Brussels, Jan 2004
The PADS project developed several production methods for a novel ceramic-based actuator. Applications include the world's first broad-spectrum digital loudspeaker and potential as an optical shutter for cameras.
The technology of sound has improved dramatically since the days of scratchy gramophone records and crackling telephone lines. Today, almost every element of sound reproduction is digital – except for loudspeakers. These are still based on 19th century technology, and are the weakest link in the chain between recording sounds and reproducing them.
Inside most loudspeakers an electromagnet vibrates the loudspeaker cone to produce sound. Digital signals are converted into an analogue format to drive the actuator. Distortion is unavoidable, and the purity of digital sound is lost.
PADS has brought loudspeaker technology into the 21st century through the collaborative production of a prototype digital loudspeaker. At its heart is a ceramic actuator, patented by UK-based start-up 1 Limited. Made from a curved, coiled ceramic strip, the Helimorph device 'translates' an electrical voltage into physical movement of up to several millimetres. One side of the ceramic strip expands in an electrical field and the other side shrinks, just as a bimetallic strip bends in response to temperature changes.
The UK company initiated the project in 2000, bringing together European expertise to develop production methods for the actuator and explore potential applications in digital loudspeakers, and in an aerial that would use the actuators to move and track low-Earth-orbit satellites.
Two academic partners researched alternative production methods for the Helimorph actuator: The University of Birmingham used viscous plastic processing to make multi-turn and tapered Helimorph actuators that provide high acoustic output. The Friedrich-Alexander University in Erlangen applied tape-casting to make a multi-layered actuator that would respond to much lower voltages, such as those found in mobile phones.
Dutch company Haikutech used a third technique to manufacture an actuator called the S-morph. Another project participant, Bookham Technology, used this device as a potential optical shutter driver.
" These three new ways to produce the actuator will be important in any commercial developments," says project co-ordinator Dr Anthony Hooley of 1 Limited. "Each has its own benefits and cost constraints, depending on the precise design and intended end-use of the actuator."
Another academic partner, the University of Manchester Institute of Science and Technology (UMIST), modelled the actuators' dynamic and operational behaviour. This provided an indispensable tool for analysing performance and assessing possible loudspeaker designs.
1 Limited incorporated the Helimorph into an acoustic transducer about 25 mm in diameter. The company also developed the necessary electronics and software to interpret Dolby and Digital Theatre System (DTS) multi-channel digital audio signals and calculate the transducer pressure patterns. The prototype loudspeaker developed in PADS has an 80-transducer array. It produces high-quality sound, with peak pressures comparable to conventional domestic speakers.
The transducer array provides an additional feature for the digital loudspeaker. "You can shape wave patterns and send them in different directions," explains Hooley. "We have ended up with a system that can produce surround sound by bouncing waves off the room walls and ceiling, so they reach the listener from different directions. Instead of a room full of boxes and wires, you need just one speaker in the middle."
And the innovation did not stop there. The project also decided to incorporate a new method of sound absorption into its speaker. The Laboratoire Ondes et Acoustique (LOA), part of the University of Paris, brought its expertise in sound measurement to the project and optimised a silica-based aerogel. Measurements confirmed that multiple layers of the aerogel outperform conventional sound absorbers, particularly at low frequencies. The use of an aerogel sound absorber makes the digital loudspeaker more compact than conventional models.
Miniature autofocus system
"There were a lot of changes over the course of the project," Hooley recalls. "One potential partner dropped out just a week before we submitted the proposal. Then Marconi's optical components business was bought by Bookham, and that changed an entire strand of the project from the antenna to optical shuttering. Because we were breaking new scientific and technical ground, we never knew what to expect. We had to be flexible, and steer the project in new directions as new discoveries and understandings arose, all the way to the end of the project."
Perhaps the biggest change in direction has occurred since the completion of the PADS project in March 2003. Although work on digital loudspeakers and acoustic transducers continues, 1 Limited is currently pursuing applications in autofocus systems for mobile phones and miniature digital cameras. "It is estimated that more than 40% of mobile phones sold in 2004 will incorporate digital cameras. Potentially, we may see the Helimorph in 100 million phones worldwide within a year or two," says Hooley. "That is a considerable market to reach in the space of just a couple of years!"
The PADS project has helped to develop commercially viable applications for the innovative actuator technology. "If half the world's mobile phones end up with a component invented in Europe, the economic impact could be considerable – especially if the ceramic is manufactured in Europe as well."