How to test: hold the coin at its center in a
coin ping-test holder so the rim can ring freely — that gives the cleanest result. As a last resort, balance it
flat on a fingertip at its center, touching as little as possible. Either way, strike its edge with a striker close to
the microphone. Don't use a fingernail — it damps the ring.
Your browser kept voice processing (echo cancellation / noise suppression) enabled on the
microphone — common on iPhone/Safari. Frequency readings usually still work, but ring
volume and sustain may be distorted.