Besides the imaginary air piano, there is a motion-activated musical instrument interface called an "AirPiano" which is controlled by waving the hands above the top.
[3][4] The AirPiano is a musical computer interface (from 2007) which allows playing and controlling software instruments simply by moving hands in the air over the device,[3] connected by USB cable.
[4] Above the AirPiano is an invisible matrix of virtual keys plus faders, each pre-assigned with MIDI messages and waiting to be triggered, via infrared sensors.
[4] The length of each triggered note is determined by the time the hand is placed over the corresponding virtual key.
The AirPiano technique has been filed as a Provisional U.S. Patent Application (Number: 60/989,986).