It is possible that excitation by plucking from right to left across the cello body maximises loudness at the expense of decay duration, and that plucking directly towards or away from the cello body maximises decay duration at the expense of loudness. The proposed explanation for this is that the cello bridge, in rocking from side to side to incite vibration of the body, is at its most efficient (releases energy fastest) when excited by a sideway movement. If the bridge is impelled to rock backwards and forwards, it uses the excitation energy more slowly. If this proposition is true, a mixed output is heard for excitation angles between these two extremes.
‘Bowing’ or stroking the strings at a vertical angle towards the bridge or nut does not incite transverse string vibration. Instead, the sound of the bowing/stroking action is amplified by the contact between exciter and cello body. Mixing some circular or horizontal movement with the vertical movement results in a mixture of string vibration and bowing/stroking sound.
The set up of the bridge and the strings has been developed to amplify lateral displacement (left↔right across the cello body). However, the string reacts to displacement at any angle between the lateral and the perpendicular plane towards↔away from the cello body. I refer to these two excitation angles as ‘horizontal’ and ‘vertical’ respectively.
The following theory has been adapted from an investigation into plucking angles in guitars by Erik Jansson. Bibliography link see Jansson (1983) pp.7-26. The results are opposite in the cello’s case because the guitar bridge vibrates towards-away from the fingerboard rather than laterally. The fact that the guitar bridge is fixed to the body accounts for some deviations in behaviour. Changing the excitation angle of the struck and possibly the bowed string might follow the same pattern. Changing displacement angle might change the nature of the coupling between strings, bridge and body in the following way:
The amount of horizontal displacement has a greater influence on initial amplitude than vertical displacement. This is because horizontal displacement causes lateral vibration of the string and lateral movement at the bridge. This lateral movement directly driving the cello body is the most efficient way that the bridge can act as an amplifier in transferring excitation energy to sound. Excitation energy is spent quickly; tones are loud with short decay.
The excitation energy of vertical excitation is spent more slowly. The bridge rocks towards/away from the tailpiece and transfers energy to the body less quickly. This implies a longer sound with a reduced attack. Angles between the two extremes imply a mixed output. In other words, ‘decay duration’ and ‘initial loudness of attack’ are, for fixed excitation energy, inversely proportional to one another.