Digital speakers that can project three dimensional sound across a room are being developed using new springs built from piezoelectric ceramics. Applying an electrical current to the material forces the spring to expand causing a vibration that produces a coherent sound image away from the speaker.
Piezoelectric materials such as quartz are used in digital watches and computers to produce stable vibrations at high frequencies. Polycrystaline ceramics, such as PZT (lead zirconate titanate), can be made to mimic the behaviour of these natural monocrystaline materials by polarising the crystals within the ceramic. This is done by applying an electric field to the material at high temperature to align the microscopically small piezoelectric domains within the material. This produces a net polarisation in the direction of the electric field which, when a smaller electric field is later re-applied to the material, causes a strain in the direction of the polarisation. In traditional speakers, these materials are used in transducers to convert the digital pulse into a vibration that expands and compresses the air to form an audible sound wave.
By using helical springs of the material, items once though to be nothing more than a laboratory novelty, the development team at the University of Birmingham and commercial company 1ŠLimited is breaking new ground. The new transducers developed by the team are constructed from a traditional piezoelectric ceramic coiled like a spring around an inner section containing a bearing and a moveable core. When an electric current is applied, the spring attempts to coil or uncoil, affecting the pressure inside the linear bearing. If the coiling is made greater at one end than the other, the core will be forced to move along the axis of coil in line with the applied voltage. This movement, similar to the movement of a hand squeezing a wet bar of soap, generates a vibration of the air next to it and therefore producing noise. An array of these devices, through a trick of human audio perception, can be used to produce a coherent sound image away from the speaker resulting in a three dimensional sound profile.
Prototypes of the ceramic helices have already been made and work in the way expected, says Dr David Pearce from the University of Birmingham. ³Further work is progressing on assembling the whole device into a working actuator,² says Pearce. ³Working prototypes of these speakers will hopefully be produced in the next couple of years, with marketable products in the years after that.²
This item is due to appear as an article in the December issue of Materials World, the journal of the Institute of Materials.(Due for publication on 1.12.1999)
Notes For Editors 1. Brief contents of Materials World, The journal of The Institute of Materials, are also available on the web: www.materials.org.uk