A single amplitude envelope misses that harmonics decay and modulate at different rates

Evidence

The amplitude envelope is not sufficient to represent the behaviour of different spectral parts of the sound produced by many musical instruments. The spectrogram (see Section 3.2.6) can be used to visualize instrument sounds. Figure 6.5 shows the magnitude envelopes of the harmonics of a kantele, a Finnish plucked string instrument. The plot shows the typical characteristics of the kantele sound; it has strong decay at higher harmonic partials, and it also has strong amplitude modulation at the harmonics, which appears with different modulation frequencies for different harmonics. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 687

Categorization

• XREF: Plucked-string partial behavior parallels what was covered on plucked strings generally; connects to instrument synthesis questions.
• QUESTION: Why do different harmonics of the kantele modulate at different frequencies — does this trace to specific sympathetic resonances or string nonlinearity?