Shrill whistles burst periodically from the corner of the lab on the second floor of the James Fletcher Building. Some are steady, others warble slightly like a mechanical bird song.
Their cries interrupt undergraduate Matt Emmi as he describes the work going on behind him.
Over three years ago, he took a class on the physics of audio and video, and then came to work for the man who taught the course.
“The professor is always doing interesting things,” he said.
He explains that the two students making noise in the corner behind him are experimenting with small devices that turn heat into sound.
Orest Symko—the professor—wearing a tie under a brown wool jacket with elbow patches, arrives to confer with Emmi.
Once back in his office again, Symko sits at his desk. Lazy notes of Brazilian jazz drift from the stereo near the window.
He introduces his work in thermoacoustics with a touch of history. Heat engines have been around since the 1800s, he explains.
To demonstrate he stretches out his palm and places the circular base of a pinwheel-like machine on it.
Several degrees of warmth from his hand drive a piston into action. It spins a clear plastic disk, decorated with two strips of hologrammatic silver.
Just as heat produces motion, it can also generate sound, a phenomena first noticed by glass blowers more than 200 years ago.
Now Symko is using a similar principle to create tiny refrigerators that could be used to cool the circuitry in computers and other electronic devices.
He has carried a handful of the small devices from the downstairs lab and scattered them on the desk in front of him.
Made of a variety of metals, some vaguely resemble cylindrical bits of pipe, closed at one end with white teflon bands around the center.
With his fingertips, Symko dips the end of one of the metal devices into liquid nitrogen. This creates a difference in temperature and drives heat flow along its length. Wisps of vapor lap at his fingers, and after several seconds he lifts the device.
A shrill tea-kettle-like whistle emerges from the open end of the inch-long device. He explains that the flow of heat between the warmer and the cooler ends creates the sound.
“The beauty of this is that you have heat that is so disorderly and you have sound which is orderly and one frequency,” he explains.
Rapid oscillations in air pressure compose sound. When pressure goes up, air compresses and temperature rises. When pressure goes down, the air expands and cools.
“If you wanted to redesign humans and you didn’t like their ears, you could have used temperature sensors,” Symko says smiling.
His devices exploit the relationship between sound and heat. And if a difference in temperature could produce sound, couldn’t noise create a difference in temperature?
“In physics we always ask questions on both sides,” Symko explains.
Several clear plastic devices lying among the metal ones originated after this question. They are part of a new class of refrigerators.
He lifts one to illustrate. Its body is made of plastic, and attached to the end of this one is a black cone, or speaker. Here sound produces cooling.
In the past, many refrigeration systems relied on freon or one of several chlorofluorocarbons. However, most developed nations agreed to end their production by 1996 in response to fears that CFCs damage the ozone layer.
Symko’s small refrigerators use simple gases like air as a medium, and they have minimal moving parts. They function on the same principles that draw sound from his heat-driven devices.
The body of the mini refrigerator intensifies sound just as the body of a violin amplifies a note. And like the violin, the device resonates at its own natural frequency as soundwaves travel through it.
The pulses of sound pump heat in one direction through a barrier inside the device. As a result, one side of the barrier warms while the other cools, functioning like a refrigerator.
“What really is neat about [this research] is that it has the potential to be something that just about every single consumer product that’s based on electronics could use,” Emmi says.
He goes on to explain that the only current method for cooling circuitry is to blow air across it. Fans are both noisy and inefficient, and they can not bring cooling below room temperature because they are only working with ambient air. Both are problems that could be overcome with the new class of refrigerators.
In fact, researchers have already developed full-sized, sound driven refrigerators.
“What’s new about our approach is that we are making everything small, so small that it could be used in microcircuits in laptops,” Symko says.
People want more devices on their computers, and they want them to go faster. Increased work for computer circuits creates more heat that must be disposed of. Symko gestures to a Macintosh cube behind him.
“The next generation will probably have problems getting rid of the heat,” he says.
Mini-refrigerators could be the answer. He picks up the smallest device on the table, less than half of an inch long. Its pitch is ultrasonic, beyond the human range of hearing.
“We’ve been collaborating with a local company, and we’re just about ready to start making prototypes,” he says.
Researchers elsewhere have linked the refrigerators to the heat driven devices “so that heat would produce sound and that sound would produce cooling,” Symko says.
In effect then, heat produces cooling.
At the moment Symko’s work in the lab downstairs focuses on experimenting with different geometries and materials. It can be a tedious process.
“We find out that something works, but we won’t know why,” explains Emmi. The challenge then is to figure it out.
