Sound is all around us continually, but most people don’t think of it as a physical presence. We hear sounds, rather than touch them. This is why the concept of acoustic levitation is difficult to grasp, considering that sound is invisible most of the time.
The combination of sound and levitation introduces properties that cause solids, gases and liquids to float. The process can occur in normal gravity on earth, or in reduced gravity, such as in gas-filled enclosures in outer space.
The phenomenon has been likened to a magic show, but in reality, it’s not a trick and it isn’t caused by sleight of hand. It’s a scientific reaction – and to understand how it works, we must first learn about the links between gravity, air and sound.
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Why do sounds occur?
Gravity is the force that makes objects attract and move towards one another. Isaac Newton’s famous law of universal gravitation, first published in the 17th century, states every microscopic particle in the universe attracts every other particle. The bigger the object, the more strongly it will attract other objects.
Like liquids, air is made of microscopic particles. It moves in the same way as water – in fact, aerodynamic tests can actually take place underwater, rather than in the air. Particles that make up air simply move faster than those in liquids.
Sound is a vibration that travels through various mediums such as gas, liquids or a solid object. It is created by an object moving or changing shape rapidly, such as a bell vibrating in the air when you strike it. As it vibrates, it pushes the air molecules next to it, increasing the pressure in the air in that region, creating a compression. As the side of the bell moves back, the molecules are pulled apart. This creates a region of lower pressure, known as a rarefaction.
As the bell repeats the process, this creates a rapid series of compressions and rarefactions, making the sound. Every repetition of one compression and one rarefaction creates one wavelength of sound. This sound wave travels as a result of the moving molecules pushing and pulling other molecules around them. Without the molecules moving, the sound wouldn’t travel – which is why there’s no sound in a vacuum.
How do sound and levitation work?
Research into sound and acoustic levitation is ongoing, with many scientists reporting they have used sound waves to levitate liquid droplets and tiny particles. This has been achieved by using multiple vibrating plates to create different frequencies, moving an acoustic field when the particles are trapped inside. To date, the technique has been unable to lift large or heavy objects.
Even in the 21st century, scientists don’t yet know if such a thing will ever be possible, despite some minor breakthroughs. In experiments, scientists have levitated lightweight polystyrene balls that are bigger than the wavelengths used to elevate them. This is seen as a step forward in this field of research.
In 2016, a combined team of researchers from the UK and Brazil levitated a 50mm polystyrene ball several centimetres into the air. It floated for the duration of the generation of the soundwaves. In 2017, a group of researchers at the University of Bristol levitated a larger 2cm polystyrene ball.
In Illinois, scientists at Argonne National Laboratory have been using sound waves to levitate droplets of solutions containing various pharmaceuticals. Using acoustic levitation aids the production of the pharmaceutical products.
Researchers have investigated numerous different drugs. The laboratory’s Technology Development and Commercialisation Division is now pursuing a patent for the method.
The scientists are also negotiating a partnership with the pharmaceutical industry to further develop the technology. They believe it has the potential to be licensed for commercial development.
Can louder sounds move larger objects?
Sound waves can be relatively powerful – for example, in an air duct, they can cause dust to collect in a pattern that corresponds with the wave. When a wave reverberates through a room, it can cause objects to noticeably vibrate.
Research has also found that low-frequency waves in buildings can cause occupants to feel nervous – in some cases, people have reported a building to be haunted as a result! However, the sound waves required to move an object off the ground must be significantly more powerful than the ordinary ones. This is where the problems arise.
Research into whether louder and more powerful sounds can move heavy objects, such as a person, for example, suggests this will be very difficult, if not impossible. Increasing the amplitude of the wave can make the sound louder, but it doesn’t alter the shape of the waveform. It therefore doesn’t make it much more powerful physically.
Extremely intense sounds, even those so loud that they are painful to the human ear, won’t lift large objects. They can cause different reactions in the substances they travel through, due to becoming distorted. They can cause shock waves, like sonic booms – the loud sound we hear when an object travels through the air faster than the speed of sound.
Other effects to be taken into account include acoustic streaming (the constant flow of the fluid that the wave travels through) and the acoustic saturation, when the matter can no longer absorb any more of the sound wave’s energy.
The volume of sound required to produce any kind of levitation must be louder than 150 decibels. The sound from a loud nightclub with pulsating music is around 110 dB, so anything louder than this will begin to feel uncomfortable.
A complex field of study
Acoustic levitation is a very complex field. It isn’t simply a case of aiming a high-powered transducer at a reflector. The sounds must also be the relevant frequency to create the required waveform.
Ongoing research is continuing to develop new set-ups for acoustic levitation systems. Although the initial findings prove levitation through manipulating sound waves can be achieved, the research is still in its infancy. It may unearth more methods of using sound to overcome the forces of gravity – and some may be more successful than the techniques currently in use.