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Keyboard expression
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Keyboard expression often shortened to expression is the ability of a keyboard instrument to respond to the dynamics of the music or change the tone of the sound in response to the way that the performer depresses the keys of the musical keyboard. Keyboard expression types include velocity sensitivity, which responds to how hard or fast the keys are pressed; pressure sensitivity, which responds to the force with which a key is held down after the initial impact; and displacement sensitivity, which responds to how far down a key is depressed.
Different keyboard instruments offer a range of different types of expression.
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Encyclopedia
Keyboard expression often shortened to expression is the ability of a keyboard instrument to respond to the dynamics of the music or change the tone of the sound in response to the way that the performer depresses the keys of the musical keyboard. Keyboard expression types include velocity sensitivity, which responds to how hard or fast the keys are pressed; pressure sensitivity, which responds to the force with which a key is held down after the initial impact; and displacement sensitivity, which responds to how far down a key is depressed.
Different keyboard instruments offer a range of different types of expression. Acoustic pianos such as upright pianos and grand pianos are velocity sensitive. Baroque-style clavichords and professional-level modern electronic keyboards are pressure sensitive; the degree that the performer is pressing the key after the initial strike will produce effects such as vibrato or swells in volume. Old tracker pipe organs and modern electronic organs are displacement senstive; if the keys are only partly depressed, a quieter tone is produced.
Velocity sensitivity
The piano responds to the force with which the keys are initially pressed by changing how hard or fast the hammers strike the strings, which in turn changes the tone and volume of the resulting note or chord; it is velocity sensitive. Several of its predecessors, such as the harpsichord, were not velocity sensitive than the piano. There is some confusion relating to the term pressure sensitive, with some using it as a synonym for velocity sensitive. To avoid this confusion, pressure sensitivity is sometimes called aftertouch. Both velocity and (true) pressure sensitivity are supported independently by the MIDI standard.
In general, only the top of the line electronic keyboards implement true pressure sensitivity, while most professional-quality electronic keyboards support velocity sensitivity. Most inexpensive electronic keyboards, such as toy electronic keyboards and basic learning keyboards made by Casio and Yamaha in the $100 USD price range do not have velocity sensitivity. Some manufacturers use the term "touch sensitive" in their advertising. Even on a touch-sensitive keyboard, not all digital instrument sounds may incorporate velocity sensitivity into the sound's envelope; for example, the digital pipe organ sound often has no velocity sensitive effects, in imitation of the real instrument. The manufacturers and distributors of some inferior keyboards incorrectly describe their purely velocity sensitive instruments as pressure sensitive.
Pressure sensitivity
The clavichord and some electronic keyboards also respond to the force with which a key is held down after the initial impact; they are pressure sensitive. This can be used by a skilled clavichord player to slightly correct the intonation of the notes when playing on a clavichord, and/or to play with a form of vibrato known as bebung.
Unlike in a piano action, the tangent does not rebound from the string; rather, it stays in contact with the string as long as the key is held, acting as both the nut and as the initiator of sound. The volume of the note can be changed by striking harder or softer, and the pitch can also be affected by varying the force of the tangent against the string. When the key is released, the tangent loses contact with the string and the vibration of the string is silenced by strips of damping cloth.
By applying a rocking pressure up and down the key with the finger, a performer can slightly alter the vibrating length of the string itself, producing a vibrato quality known as Bebung. While the vibrato on fretless string instruments such as the violin typically oscillates in pitch both above and below the nominal note, clavichord Bebung can only produce pitches above the note.
Bebung was often not explicitly indicated in sheet music; the assumption was that, like other ornaments, Bebung was to be applied at the performer's discretion. Where Bebung is indicated, it appears as a series of dots written above or below a note, with the number of dots indicating the number of movements of the finger.
On electronic keyboards and synthesizers, pressure sensitivity is sometimes called aftertouch. The aftertouch sensors detect whether the musician is continuing to exert pressure after the initial strike of the key. The aftertouch feature allows keyboard players to change the tone or sound of a note after it is struck, the way that singers, wind players, or bowed instrument players can do. On some keyboards, sounds or synth voices have a preset pressure sensitivity effect, such as a swell in volume (mimicking a popular idiomatic style of vocal performance with melodies) or the addition of vibrato. On some keyboards, the player can select which effects are applied when aftertouch is detected. This allows a performer to custom-tailor the effect that they desire. It may also facilitate the imitation of various non-keyboard instruments. For example, a keyboardist who wishes to imitate the sound of a heavy metal guitar solo could use a distortion guitar sound, and then set the aftertouch feature to apply a pitch bend to the note.
Displacement sensitivity
A third form of sensitivity is displacement sensitivity. Displacement sensitive keyboards are often found on organs. Most mechanical organs, and some electrically actuated organs, are displacement sensitive, i.e. you can press a key part way down, and the corresponding note (pipe, reed, or the like) in the organ will produce some lesser amount of sound than when the key is fully depressed. In some organs, the pitch and/or tone colour may also be subtly altered. Small tabletop organs and accordions often respond similarly, with sound output increasing as keys are pressed further down. Even the small circular accompaniment ("one button chord") keys found on accordions and on some organs exhibit this phenomenon. Accordingly, some electrically actuated organs have retained this form of keyboard expression:
Other more sophisticated forms of sensitivity are common in organ keyboards. Both the Pratt Reed and Kimber Allen 61-key (5-octave) keyboards have provision for installing up to nine rails, so that they can sense various amounts of displacement, as well as velocity in various regimes of distance from the top to the bottom of the key travel of each key. Some modern instruments such as the Continuum, a MIDI controller for keyboards, have extremely sophisticated human interface schemes, allowing dynamic control in three dimensions. In principle, displacement can be differentiated to get velocity, but the converse is not entirely practical, without some amount of baseline drift. Thus a displacement sensing keyboard may have greater versatility when it is desired to have both organ and piano behaviour in an input device.
Other types
Acoustic pianos have expression pedals which can change the response or tone of the instrument, such as a sustain pedal and a soft pedal. On small upright pianos, the "soft" pedal lowers a sheet of felt in front of the hammers. On grand pianos, the hammers are moved further back from the strings. The sustain pedal allows all of the notes of a chord to sound. In addition, since the dampers are not applied, it allows octave and fifth overtones to vibrate sympathetically, producing a richer sound. Most electronic keyboards also have a sustain pedal which allows notes and chords to be held; however, only the highest-end digital keyboards reproduce the sympathetic vibration effect.
Electromechanical keyboards and electronic keyboards offer a range of other expression devices. Electromechanical keyboards such as the Hammond organ offer additional means of keyboard expression by modifying the starting, stopping, or speed of the rotating Leslie speaker or by engaging a variety of vibrato or chorus effects. Digital "clones" of Hammond organs offer recreations of these effects, along with other effects. The VK-9 digital organ, for example, offers a proximity-sensitive detector which can be used to trigger the Leslie speaker, a ring modulator effect, or a variety of other effects.
Some effect pedals which are used with electromechanical keyboards such as the Fender Rhodes electric piano or digital keyboards respond to the velocity with which the keys are struck. Examples include velocity-sensitive overdrive pedals, which produce a clean sound for gentle keystrikes, and a distortion effect for hard keystrikes; and fixed wah-wah pedals, which respond to keystrikes by producing a wah-wah pedal effect which alters the tone.
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