The moving sound of magnetostriction

As well as revolutionise audio equipment design, a phenomenon first observed by Joule in 1842 could also offer a cheap solution to London’s first white knuckle bridge.

Since Joule’s discovery, magnetostriction has been widely researched, but it’s only recently that the observation that iron changes shape in a magnetic field has begun to be seen as a potential solution rather than a problem.

Magnetostriction is Newlands Technology’s area of expertise, and the company has just signed a collaborative agreement with design consultancy PDD to encourage uptake of its technology in a number of innovative applications.

Newlands specialises in the use of Terfenol — a compound of iron and two rare earth elements: dysprosium and terbium — which, under the application of magnetic fields, generates huge magnetorstrictive strains, amounting to more than 0.1% changes in length. Importantly, this movement can also be generated under load, converting magnetic energy to mechanical energy with efficiencies greater than 60%.

This makes the material suitable for actuation applications, and, because the magnetostrictive effect is reversible, Terfenol also has potential use as a sensor.One possible use of the technology is in stabilising London’s very own ‘galloping gertie’, the Millennium bridge. Engineers at Ove Arup are faced with an expensive and time-consuming re-engineering process. Newlands believes magnetostriction could be the answer. Indeed, tests have shown that a magnetostrictive actuator placed at one end of the bridge could be used to negate the bridge’s oscillation.

The technology also has potential wherever a small amount of high-powered movement is required in a compact space envelope. The power/displacement ratio is enormous, with output forces of upto 1.5 tons achievable from a small mass of material. Any reciprocating, rotating or linear device could be replaced by a Terfenol engine

However, perhaps the most exciting application of this technology was recently demonstrated to DE at PDD’s West London offices.

In a presentation which began like any other, PDD’s Design Director, Brian Smith, announced that we heard his amplified voice not through conventional loudspeakers, but through the windows. Tiny magnetostrictive actuators attached to the windows and electrically connected to an audiofrequency signal source had turned the windows into loudspeakers.

Indeed, the actuators will cause any surface which will vibrate at audiofrequencies to radiate sound. Thus, doors, tables, walls, ceiling panels, and even your own teeth can be turned into a loudspeaker.

This opens up a range of intriguing design possibilities.

The technique means that sound is radiated evenly over the whole of a surface, ensuring that the power is distributed more uniformly than with conventional loudspeaker systems. So, rather than having a single sound source that is intrusively loud at one location, sound can be spread about, using, for example, the ceiling panels of a club to distribute music across the whole room.

A similar approach could be used in public address systems. PDD envisages tannoy applications where the fabric of the building is used to transmit sound.

The technology is expected to generate excitement amongst manufacturers of audio equipment who could use it to build music systems into everyday objects. ‘We are very excited about the opportunities created with this original technology’ commented Ivor Tiefenbrin of Linn Products, a manufacturer of sound and vision entertainment systems.But applications don’t stop here, actuators could be used by banks to apply white noise to windows and counter laser espionage devices, and mobile phones could be designed which are completely sealed and therefore totally waterproof.

Finally, as sound can also be transmitted via what is known as ‘bone conduction’, hearing aids could be designed which negate the need to transmit sound via the diaphragm bones of the inner ear. By vibrating the mastoid sound can be transmitted directly to the cochlear.

Newlands Technology

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