High relief coins present a question that occasionally surfaces among collectors: if the ping test relies on a coin's geometry to determine its acoustic signature, what happens when that geometry includes dramatic three-dimensional features?
The short answer: high relief designs do alter resonance characteristics—but not in ways that undermine acoustic authentication.
How Relief Affects Vibration
When a coin is struck, it vibrates at frequencies determined by three factors:
- Material properties — specifically density and stiffness (Young's modulus)
- Overall geometry — diameter and average thickness
- Mass distribution — how material is arranged across the coin's surface
High relief coins redistribute mass compared to standard strikes. The raised design elements concentrate material in certain areas, creating regions of varying thickness. This changes the coin's vibrational modes—the patterns in which it oscillates when struck.
In practical terms, this means:
- Some resonant frequencies may shift slightly higher or lower
- The relative intensity of different frequency components may change
- The overall acoustic "character" of the coin differs from a standard relief version
What Doesn't Change
The fundamental relationship between material composition and acoustic response remains intact. A high relief coin struck from the correct alloy will still resonate within a predictable frequency range for that material. What changes is the specific pattern of frequencies—not whether the material can be identified.
Consider: a high relief gold coin contains the same gold alloy as its standard relief counterpart. Gold's stiffness and density are fixed physical properties that cannot be altered by the minting process. The relief design redistributes the same mass into a different shape, shifting frequencies—but the acoustic signature remains identifiably gold.
A counterfeit made from tungsten or base metal would exhibit fundamentally different acoustic properties, regardless of whether it copies a high relief or standard relief design.
Practical Implications for Testing
High relief coins require adjusted expectations when performing acoustic authentication:
Reference Calibration
If using a database-driven application like Pingcoin, the reference values should ideally come from the specific coin variant being tested. A Saint-Gaudens High Relief will produce different frequencies than a standard Saint-Gaudens—both are genuine, but their acoustic profiles differ.
Comparison Testing
When comparing two coins by ear, ensure both are the same variant. A high relief coin compared against a standard relief example of the same type will sound noticeably different. This difference reflects geometry, not authenticity.
Focus on Material Signature
Even without perfect reference values, the ping test can identify whether a coin is made from the correct material. The frequency shift from relief variation is small compared to the difference between precious metal and common counterfeiting materials (base metals, tungsten, or composite cores).
Which Coins Exhibit This Effect?
Several notable high relief issues exist in the bullion and numismatic markets:
- Saint-Gaudens Ultra High Relief — The 1907 original and modern commemorative restrikes
- American Eagle Ultra High Relief — The 2009 issue with exaggerated design elements
- Perth Mint High Relief — Various limited editions with enhanced depth
- Canadian Maple Leaf Special Issues — Select premium versions with sculptural features
Standard bullion coins—American Eagles, Maple Leafs, Krugerrands, Britannias—maintain consistent relief across years, making them straightforward to test with established reference values.
Does Relief Height Matter?
The magnitude of frequency shift correlates with how much mass is redistributed. Coins with modest relief variation (typical year-over-year minting changes) show negligible differences. Dramatic ultra-high relief coins may show measurable shifts.
For most practical authentication purposes, this variation falls within acceptable tolerance. The ping test is not looking for exact frequency matches—it's looking for material-appropriate resonance behavior. A coin made from gold will resonate like gold whether the design is flat or deeply sculpted.
The Bottom Line
High relief coins can be authenticated acoustically. The design alters frequency patterns but does not prevent material identification. When testing these coins:
- Use reference values specific to the coin variant when available
- Compare against known authentic examples of the same issue
- Recognize that frequency shifts from relief are smaller than shifts from counterfeit materials
- Combine acoustic testing with weight and dimension verification
The physics remains reliable. The geometry is simply more interesting.