Haptics via Ultrasound
All manner of approaches are being fielded these days to try and create a workable haptic interface, that allows the true feeling of touch to an object that is only virtually there. One of the more esoteric, is the use of sound to shape the pressure fields in the air. Specifically, the use of ultrasound.
Termed acoustic radiation pressure, this form of haptic feedback is actually a creation of very high frequency sound pulses. A speaker system sets up a specific acoustic wave that creates a difference in pressure density at a given location above it. Modulating the sound carefully around the speaker allows shapes to be made in the air, that exert differing amounts of pressure on anything passed through there - such as body parts. This replicates the feeling of brushing wind, or a ghostly almost-solid object.
An immediate problem of course is that almost-solid status. There is only so far you can push using sound, before the vibrations start to damage listening equipment nearby - such as the eardrum. Still, a group of Japanese researchers have produced a prototype that is capable of creating ghostly objects in the air, without reaching this point - and from a single, somewhat complex speaker disk. If more disks were added, perpendicular to the existing one, then it seems highly likely that an actual three dimensional 'ghost object' or ghostly landscape could be created, with some degree of fidelity.
The researchers from the University of Tokyo, led by Takayuki Iwamoto demonstrated their first prototype in 2008. The basic construction of their speakers is a group of concentric rings of ultrasonic transducers arranged into an array on a pad. These transducers can be set to each emit a different modulated ultrasound pulse, allowing each to set up a pressure wave pushing through the air. Thus a single surface can be created at a defined point above them. No work has yet been done on the possibility of multiple pressure waves, so only simple objects can be crafted - no surfaces behind surfaces at this point.
The team's prototype systems all include a camera which tracks the position of a user's hand and shifts the output from the transducers to move the focus around with the hand. The result is a feeling of tracing the edge or surface of the virtual object.
The system is still under active development however, and the array geometry is still being tweaked, to give the most control over the possible outline of virtual shapes. In theory at least, at this point something as complex as a moving hand might be possible. Likewise the amount of power the device can utilise at any given time is also under refinement.
Professor Iwamoto envisions marrying the approach with 3-D modelling software, social VR and gameworlds, hoping to increase the feeling of immersion whilst other approaches to haptics, mature. Among its advantages, are that as the virtual object exists in physical space as a collection of pressure waves, multiple people can interact with it at the same time. The camera element allowing real-time changes to the pressure waves, and thus the object.
In essence, it is a very basic holodeck type technology - a type of constrained forcefeed made of ultrasound.
Non-Contact Method for Producing Tactile Sensation Using Airborne Ultrasound (Original Paper) (PDF)