TechArchive

Original article date: May 2000

With their dated technology, even the best speakers are the weak link in any Hi-Fi system. But here’s a digital alternative!

If the company 1…Limited has its way, there is going to be a revolution in loudspeakers in the next few years. Conventional loudspeakers are the weakest link between recording sound and reproducing it – they are still based on magnets and moving coil technology invented in the 1920s. 1…Limited is developing a revolutionary digital loudspeaker which will incorporate new transducer technology using helical springs made of piezoelectric ceramics.

The company defines a Digital Loudspeaker (DLS) as a speaker which takes digital signals, for example from a CD player, as its input and emits digital pulses of sound. It contains no electrical digital to analogue converter: that task is left to the human ear. Speakers using piezoelectric ceramic springs will be able to project three dimensional sound across a space and do it in a way that can ’steer’ a sound in a particular direction.

Tony Hooley at 1…Limited came up with the idea of using helical piezoelectric ceramic springs in digital speakers. Quartz is a natural piezoelectric material used in digital watches and computers to produce stable vibrations at high frequencies. Polycrystalline ceramics, such as PZT (lead zirconate titanate) can be made to mimic the behaviour of natural monocrystalline materials by polarising the crystals within the ceramic.

An electric field is applied 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. The problem lies in the manufacture of helical springs from this material. Such a complex shape couldn’t be machined from a solid block, for example. Hooley found a partner in Dr David Pearce from the University of Birmingham, whose experience in the field of piezoelectric materials gave him the confidence to build the helices.

The helical springs start in powder form mixed with polymer and solvent. High temperature metal electrodes are applied, simply using screen printing techniques. The plastic material can then be formed into virtually any shape. As the solvent dries, the material solidifies and a certain amount of predictable shrinkage occurs. The polymer is then removed through slow heating to 600degC, followed by sintering of the remaining ceramic at between 1100 and 1200degC.

Good vibrations

The spring coils around an inner section which contains a novel low-friction contact bearing and a moveable core. The coil tries to coil or uncoil when an electric current is applied, affecting the pressure inside the linear bearing. If the coiling is greater at one end, the core will be forced to move along the axis of coil in line with the applied voltage. This is rather like a hand squeezing a wet bar of soap. The movement generates a vibration of the air next to it and therefore produces noises.

When an array of these devices are placed together they can produce a unique way to shape sound. Not only can they produce a coherent sound image away from the speaker, but the transducers can be driven by clever digital processing, so sounds can be aimed in a particular direction. This will have applications in PA systems, as well as audiovisual areas like cinemas. The prototype has already been built to prove that the ceramic helices work in the way the researchers expected them to, but there is still some way to go. The small transducers will have to be mass-produced in a cost-effective way. Besides the springs – and there are companies working on other aspects of the design – the low-friction contact bearing which transmits force from the helix to the core must be able to move very quickly. The current design being developed by Ranier in Cambridge uses an inflated tolerative polymer, rather “like a balloon rolled in on itself,” as Pearce describes it. But the material needs to be developed further.

1..Limited and the University of Birmingham have teamed up with a number of other companies and academic institutions in a Framework 5 European Programme, PADS, to develop this exciting technology to a fully operational prototype in a couple of years time. Marketable speakers should follow a couple of years after that, offering pure digital sound.

For further information, the University of Birmingham IRC in Materials can be contacted on: 0121 414 7836, and 1…Limited is on: 01223 422290.

• University of Birmingham IRC in Materials
  Tel: 0121 414 7836
• 1…Limited
  Tel: 01223 422290

May 2000