Aftertouch is the amount of pressure applied to a key after a note has been played. The pressure is applied to the note after the key is struck and while it is being held down or sustained.
Aftertouch is a keyboard expression that is often routed in order to control the vibrato, volume and other parameters.
Therefore, aftertouch makes it possible to control the sound after striking a tone, typically with with a vibrato kind of Modulation effect.
Aftertouch expands the musical expression and will get a little more into keyboard expression later on this post.
There are basically two types of after touch:
Channel aftertouch and polyphonic aftertouch.
Most newbie keyboard players that I coach usually ask me the logic behind aftertouch and this prompted me to come up with a full blog post so that anyone with a similar question can benefit from this discussion.
Therefore I urge you to continue reading so you have a better sense of keyboard expression.
With that said let’s get into the types of aftertouch.
Types of Aftertouch
Channel aftertouch simply refers to the average amount of pressure applied to any keys that are held down; it is independent of which key or how many keys are held.
On the other hand, Polyphonic aftertouch is specific to each key, meaning that the MIDI data that conveys it will contain both a note number and an amount; the channel aftertouch message or data contains only an amount.
Relatively few synths and controller keyboards implement polyphonic aftertouch because it requires a more expensive mechanism.
Keyboard expressions
Now that we have an overview of the two significant types of aftertouch let’s get into keyboard expressions.
Keyboard expression is simply the ability of a keyboard or musical instrument to change its tone or other qualities of the sound in response to velocity, pressure or other variations in how the performer depresses the keys of the musical keyboard or synthesizer.
Expression types include:
Velocity sensitivity — this is simply how fast or hard the keys are pressed. You’ll notice that when you strike a key fast and hard you get a sharp bright sound. If you strike the key much slower, you’ll get a more softer sounder.
Aftertouch, or pressure sensitivity — the amount of pressure on a key, once already held down. You can most likely hear the key change its expression as you apply pressure to a key that’s already held down.
Displacement sensitivity — this is the distance that a key is pressed down.
Keyboard instruments offer a range of expression types.
Acoustic pianos, such as upright and grand pianos, are velocity-sensitive—the faster the key strike, the harder the hammer hits the strings and the louder and sharper the sound that you get.
Baroque-style clavichords and professional synthesizers are aftertouch-sensitive—applied force on the key after the initial strike produces effects such as vibrato or swells in volume which is a cook effect and adds an interesting twist to a note.
Tracker pipe organs and electronic organs are displacement-sensitive—partly depressing a key produces a quieter tone while fully depressing a key produces a louder tone.
Pressure sensitivity
On electronic keyboards and synthesizers, pressure sensitivity is usually called aftertouch.
According to Wikipedia, The majority of such instruments use only channel aftertouch: that is, one level of pressure is reported across the entire keyboard, which affects either all the notes that are pressed including those not being pushed into aftertouch or a subset of the active notes in some instruments that allow this level of control.
A minority of instruments have polyphonic aftertouch, in which each individual note has its own sensor for pressure that enables differing usage of aftertouch for different notes.
Aftertouch sensors detect whether the musician is continuing to exert pressure after the initial strike of the key or is simply steady holding down 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.
Check out more of this information about keyboard expressions on Wikipedia and sound on sound.
Final Thoughts
Keyboard expressions are vital part of music and generally make it more interesting.
This post was focused on understanding what Aftertouch is.
Therefore Aftertouch is simply the amount of pressure exerted on a key after a note has already been played. The pressure is applied to the note after the key is struck and while it is being held down or sustained.
Aftertouch is a keyboard expression that is often routed in order to control the vibrato, volume and other parameters.
There are two types of aftertouch, namely Channel aftertouch and Polyphonic aftertouch.
Channel aftertouch is basically the average amount of pressure that can be applied to any key or keys that are held down; it is independent of which key or how many keys are held.
Polyphonic aftertouch is specific to each key, meaning that the MIDI data that conveys it, will contain both a note number and an amount; the channel aftertouch message or data contains only an amount.
