Keyboards are a necessary part of today's computers, right?
Maybe not for much longer. A group of European scientists have used acoustic sensors to turn wooden tabletops
and even three-dimensional objects into a new type of computer interface.
(Credit: Image courtesy of ICT Results)
Sound vibrating a windowpane or through a tabletop is something most people experience daily. Sound waves travel well
through most solid materials. Now, European researchers have exploited the excellent propagation of sound waves through
solids to turn everyday objects – including 3D objects – into a new kind of computer interface.
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By attaching sensors to solid materials, researchers from TAI-CHI, a project working with Tangible Acoustic Interfaces for
Computer-Human Interaction, were able to locate exactly and track acoustic vibrations. Tapping on discrete areas of a
whiteboard could generate musical notes on a computer. Tracking the sound of a finger scrawling words on a sheet of
hardboard could translate, in real time, into handwriting on a computer screen. There is no need for overlays or intrusive
devices.
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Sensing vibrations in a solid and converting them to electrical pulses is the easy bit. Exactly locating the source of
that vibration in a solid material is where it gets complicated. The problem is that the complex structures of solids make
wave propagation difficult to model. Wood knots in a desktop, for instance, will alter how acoustic vibrations disperse.
Reading the signals
The TAI-CHI team investigated four main technologies. Time Delay of Arrival (TDOA) uses three or more sensors and compares
the difference in arrival times of an acoustic wave at each of the sensors to establish location. In fact, the concept of
TDOA has been around for about 100 years. Provided you know the propagation velocity of acoustic waves through the solid
material, TDOA provides a very practical, if rather expensive, solution.
Time reversal, on the other hand, needs only a single sensor. It works on the notion that each location on the surface of a
solid generates a unique impulse response which can be recorded and used to calibrate the object. Time reversal works on
3D objects just as well as flat surfaces.
MUlti-Sensor Tracking through the Reversal of Dispersion (MUST-RD) requires a deep understanding of the wave-dispersion
properties of the solid. The dispersion curve of acoustic waves moving through the material under test is compared to a
database of dispersion curves for common materials. From the comparison, the location of the vibration source can be
calculated. (MUST-RD can also be used to give a crude estimation of a material type.)
Finally, TAI-CHI researchers worked with in-solid acoustic holography. Using sound pressure, sound intensity or particle
velocity to calculate position and time, a sound source can be mapped and visualised in much the same way as an infrared
camera can map heat sources. Some of the TAI-CHI researchers also experimented with a combination of acoustic localisation
and Doppler tracking to locate and track sound sources moving through the air.
The range of researchers brought together by the project, part-funded by the European Commission – in Germany, France,
Italy, England, Wales and Switzerland – was an important factor in its success, according to TAI-CHI coordinator, Dr Ming
Yang of the University of Cardiff.
Specialist solution
Tangible acoustic interfaces like this are not going to replace keyboards and computer mice in the near future, says Dr
Ming Yang. But in specific environments where keyboards are impractical – perhaps in very dirty environments or in
hospitals where a keyboard might be a hiding place for bugs – TAIs could provide an elegant solution.
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“Time reversal is a beautiful technology,” he says. “Unlike TDOA, it works with any object and it does not require
special materials. Because it needs only a single sensor and a normal computer, it is very simple and cost-effective.
One spin-off company from the University of Paris is working on commercial applications for this.”
Other technologies, such as acoustic holography, show great promise but are not ready for commercialisation.
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CeTT, a Swiss member of the consortium, has put together a TAI-CHI Developer’s Kit, comprising algorithms developed
during the project, software and hardware, as a one-stop-shop for application developers looking to build on TAI-CHI
breakthroughs.
Other applications include a wireless sensor using Bluetooth technology that Dr Ming Yang would like to develop with
commercial partners.
The time-reverse technology is the project’s major breakthrough, according to Dr Ming Yang. “Before, people were only
working on easy materials. We have developed it for metal, plastic and board. We have a really interactive interface.”
Note: This story has been adapted from a press release issued by ICT Results.
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