When MC Hammer rapped ‘You can’t touch this’ little did he know of the work being carried out by a group of scientists at Bristol University. The team led by Dr Ben Long and colleagues Professor Sriram Subramanian, Sue Ann Seah and Tom Carter have produced an ultrasonic sound system able to generate 3D shapes in mid-air that can be felt.

The apparatus consists of a 16 x 20 array of ultrasonic transducers. The phase of the signals driving the individual transducers is altered to produce a shaped pressure wavefront. This is similar to the technique used in phased-array radar systems which steer a radar beam by altering the drive signal phase to an array of transmitting antennae. The team from Bristol have demonstrated how the system can produce a ‘pressure profile’ of a 3D shape in space that is tactile. By aiming the array of ultrasonic transducers at the surface of an oil bath the team show how the pressure waves making up basic three dimensional shapes such as pyramids and spheres can be made visible on the oil surface.

Taking into account ultrasonic wavelengths it is unlikely that much small detail can be represented but according to Dr Long “Touchable holograms, immersive virtual reality that you can feel and complex touchable controls in free space, are all possible ways of using this system. In the future, people could feel holograms of objects that would not otherwise be touchable, such as feeling the differences between materials in a CT scan or understanding the shapes of artefacts in a museum”.