(I can not confirm that the photos used for this example are from an aftertouch-enabled keyboard. I am using these photos for demonstration purposes only.)

Keyboard Aftertouch

The top layer of a keyboard is the obvious and most visible layer: the tone-producing key “button”. When a key is pressed, a tone is produced. Depending how advanced the keyboard is, would determine how much control the musician can have on producing the tone. For example, on more technologically advanced keyboards (and usually much more expensive), a musician could produce a tone of varied loudnesses, or varied percussive attacks of the tone. Versus, a cheaper keyboard that produces the same loudness and attack no matter how differently you press the key.

All of these elements pertain to the tone-producing layer of interaction on a keyboard.

Keyboard Aftertouch: button level

The “aftertouch” layer of interaction occurs only after a tone is produced in the upper layer. Presumably there are sensors underneath the key that determine when it is pressed to produce a tone, and also sense when slightly more pressure is given to the press to activate aftertouch. Aftertouch can only be activated once a tone is generated. This fact breaks one of the rules of determining a SUI: interface independence. The upper layer of interaction must be used in order to get to the lower layer of interaction.

Keyboard Aftertouch: touch level

Aftertouch is not a SUI for multiple reasons:

• Multiple interfaces? The upper layer produces a tone, while the lower layer augments the tone. Both are related and interfacing with the tone generating interface of the keyboard. Are opposed interfaces necessary for a SUI?
• Both interfaces described here occupy the same physical space, but it is unclear whether the interfaces described here are separate and plural.
• There is no interface independence. Aftertouch can not be activated without first interacting with the upper layer of interaction.

Keyboard Aftertouch: SUI levels