ETREMA: How Etrema Makes Windows Whisper

by Cora Daniels

2003

Terfenol-d is a specially formulated metal alloy made from rare earth metals that changes shape—up to 20,000 times a second—in the presence of a magnetic field. To make Whispering Windows, the scientists from Etrema wrap a piece of Terfenol, about the size of a splinter, in copper coil. The coil carries an electrical current, which creates a magnetic field, causing the Terfenol to vibrate quickly. This small motor is then placed into a disc-shaped chrome case, which also contains an audio jack into which a user can plug a CD player, for instance.

Most speakers push the air directly in front of them; the resulting waves in the air produce sound. Terfenol is so powerful that the discs are capable of vibrating an entire tabletop, wall, or window. Those vibrations—though invisible—displace a large amount of air, producing a uniform, rich sound. The surface stops vibrating when the disc is lifted, and the noise ceases.