A new device eavesdrops on objects to take their temperatures. Hot objects not only glow, but also softly hum. The hum is generated by the rapid jitters of particles that make up the hot object.

If human ears were keen enough to hear this noise, “it would sound like radio static,” says Tom Purdy of the University of Pittsburgh. “The hotter [an object] gets, the louder it gets.”

Purdy, along with Robinjeet Singh of the University of Maryland in College Park, created an acoustic thermometer that senses the intensity of heat-generated sound emanating from nearby objects.

The heart of the device is a one-square-millimeter sheet of silicon nitride. That sheet is suspended within a window cut in the center of a silicon chip, which transmits sound waves better than air.

In experiments, the physicists deposited blobs of an epoxy material on the chip’s surface around the silicon nitride sheet.

When heated with a laser, each epoxy blob gave off sound waves that rippled through the chip to the sheet, causing the sheet to vibrate. The hotter the epoxy blob, the stronger its sound waves, and the more intense the silicon nitride’s vibrations.

Bouncing a laser beam off the sheet and measuring the beam’s angle of reflection allowed the researchers to track the sheet’s motion, and therefore the temperatures of the epoxy blobs.

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