According to the FOURIER THEOREM, periodic sound may be shown to consist of SINE WAVEs in the HARMONIC SERIES, where the Fourier coefficients ... it will be seen that the spectrum is always changing ...

As supported by the LAW OF SUPERPOSITION, sine tones of varying amplitudes will combine to form a complex harmonic SPECTRUM ... See: COMPLEX TONE, FOURIER THEOREM, HARMONIC SERIES, SINE WAVE. Compare: ...