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Flat-pack loudspeakers

February 13, 2002

IMAGINE the din. Every cardboard-packed item in a supermarket could soon be blaring out reduced-price messages or playing advertising jingles. It`s just one of the uses being touted for a build-it-yourself, flat-pack loudspeaker developed by British company NXT.
        
The Cambridge-based firm`s collapsible speakers are made of "acoustic fluted composite"-otherwise known as corrugated cardboard. Versions designed to be used with home sound systems are pyramid-shaped and stand about 70 centimetres tall. Advance demonstrations for New Scientist of a £30 pair revealed surprisingly high-quality sound that was certainly loud enough to dance to. The sound could easily be coming from a mini hi-fi system with normal speakers.
        
But why would anyone want cheap flat-pack speakers? NXT claims students might want to carry them easily from one lodging to another, and their low cost means children can have them in their bedrooms without parents worrying that boisterous play will damage them. Adults can use them instead of expensive hi-fi speakers for parties or barbecues in case drunken guests spill drink on them or tread on them.
        
Until the mid 1990s, all loudspeakers used a piston effect to pump out sound, until NXT worked out how to generate sound by moving a flat panel. It invested many millions in finding lightweight materials stiff enough to ensure faithful sound reproduction. The company also worked out how to find the best place on the panel to put the small electromagnetic exciter that converts electric signals into acoustic waves. It now leads the field in flat-panel loudspeakers.
        
Last year, NXT discovered that it could make ordinary cardboard boxes produce reasonable sound quality-given a decent enough exciter. So the company designed a roughly tetrahedral box speaker (see Graphic) that folds flat-ideal for stacking, and easy to carry, store and assemble at home.
        
David S. Smith, a European company that specialises in cardboard packaging, helped develop a tough corrugated cardboard with the correct frequency response. The speaker is made from a single sheet of the board, pre-folded down four lines to form a three-sided pyramid. The fourth crease allows one face to fold inwards and collapse the structure. It can be printed with any design you want, and topped with a weatherproof varnish.
        
NXT uses a flat exciter coil, 2.5 centimetres wide, powered by a neodymium electromagnet. It sits on the inside of one face of the pyramid at its "sweet spot". Hook the exciter up to a 15-watt amplifier and it vibrates, making the whole pyramid flex to produce sound. "We are already working on smaller versions," says NXT`s Jon Vizor. "Cornflake packets will come when we have got the right material."

Author: Barry Fox

New Scientist