The sound produced by musical instruments is not a single simple harmonic, but a superposition of many simple harmonics with different frequencies and amplitudes. Do you know Fourier series? ) different tones are determined by the basic frequency of musical instrument vibration, and the basic vibration frequency of do re mi is different. The timbre of different instruments is different because the harmonic frequency and amplitude of different instruments are different. The material of musical instruments determines that the vibration of sound waves of musical instruments has different harmonic frequencies and different harmonic amplitudes. That is, the material determines the timbre of the instrument.

Fourier series is described in mathematical language: non-sine wave can be regarded as the superposition of a series of sine waves with different frequencies. It seems that you have to use your head to understand my answer above!