Coupled string and board modes produce wavering sound rather than steady tone

Evidence

The pendula shown in figure 12.7 differ a little in length, making their frequencies different. One of the two has all the energy initially. The result reveals a decaying oscillation in the initially quiescent soundboard mode. Since the string (represented by the initially energized pendulum) itself does not sound, the blue trace represents the (wobbly, wavering) sound that would be heard coming from the soundboard. This example, though it treats only two modes (one string and one soundboard coupled together) does not bode well for isolated resonances. — Eric J. Heller, Why You Hear What You Hear- An…, 1170