This models a stiff piano string, whose partials are stretched sharp by string
stiffness: fₙ = n · f₀ · √(1 + B · n²)
Real piano strings have a small inharmonicity coefficient B (≈ 0.0001–0.0005 for the
midrange, larger in the bass and treble). Set f₀ and B below, hit
Play, and hold a phone or laptop mic up to your speaker. Open
Overtone in another window — the tuner should recover roughly the
B and f₀ you dialed in here, letting you validate it without
touching a real piano.