Using acoustic and piezo-electric sensors, the system compares the sound made by tapping the screen to a “database of reference acoustic signatures” to determine which key is being used, according to the patent filing.
Three sensors within the keyboard – one at the top and two at the sides – would be “sufficiently precise” to triangulate the position of the tap, said the filing.
The database should also be able to differentiate between taps and innocuous contact such as resting on the screen.
A traditional Qwerty layout would be displayed on the screen below the glass for users to tap away on, but the graphical display means software designers can switch between traditional input layouts and bespoke designs.
The patent filing called touchscreen keyboards “less reliable” than traditional ones, saying they “often require that a user tap on the screen several times before detecting the command” and can't distinguish “when a user is merely resting on the surface of the device or actively selecting a letter”.
The concept isn't new: the Acer Iconia Touchbook announced earlier this year has a dual-screen design, with the secondary screen being used as a touchscreen keyboard. And Apple, of course, offers a touchscreen keyboard on the iPad.
According to Patently Apple, the company has been investigating such keyboards since at least 2009. The company filed its first keyboard patent that year, and followed it up with two more this year.
The patent could provide a clue for future designs of Apple laptops, with the company gradually implementing touch features into successive releases of OS X. It could also potentially be used in a patent war with other tablet and laptop manufacturers.