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Gold APMEX Round 1/4 oz — ping test & specs

A generic 1/4 oz .9999 gold round distributed by APMEX. As a private product its diameter varies by maker (≈21.7 mm nominal); its quarter-ounce mass and purity still give a consistent ring.

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

Official specifications

Weight7.776 g
Diameter21.7 mm
Composition.9999 fine gold

Source: private mint (APMEX round). Coin Pinger checks measured weight and diameter against these values.

Expected resonance range

Provisional confidence*
ModeExpected ringRecommended variance
c0d24.22–5.07 kHz±9.1%
c0d39.82–11.66 kHz±8.6%
c0d417.17–20.40 kHz±8.6%

A genuine Gold APMEX Round 1/4 oz 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). The higher modes also carry a small thick-plate centre correction, since a real coin is not infinitely thin. The provisional-confidence tag reflects how well-established this metal's constants are: Computed from .9999 gold's annealed modulus — the least-certain material input, so the band is widened and this reference is best confirmed by calibrating a coin you know is genuine. 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 APMEX Round 1/4 oz 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 APMEX Round 1/4 oz

  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 APMEX Round 1/4 oz 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 APMEX Round 1/4 oz is 21.7 mm across in .9999 fine gold, which puts its fundamental (c0d2) near 4645 Hz.

A perfect disc's modes sit near 1 : 2.3 : 3.9; this coin measures 1 : 2.31 : 4.04. 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 ≈10600 Hz instead of 4645 Hz. Density can fool a scale; the ring can't be faked without the right metal.

We publish this as a Provisional confidence reference. Computed from .9999 gold's annealed modulus — the least-certain material input, so the band is widened and this reference is best confirmed by calibrating a coin you know is genuine.

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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