nt patterns of sound waves to find the ones that produced the signature combination of a low-pressure region surrounded by high-pressure zones.
They found three different acoustic force fields that can twirl, grab and manipulate objects. One works like tweezers and seems to grab the particles in thin air. Another traps the object in a high-pressure cage. The third type of force field acts a bit like a swirling tornado, with a rotating high-pressure field surrounding a low-pressure, quiet "eye" that holds the object in place, the researchers report this week in the journal Nature Communications.
To accomplish this task, the team used a tiny array of 64 mini loudspeakers, made by a company called Ultrahaptics, that produce exquisitely timed sound waves with accuracy to the microsecond level. Past acoustic levitation systems have used two or four arrays of these transducers to essentially surround the system, but the researchers' models allowed them to create the same force field using just one array. The team demonstrated their tractor beam using tiny balls of polystyrene, the same material used in packing peanuts.
Wavelength and intensity
The size of the low-force region depends on the wavelength: The longer the wavelength, the larger the region of low pressure. The sound intensity determines the maximum density of an object that can be pushed and pulled by the acoustic force, Drinkwater said.
In this instance, the sound waves operate between 140 and 150 decibels. That would be an ear-splitting volume if people could hear it, but the sound waves operate at 40 kilohertz, at a wavelength of about 0.4 inches (1 centimeters), well above the human hearing range but audible to dolphins and dogs.
"I think, if you pointed this device at a dog, it would hear it for sure," Drinkwater said. "It wouldn't like it; it would run away."
The team currently levitates lightweight polystyrene balls that measure up to 0.2 inches (5 millimeters) across. But for the system to be useful for medical operations, the team would need to miniaturize it to manipulate objects on the micron scale. Doing so would mean using higher-frequency sound waves -- a relatively simple tweak, Drinkwater said.
"The fact that we do it as a one-sided system is so important," Drinkwater said. "To get at the body, you have to apply it to one side."