How does the piano action work?

The piano action is an amazing mechanism.  Its crucial task is to transfer musical energy seamlessly from the pianist’s fingers to the piano strings, allowing the pianist’s musical intentions to be transferred accurately and artistically through the instrument.  To accomplish this “seamless transfer of energy” properly, all 8000+ parts of the piano action must remain stable and work together perfectly at all times – over the course of time.  Even the smallest dimensional or positional change in any component part can hurt piano performance by preventing the action mechanism from working properly.  When problems occur, the player will sense the loss of optimal tone and touch.  Performance suffers – and the music produced will be far less satisfying for player and listener than it could have been. 

When a piano key is pressed, a corresponding hammer rises to strike the strings causing the highly-tensioned strings to vibrate.  The energy of the vibrating string is transferred to the soundboard through a component called the “bridge” that is fixed to the top of the soundboard.  Because  the soundboard is mounted with tremendous tension (similar to the tensioned head of a drum), it resonates intensely when sound energy is received.  It is the resonating vibrations of the soundboard that a listener hears as “tone” when a piano is played. 


When a grand piano key is depressed, the other end of the key lifts upward with equal force.  This key movement lifts a screw known as the “capstan.”  The capstan is located under a multi-faceted hinged assembly called the “wippen” that contains an L-shaped part called the “jack.”  The upward force of the capstan causes the wippen to rise, thrusting the jack upward into another component called the “knuckle” that is fixed to one end of the hammer shank. As the knuckle and hammer shank are forced upward, the felt hammer (that is fastened to the other end of the shank) is propelled toward the string to create the hammer strike.  From this description, one can see that the flow of energy from “a key being pressed” to “a string being struck” is actually an intricate series of progressive steps that lead to the ultimate hammer strike. 


The hammer strike, however, is only the beginning of the process for creating piano tone.  The hammer must bounce off (or “release”) from the string immediately so the strings can vibrate.  This hammer release must occur even if the player holds the key down. Without this release, the hammer would remain pressed against the strings, muting any tone that may be created and producing only a dull “thud” instead of resounding tone.  What makes this hammer release possible? As the jack raises the knuckle, it engages a component called the “let-off button” that causes the jack to release from the knuckle at the precise moment before the hammer hits the string.  This “let-off” allows the hammer to fall away so the strings can vibrate freely after the strike. 


But the hammer release presents another potential issue.  What if the player wants to repeat the same note immediately?  If the hammer drops all the way down to its initial rest position, it would take too long to reset and play a quick repeated note.  To solve this problem, the end of the key is fitted with a component called the “backcheck” that rises as the key is pressed and catches the hammer as it rebounds from the string.  The raised backcheck holds the hammer just under the strings awaiting the possibility of another strike.  If the note is repeated, the key only needs to be raised slightly for the jack to reset for another immediate strike.  This inventive design makes a series of rapid repeated notes possible.


But there is one more problem the action must solve.  The pianist must be able to not only make notes sound, but also make them stop sounding.  When a key is not being played, a component called the “damper” rests on top of the corresponding strings to prevent them from resonating with other vibrating strings (a phenomenon called “sympathetic vibration”).  When a key is initially pressed, the other end of the key pushes up on a component called the “damper lever” which immediately lifts the “damper felts” off the strings for that note.  (You can now see that two things must occur simultaneously to keep a string vibrating – the hammer must release and the damper must be raised).  While the key remains pressed down, the damper hovers above the strings allowing them to vibrate.  When the player releases the key, the damper falls back down onto the string to silence it.  In this way, the damper mechanism allows the player to control the length of each note.  In addition, the pianist can also use the “damper pedal” below the keyboard to lift all dampers at one time.  The damper pedal is also called the “sustain pedal” because it allows the sound of all notes to be sustained (or continually resonating) at the same time.  When the damper pedal is released, all dampers fall and the sound stops for all notes (except those that are still being sustained by keys that are depressed).

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