The acoustic radiation force (or ARF) has been used to levitate objects in air since the 1980/90’s when Eugene Trinh did his pioneering work at the Jet Propulsion Lab. By the way Trinh lived the dream and became an astronaut flying on the space shuttle in 1992. At the time, they and others were interested in creating micro-gravity environments (i.e. mini versions of space on earth). However, more earthly applications in container-less processing emerged.
My work has focused on using a large number of sources to create a phased array. This type of device can then form a diverse range of acoustic fields. As the ARF depends on the gradient of the sound pressure, by controlling the acoustic field we can control the forces on an object. If these forces are large enough then we can defy gravity and levitate small objects. This requires high sound levels so we operate our devices in the ultrasonic frequency range where they can’t be heard. To levitate low density objects is easy and any good tweeter loudspeaker can do this if set-up correctly (see my instructions). Levitating more dense objects is harder and requires more specialist sound/ultrasound sources and power amplifiers. The standard approach to acoustic levitation is to use a small number of high intensity sources, such as Langevin horns driven by a powerful power amplifier, to create a high intensity one-dimensional standing wave field. Such an apparatus is now a classic and great for demonstrating this amazing acoustic phenomenon. Small and dense objects then become trapped at the nodes (zero pressure points) in the acoustic field.
Building on this classic apparatus, we have shown that, as an alternative, you can use many lower intensity sources to form an array. This type of device, which we named Tinylev, turns out to be a much cheaper way of making a levitator. Low cost parking sensors can be used as the sources and cheap electronics designed for audio applications can be used as the sine wave generator and amplifier. Also, the inputs to the sensors are quite low power, making it inherently safer than the traditional Langevin horn devices. The total cost of building a Tinylev is about £50 and with this you will be able to levitate small plastic objects and insects, a capability previously only possible with expensive research-standard lab equipment. If you are interested, you can now buy a kit which when assembled replicates our device. Or you can just buy the parts yourself and construct the device following these instructions. This type array-based levitator has proved very useful for many different projects including a futuristic food experience!
With an array and careful control of the outputs of each source we can form almost any sound field imaginable. The limit is that the sound field cannot have any texture smaller than an acoustic wavelength. However, we can change the wavelength by changing the frequency so there really are an infinite number of possibilities. Our breakthrough in 2015 was to show that there are three basic shapes of acoustic field that can be used to form a stable tractor beam. We called these the Twin, Vortex and Bottle traps. Using these new ideas we recently showed that a tractor beam could be made for about £70 from readily available components. This is a somewhat more challenging project than Tinylev, but if you wish to follow the instructions you can build your own battery-powered acoustic tractor beam.
The other significant challenge in this field that I am currently working on is the levitation of objects larger than the acoustic wavelength. Various attempts are currently underway to try and break this limit, including a field-optimisation approach that seems to be yielding some success, although levitation with this approach is currently only possible for a limited period of time. We are exploring the use of rapidly counter-rotating acoustic vortices and the results are very exciting.
Bruce Drinkwater 