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In trying to connect the observations of the hammer vibrations with the tone generation, we automatically return to the question of the pianist's "touch" and the conflicting opinions of physicists and pianists hinted at in the introduction. Before trying to answer, we must first clarify the question. Physicists usually claim that as the mechanical contact between the key and the hammer is broken before the hammer strikes the string, the pianist can only influence the final velocity of the hammer and thereby the loudness, and nothing more. Many pianists on the other hand, claim that important shadings in the character of the notes can be achieved by applying different types of touch, and that such a skill is an important component of the art of piano playing.
First of all it is important to realize that the term "touch" probably is used with several different meanings. If a pianist is said to have a "beautiful touch," it may refer to the way a melody part is lifted above an accompaniment, or how certain notes in a chord are emphasized. This seems mainly to be a question of the timing and strength of certain notes relative to other notes, factors which have been shown to separate the artist from the amateur. But in certain connections, a "beautiful" or a "bad" touch can refer to the character of a single note at a given dynamic level, a topic which has interested prominent pedagogues and even created tensions between pianists of different schools.
Surprisingly, a strong candidate for part of the answer has nothing to do with the normal string motion. It is probably so that a characteristic percussive component ("thump") at the onset of the note plays a decisive role for the character of the piano tone. This "thump" is generated by the key as it hits the stop rail on the key frame. The impact shock excites the keybed (the supporting surface under the action), and partly also the soundboard and iron frame.
The significance of the "thump" sound is illustrated in sound example 8, in which the normal airborne sound of a grand piano and the sound of the string vibrations in isolation can be compared. The listener will probably agree that the normal piano sound as recorded in the room has a certain resemblance with the sounds in a blacksmith's shop. The string sound component on the other hand, lacks something of the interesting piano character, resembling a plucked string more closely than a struck one. Once these components have been identified, they are usually easy to distinguish in all piano tones.
The "thump-component" is undoubtedly excited differently depending on the touch and could be assumed to be characteristic of a pianist's way of playing. The importance of this component of the piano sound is further illustrated by knowing that the recognized piano manufacturers select the wood for the keybed with great care in order to achieve the right "thump" quality.
Nothing definite can be said yet about the pianist's ability to influence the tone quality by controlling the motion of the hammer It is true that the entire history of the hammer motion during acceleration can be very different depending on the way the key is depressed (cf. Fig. 11). But this observation does not automatically imply that the interaction between hammer and string is influenced in some way. As mentioned, the hammer is flying freely the final distance before string contact. However, we have recently observed that the motion of the freely flying hammer can be somewhat different for slightly different types of touch (see Fig. 14). This is due to the "ripple" resonance, which is seen to influence the hammer motion during the last milliseconds before string contact, i.e. after "let-off." In this way, it is possible that a hammer resonance could serve as a "memory" of the history of the hammer motion. This would allow the pianist to have at least an indirect influence on the very final part of the hammer motion, even after mechanical contact between key and hammer has ceased.
Fig. 14. Comparison of the vertical motion of the hammer head before string contact for slightly different types of touch and regulations of the let-off distance ("staccato-touch," mezzo forte, C4). The dashed lines indicate the moment of contact between jack and escapement dolly ("let-off" begins). Note that the motion of the hammer head can be influenced by the "ripple" mode during the short interval between this line and string contact. The curve at the top corresponds to a longer setting of the let-off distance (3 mm) than normal.
However, we still have to be very careful in the interpretation of these results. Although the observations suggest that the hammer can make a rubbing motion against the string during contact, it has not yet been shown that such a motion takes place. Even if it does, it remains to be shown that a motion of this kind could influence the string excitation and hence the tone quality.
In summarizing the present results, we cannot rule out the possibility that the pianist's touch may have an influence on the character of the piano tone. As regards the initial "thump" it seems rather clear that it contributes to the character of the note, while an actual influence on the tone character via the hammer motion has not been verified yet.
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This lecture is one of Five lectures on the Acoustics of the piano
© 1990 Royal Swedish Academy of Music