We humans are a rather fantastic species. If we decide to be that is; otherwise we can also be quite the pest. One of the things that fascinates me about who we are is that if we find something captivating we simply pursue it, regardless of whether or not there is a direct reward. It’s called “play” and is its own reward. And if “play” gets expensive it’s called “fundamental science”.
Acoustophoresis, or acoustic levitation is such an example where labs around the world are so fascinated by the prospect of detaining and moving an object by sound waves alone that the question of what to do with that can safely be left unanswered. By generating standing waves of sufficient intensity a sufficiently light object can be suspended in space in the nodes of these waves. While being attracted by gravity, the energy required to overcome the sound pressure by leaving the node is simply larger than the energy gained by moving down the gravitational field. Going upward and out of the node is another impossibility, since this creates an immediate downward force through pressure. The particle is therefore held captive in minuscule oscillations, not knowing which way to turn. This suspension works with liquids drops and solids.
The frequencies of sound waves forming a standing wave determine the physical distance between the nodes and thus have to be appropriate for the size of the object. For most applications that levitate rather small objects these frequencies are well above human hearing in the ultrasonic spectrum, adding complete silence as an additional attribute to what makes this technology mysterious and captivating. Try this with anything larger than a ping-pong ball and it will get noisy quickly, posing a threat to also levitate the researchers and any lab equipment.
By changing the frequency the nodes shift and a particle can be moved up and down to some degree. This is now a relatively cheap trick compared to some of the remarkable work done by Daniele Foresti, Majid Nabavi, Mirko Klingauf, et al. at my alma mater, the Suisse Federal Institute of Technology, in Zürich. By focusing the nodes from several transducers onto specific points by varying their frequency, several objects can be levitated and moved in three-dimensional space.
When thinking about this, I thought there must be many existing technical applications and I went into this article with the enthusiasm to find out what's being done. My conclusion is that there currently aren’t any applications but some prospects of a brave new world are being talked about.
The video above held the immediate promise to take some of the messiness out of human procreation, so at least I thought. There are many reasons to not bring a substance of high chemical purity in contact with other substances and acoustic tweezers sound great but there are also many other ways of skinning that cat. Objects that conduct electricity are already being levitated magnetically; small objects can be levitated optically by shooting photons at them, and electric fields can provide electrostatic levitation. Lastly anyone who has ever seen a kid suspend a Ping-Pong ball on a jet of air blown into a small tube has experienced aerodynamic levitation. Of all these ways to make stuff hover, acoustic levitation arguably has the most challenges to face.
While looking for additional information I found that there are a few brave men and women at the University of Southampton who want to heal our aging knees by letting cartilage cells grow while suspended in sound.
There are other applications that are being discussed: detection of cancer cells in human blood by moving one cell at a time through sound radiation pressure and then scanning it for defective DNA. Semiconductors that require a high grade of purity could be handled by what is called container-less transport. Acoustic tweezers used to manipulate small amounts of fluid without contamination are definitely on the way.
And then there are always those guys who claim that they can levitate themselves through the sound of their voices. Anyone else here think they may be full of something other than music?
On an unrelated note, I once heard of a supposedly rather obtuse gentleman in the suburbs of Chicago, who attempted to create an automobile alarm which when tripped, emitted a specific tone that would cause immediate sphincter release. I think that the frequency was near 16Hz. In theory it sounded like a highly desirable feature to add to anyone’s car, but judging by the number of car alarms being set off accidentally, one might not feel safe to leave the house without sporting an adult diaper. Further if there were any degree of market penetration of this feature, we could assume that one car alarm would set of others causing a chain reaction and a veritable citywide crap storm.
I asked myself what was the largest object to ever be levitated. Wikipedia speaks of a few kg, but I did not find a good reference to that. So once again, I will correct Wikipedia unless anyone disagrees:
The largest object to truly ever be lifted off the ground by sound according to my own original research weighed 2,150kg, was over 8m long and made a buzzing sound, which gave it the nickname buzz bomb, or doodlebug. The V1 flying bombs the German’s used to attack London amongst other cities used a Pulse Jet. This Jet can best be considered a gigantic Subwoofer driven by explosions. An air inlet is opened and shut at high frequencies and while shut, explosive gas is injected into a combustion chamber behind it and ignited by a spark plug, sending a sound wave-front out the exhaust. The frequency of these cycles was to be perceived as sound and not individual explosions.
My immediate question as the founder and owner of a loudspeaker company was if this could be redesigned to play music rather than threaten the English. I don’t see why not but do suspect a rather large amount of distortion. If you enjoy a very loud sound of a rather large flying beetle, then by all means check if a pulse drive is right for you. For all else, we make loudspeakers that are much smaller, less loud and can be operated outside military compounds. They're also a lot of fun.
Arved Deecke is founder of the Danish / Mexican Loudspeaker company KVART & BØLGE that makes audiophile quarter wave loudspeakers and sound systems at a price anyone can afford. In his free time he blogs about all things related to sound, music and audio.