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Gold Double Eagle $20 — ping test & specs

The Saint-Gaudens $20 Double Eagle (1907–1933) is a classic 90% gold US coin holding just under one ounce of gold. Its copper alloy and larger 34 mm size shape its ring.

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Opens the free tester with this coin preselected — audio never leaves your device.

Official specifications

Weight33.436 g
Diameter34 mm
Thickness2.41 mm
Composition.900 gold (90%)

Source: United States Mint. Coin Pinger checks measured weight and diameter against these values.

Expected resonance range

Good confidence*
ModeExpected ringRecommended variance
c0d23.48–4.39 kHz±11.5%
c0d38.06–10.08 kHz±11.1%
c0d414.04–17.54 kHz±11.1%

A genuine Gold Double Eagle $20 rings within these ranges at its c0d2/c0d3/c0d4 vibration modes, fixed by its size and metal. The recommended variance is how far a genuine specimen's ring can sit from each center — built up in quadrature from real manufacturing tolerance and temperature (under 1%), phone-mic capture reproducibility (≈4%), an extra allowance on the fundamental for minting-asymmetry splitting, and a per-metal model-uncertainty term (largest for .9999 gold, whose modulus is least certain). This coin's band is widened further because circulated 90%-silver classics lose mass and edge crispness with wear, spreading their ring. The higher modes also carry a small thick-plate centre correction, since a real coin is not infinitely thin. The good-confidence tag reflects how well-established this metal's constants are: Computed from 22k-gold alloy constants, which are slightly less certain than silver; the band is set accordingly. These are guidance bands, not exact values — see the methodology for the exact formula, or calibrate your own coin in the app for a specimen-exact reference and tighten each peak per coin.

* These are computed guidance ranges. Certain your Gold Double Eagle $20 is genuine but it won't match? Send us its ring — we'll dig in and improve the reference.

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How to ping test a Gold Double Eagle $20

  1. Hold — grip the coin by its center (a nodal point) in a coin ping-test holder so the rim rings freely; a fingertip at the center is a last resort.
  2. Tap — strike the edge with a striker near the microphone.
  3. Read the verdict — MATCH or NO MATCH against the peaks above, plus weight and size checks.
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The physics of the Gold Double Eagle $20 ping

Those 3 peaks aren't random. When you strike the coin it flexes as a free disc in fixed patterns called vibration modes — 0 nodal circles with 2, 3 and 4 nodal diameters, written c0d2 / c0d3 / c0d4. Each pattern rings at one frequency.

Where those frequencies land is set by the coin's geometry and metal. For a thin disc, frequency ∝ (thickness ÷ diameter²) × √(stiffness ÷ density). This Gold Double Eagle $20 is 34 mm across and about 2.41 mm thick in .900 gold (90%), which puts its fundamental (c0d2) near 3935 Hz.

A perfect disc's modes sit near 1 : 2.3 : 3.9; this coin measures 1 : 2.30 : 4.01. That fixed spacing is the fingerprint of a genuine struck disc — Coin Pinger rejects taps that don't fit it (keys, glass, a coin lying on a hard surface).

And it's why a fake can't hide: change the metal and √(E/ρ) changes, so every mode moves. Struck in tungsten (which weighs almost exactly right) instead, the ring would land far higher and brighter — its c0d2 would land near ≈9000 Hz instead of 3935 Hz. Density can fool a scale; the ring can't be faked without the right metal.

We publish this as a Good confidence reference. Computed from 22k-gold alloy constants, which are slightly less certain than silver; the band is set accordingly.

Free-plate modes per A. W. Leissa, Vibration of Plates, NASA SP-160 (1969, public domain). See how we measure them →
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