight and dimension checks such as the coin weight checker.