Scratch input

In computing, scratch input is an acoustic-based method of Human-Computer Interaction (HCI) that takes advantage of the characteristic sound produced when a finger nail or other object is dragged over a surface, such as a table or wall.

The technique is not limited to fingers; a stick or writing implements (e.g. chalk, or a pen) can also be used.

Scratch input was invented by Mann et al. in 2007,[1][2][3] though the term was first used by Chris Harrison et al.[4] A natural interface for musical expression operating on scratch input principles was first published and presented in June 2007.

Harrison et al.[4] proposed it to create large, ad hoc gestural input areas when mobile devices are rested on tables.

If mass produced this sensor could cost less than a dollar....Despite the limitations, the technology holds enough promise to make it into the hands of consumers.

Scratch input using geophones (contact microphones) bonded to each fingernail as a musical instrument. The awful sounds generated by running fingernails down the chalkboard are separately pitch-corrected, resulting in a pleasant-sounding musical instrument.
Scratching a stick on bricks and mortar, with vision-assisted gesture recognition. This gives better results than scratch input alone. (Public performance in Toronto, Ontario; image from the 2007 paper.)
Scratch input with ice skates is called a pagophone. [ 3 ]