FROM Bluetooth and Wi-Fi to FM and AM, wireless communication depends on electromagnetic waves—usually, radio waves. But as any motorist driving through a tunnel or under power lines can attest, such waves cannot always propagate properly past every obstruction. Sometimes, a system that used a different medium of transmission would make communication that bit easier.

As they report in the Proceedings of the National Academy of Sciences, Alex Zettl and his colleagues at the University of California, Berkeley, think they have devised such a system. Instead of radio waves they use ultrasound. By exploiting sound at frequencies above the 20 kHz limit of human hearing they can, in principle, send messages that neither affect nor are affected by human conversation and other everyday noises that take place at lower pitches.

Ultrasonic transmitters and receivers have been around for a long time. But they usually employ the piezoelectric effect, whereby an alternating electric current causes them to vibrate if transmitters or, if receivers, to generate an electric current in response to vibration. This means their range is restricted by their resonant frequencies—which in turn restricts their bandwidth, and thus the amount of information they can handle.

More conventional diaphragm-based equipment has a wider frequency range than the piezoelectric sort, but is harder to deploy ultrasonically because the diaphragms are too thick and heavy to vibrate in synchrony with the rapid oscillations of ultrasound. Dr Zettl has therefore been looking for an alternative. And he thinks he has found one: graphene.

Graphene is a form of carbon in which the atoms are arranged in a sheet a single atom deep. This lack of thickness leads to lightness which, combined with graphene’s strength (100 times that of steel) and electrical conductivity (better than any unalloyed metal), means the material has attracted a fair amount of commercial hype in recent years. It certainly looks like something thin, light and strong enough to deal accurately with ultrasound. So Dr Zettl and his colleagues have built both loudspeaker and microphone using it.

They have field-tested their microphone by recording bats in a nearby park. The animals’ calls range in frequency from 50 to 100 kHz, and the kit’s ability to record them accurately shows its potential to act as a receiver for ultrasonic “radio”. Dr Zettl has gone on to construct such a radio, using the microphone together with a graphene loudspeaker, to broadcast signals within his laboratory.

Whether a graphene version of wireless really is the best way to deal with places that the conventional sort cannot reach has yet to be determined. And on land, the need to shuffle between ultrasound and radio waves according to circumstances means the answer may be “no”. But the technology could have wide application underwater—a place that radio waves do not like to go. Swimmie-talkies for divers, anyone?