A note played on a piano contains more than a single frequency. Under the usual tuning standard, for example, A4 has a fundamental frequency of approximately 440 Hz, but its sound also contains higher partials and transient components at the onset. The relative strengths of these frequency components, together with the way they change over time, contribute to what we hear as timbre.
The same note can therefore have different qualities. A faster hammer strike usually produces a louder sound and makes the higher partials relatively more prominent, giving the tone greater brightness. This changes both the overall volume and the balance between frequency components. Pedalling, the timing of key release and sympathetic vibration in other strings further affect how the sound is sustained and decays.
To understand how touch affects sound, we need to consider more than the instant when the hammer meets the string. The hammer begins at rest and must undergo a period of force and acceleration before it acquires the velocity and kinetic energy needed to strike. The finger moves the key down, and the action drives the hammer. This takes time; it does not happen instantaneously.
When each stroke begins with the action fully returned to rest, the distance between a given hammer and its strings is essentially fixed. Playing softly or loudly does not move the strings. Provided the hammer reaches them, it must travel essentially the same distance. Yet equal distances do not imply identical force histories or equal final velocities.
Imagine starting twice with a finger resting on the key. In both cases the initial velocity is zero. Pressing down slowly on one occasion and accelerating rapidly on the other can nevertheless produce different hammer velocities at impact. The difference lies in the magnitude, duration and changing pattern of force during the driving phase, and hence in the energy transferred to the hammer. If the key is depressed extremely slowly, the hammer may acquire too little kinetic energy to reach the strings after the driving mechanism disengages. The key can then reach the bottom without producing a string strike.
Force and velocity are thus two related physical quantities in this process. Force causes the hammer to accelerate; acceleration accumulated over time produces velocity. By applying force, the pianist controls the motion of the key and action and thereby influences the hammer’s state at impact. The importance of impact velocity does not make force merely a psychological sensation.
Actual force must also be distinguished from the subjective feeling of effort. Muscular tension or continuing to press after a keystroke can make playing feel strenuous without necessarily increasing hammer velocity. A well-coordinated movement can feel easy while still applying sufficient physical force to the key. When the hammer starts from rest and its travel distance is essentially fixed, a very small driving force throughout the movement makes it difficult to achieve a high impact velocity.
Another important feature of the piano action is escapement. As the hammer approaches the strings, the mechanism driving it disengages, allowing the hammer to continue towards them under its existing motion. During a normal strike, the finger therefore cannot use that same driving mechanism to add further striking force at the moment the hammer contacts the strings.
This tells us when direct driving ends. It does not erase the force that has already acted.
The downward movement of the key overlaps in time with the hammer’s movement towards the strings. Before escapement, the finger can influence the hammer through the key and action: this is the window in which force and energy are transmitted. After escapement, both key and hammer may still be moving, but the key is no longer directly driving the hammer. Their movements therefore overlap for longer than the period of direct force transmission.
This also explains why a note can sound before the key reaches the bottom without the key still driving the hammer at the onset of the sound. Key bottom, escapement and string impact are separate events. The hammer may already have escaped and struck the strings while the key is still completing its downward travel.
Two stages of force transmission need to be distinguished here. In the first, the key action drives the hammer. In the second, the hammer makes contact with the strings and exerts force on them. The latter process may still be continuing as the sound begins. Hammer–string contact is not a durationless point: the felt compresses, the strings deform, and the contact force changes throughout a brief interval. Energy continues to pass into the strings until hammer and strings separate.
A discussion of touch should therefore give attention to more than the fact that the hammer eventually leaves the key’s direct control. We also need to understand what happens before escapement: how the finger applies force, how the key accelerates, how the action responds and how the hammer progressively acquires its state of motion before impact. This history of force and movement can be complex, and different forms of touch can produce different histories.
However, different force curves do not automatically produce audible differences in timbre. Some differences primarily change impact velocity, affecting volume and the relative strengths of the partials. Others affect finger–key noise, action noise and the overall impression of the onset. Establishing whether the preceding force history can alter the sound radiated by the strings when hammer impact velocity is held constant requires specific measurements; the player’s tactile experience alone cannot settle the question.
A 2025 study found that expert listeners could distinguish certain timbral differences between keystrokes with different key accelerations while key velocity and other measured features remained approximately constant. The researchers suggested that elastic movement of the hammer shank might contribute. However, the study did not directly measure the internal motion of the hammer, shank and strings. It therefore did not establish that impact force can be independently controlled while hammer impact velocity remains exactly the same, and the mechanism still requires further investigation.
The point of this discussion is that the force acting before escapement genuinely helps shape the hammer’s state of motion before impact. Impact velocity is crucial, but it is itself the outcome of an earlier history of force and movement. The fact that escapement occurs before string contact does not rule out an influence of touch on sound. Explaining a particular difference requires examining the transmission of force, the resulting motion and the sound actually produced.
References
Werner Goebl, Roberto Bresin and Alexander Galembo, Touch and temporal behavior of grand piano actions, 2005.
Click to access Goebl-Bresin-Galembo_JASA2005_PianoAction.pdf
Werner Goebl, Roberto Bresin and Ichiro Fujinaga, Perception of touch quality in piano tones, 2014.
Click to access GoeblBresinFujinaga2014-JASA-PianoTouchQuality.pdf
Kaori Kuromiya, Yuya Kobayashi, Masato Hirano and Shinichi Furuya, Motor origins of timbre in piano performance, 2025.
https://doi.org/10.1073/pnas.2425073122
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