Overlapping damped modes sound smooth while isolated modes produce terrible wavering

Evidence

Now consider four coupled modes, one string and three soundboard modes. First, we suppose the string is only slightly damped; the soundboard modes are more damped, but not enough to cause them to overlap. The result is the disaster on the top in figure 12.8; each of the three board modes is plotted in a separate color; the wobbly waxing and waning sounds terrible. If the damping is increased so that distinct isolated peaks no longer exist, the result is much smoother (refer to sound files overlapping.wav and nonoverlapping.wav on whyyouhearwhatyouhear.com). — Eric J. Heller, Why You Hear What You Hear- An…, 1171