As the pressure and/or the density of the stopping finger increase, fewer overtones are lost to damping when vibration is reflected at the stopping finger. Therefore, the overtone content of the tone increases with pressure and/or density of the stopping finger.
As finger pressure is reduced from ‘stopping’ to ‘touching’, at first a harmonic is introduced into the stopped string sound (the harmonic is possibly slightly sharp due to increased string tension). For further decreases in pressure, the harmonic sounds alone. As pressure is reduced further, the open string, or neighbouring harmonics might be introduced into the sound, in a multiphonic. Finger pressure is relative to harmonic order; higher harmonics are more sensitive to changes in finger pressure.
Within ‘normal’ stopped string sound, the pressure and density of the stopping finger/‘object’ influence timbre by the extent to which they dampen the string’s vibration. As pressure and/or density increase, damping decreases, i.e., less energy is lost at the stopping finger and more is reflected back to the bridge. Therefore, loudness and overtone content (since damping affects higher overtones more than lower) are proportional to finger pressure/density. Decay duration is also proportional to finger pressure/density since, as less energy lost through damping, the string is able to complete more cycles of vibration before ‘spending’ the excitation energy.
Under harmonic finger pressure, as above, increasing the density of the touching finger reduces damping. Contrary to above, however, damping increases with finger pressure. Since string vibration passes the touching finger (rather than the finger reflecting vibration, as above), damping is limited by decreasing pressure, minimizing the energy transferred to the touching finger. Therefore: overtone content, loudness and decay duration of harmonics are proportional to finger density and inversely proportional to finger pressure.
In general, a reduction in overtone content has a more pronounced effect for harmonics than for stopped strings of equivalent pitch. Since fewer overtones are present in ‘normal’ harmonic sound, the exclusion or weakening of upper partials has a larger impact on timbre in relative terms.