Sustainable materials sound good

Cellulose fibres covered with magnetic nanoparticles have been used to make ultrathin loudspeakers by researchers in Sweden. The material could provide a cheaper and more sustainable alternative to the polymers traditionally used in speakers.

Conventional loudspeakers contain a bulky magnet that is becoming harder to incorporate into our ever-shrinking gadgets. A coil attached to the speaker membrane is essential for moving the membrane and creating sound waves but the force of this coil can also reduce sound quality. The biocomposite membrane developed by Richard Olsson, Lars Berglund and their teams at the Royal Institute of Technology in Stockholm removes the need for a bulky magnet and enhances sound quality because there is no coil in contact with the membrane.

To make the biocomposite membrane, magnetic nanoparticles are securely fastened along cellulose nanofibrils to make a magnetic gel. This gel can be sprayed from a spray gun nozzle to give a membrane with a uniform spread of particles.

It is important to have an even spread of particles as an uneven spread would cause the already stiff material to become brittle. ‘The stiffness of cellulose means it is well-suited to a quick reacting acoustic membrane,’ says Olsson. Cellulose nanofibrils have mechanical properties comparable to high performance synthetic fibres and come from a renewable source – wood pulp.

The properties of the membrane can be finely tuned by attaching different magnetic nanoparticles to the cellulose nanofibrils. Incorporating magnetic particles into the membrane itself, which is only 50 µm thick, means there is no bulky external magnet. A coil still drives the membrane but does not need to be directly attached.

With only an even layer of air pressing against the membrane surface the sound quality delivered by the loudspeaker is impressive and Canadian musician Romi Mayes allowed the team to use her music to demonstrate this quality.

Wendelin Stark, an expert in functional materials at ETH Zurich in Switzerland, is excited by the new material. ‘It allows very soft, well-defined movements, as each magnetic particle draws on a fibre. Such uniform application of force allows the kind of movement required for high quality sound generation.’

The team are now working to increase the volume of their loudspeaker without compromising the sound quality.