Bass strings of piano are manufactured by winding a wire around a core string. At these strings, the bending stiffness is not negligible. This effect modifies the higher harmonics of the string compared to the harmonic spectrum of the ideal string. This inharmonicity is even more significant in upright pianos. The effect has recently received renewed attention in the literature. In the present paper, the authors present a dynamic measurement for determining the bending stiffness of the piano strings. By using the so-called inharmonicity coefficient, the spectrum of the string is predicted, and the deviation from the ideal string is determined.