Scientists have developed small robots that can fly and steer themselves using only sound waves. How does that work?
By way of background, many modern technologies use sound waves. For example, in navigation sonar sends sound pulses underwater to detect objects and map the ocean floor. Medicine uses high-frequency sound waves to create real-time images of muscles, organs, etc. It also uses high-energy sound waves to break up kidney stones without surgery. Industry uses high-frequency sound waves in liquid to scrub dirt off jewellery, medical tools, etc. and in non-destructive testing of metals and concrete.
But powering something with sound waves seems to be quite another thing.
This technology utilises what is known as Helmholtz resonance. Hermann von Helmholtz (1821-1894) was a German physicist who discovered the type of resonance that occurs when air is forced in and out of a cavity (the resonance chamber), causing the air inside to vibrate at a specific natural frequency. The size of the resonance chamber and the neck determine the resonance frequency. A couple of everyday examples of this effect: blowing across the top of a bottle results in a resonant tone, and the buffeting sound heard when driving with only a single window cracked open.
In all cases, the air inside the resonance chamber is behaving acoustically like a mechanical mass-spring system. When air is pushed into the neck of the chamber, the internal pressure rises, compressing the cavity’s air until it pushes back, overshooting into a slight vacuum that draws air back in. The resulting resonant frequency is determined by the speed of the sound, the cross-sectional area and length of the neck, and the volume of the cavity. Which means that the cavities can be “tuned” to specific frequencies.
This capability has been utilised for many years now. For example, all string instruments, from the sitar to the modern guitar and violin, generate a sound that consists of a series of Helmholtz resonance modes associated not only with the size and shape of the resonance cavity, but also the absorption properties of the material used. The aircraft industry uses resonators to reduce the noise of the engines. The car industry uses them to alter the sound of exhausts.
With these usages, especially with the stringed instruments, the pressures involved are low, so the impact is also low. The scientists on this project recognised that if you crank up the pressures, the sudden jet could become significant – in a way that the force can be harnessed for machinery. Furthermore, shrinking down the resonators would push the frequency needed to activate them into the ultrasonic range. This makes them too high-pitched for humans to hear, but, more importantly, makes it easier to focus them with precision.
All of that is very interesting and very clever – but what can we do with these new widgets?
Well, the answer right now is: not much. This is real blue-sky research. But here are some emerging thoughts:
- Unlike conventional power generation means, this one would appear to have no limit on how small they could eventually be.
- This could lead to safe, wireless navigation of tiny swimming robots inside the human body for targeted drug delivery.
- Capitalise “swarm intelligence” with micro-robots using acoustic signalling to self-organise, navigate tight spaces, and coordinate real-time tasks.
- Explore dangerous or hard-to-reach disaster zones where traditional electronics fail.
- Build soft, flexible surfaces that bend and change shape on command when they “hear” a particular frequency.
I’m not sure I like the idea of one of these widgets swimming around inside me! But who knows?