Carbon was first used in microphones and speakers in the form of grains. Now it has a new form: layers only a couple of atoms thick, generating sound from heat.
It's called thermoacoustic sound, and it dispenses with the bulky magnets that ordinary speakers need and allows for speakers that are nearly any shape and size.
The invention comes from Ji Won Suk at the University of Texas, Austin. He used a principle that has been known for a long time: put a current through most materials and they heat up. Oscillate the current and you get a temperature change that has the same frequency.
Suk did this to graphene, which consists of carbon atoms arranged in a regular pattern. It's strong and conductive, and also transmits heat very well. Suk made a layer of graphene less than a single nanometer thick and put it on glass and two different types of plastic. He then ran an alternating current through it at various frequencies, from 500 and 22 cycles per second. The result: sound.
An ordinary speaker uses an oscillating electric current to drive magnets, which then cause a diaphragm to vibrate. In this case, the graphene isn't vibrating. Rather, it is transmitting the heat energy to the air. But transmitting that heat energy moves air molecules around in a way similar to the vibrations of a speaker, so you get sound.
Suk told Discovery News that he found the best materials to use as substrates are those that are thermal insulators, so more of the energy goes out into the air (where we can hear it). Ideally a graphene speaker would be suspended in the air, but given that it's only a couple of atoms thick that isn't practical, he said.
What's practical is putting the graphene on various substrates, which can be any shape. Suk noted that the sound can also be adjusted by altering the shape of the graphene layer, which itself is flexible. He added that he hasn't tested the limits of the graphene yet — he used relatively small amounts of current, on the scale of hundreds of milliamps. (He didn't try hooking up an iPod to it yet, but there is no reason that couldn't work).
Suk's experiments appeared in the journal Advanced Materials on Sept. 18.
Credit: University of Texas / Ji Won Suk