How To Float In Three Easy Steps

An image showing the nodes and antinodes of a wave hitting a wall and bouncing back.

Have you ever put on some music and just started ascending? No, not just figuratively, but literally lifted off the ground?

Wingardium Leviosa!

Sound is nothing more than pressure changes in the air. When this pressure change travels through a medium like air, we get sound waves. Since pressure is just force applied over an area, we can theoretically use sound to apply a push or pull to objects! This force is scientifically known as acoustic radiation pressure. Research into the effects of sound pressure began in the early 1900s, when physicists realized that intense sound waves could exert forces on small objects. How does this work? When we shoot a sound wave from a speaker towards a reflector, the waves bounce back. In the image above, we can see that the solid line is the original wave that hits the wall, and the dotted line is the reflected wave. If we let this pattern continue, then this shape stays steady, and we call it a “standing wave.”  

An important thing to keep in mind is that sound waves are actually longitudinal waves, meaning the air particles travel back and forth like a spring in the same direction the wave travels. This wave in the image, known as a transverse wave, is not an accurate sound wave itself, but rather a graph of pressure vs. position. The x-axis is drawn through the nodes, meaning that there is no pressure variation at those points. The nodes represent points where pressure variation is minimal, and the antinodes represent points where pressure variation is large. Therefore, the nodes are the most stable points in this wave with almost zero pressure oscillation.

Ok, but where does this lead to? Levitation!

Nodes and Antinodes

Imagine a set up with a speaker facing upwards and a reflecting plate right above it. If we turn the speaker on to play a constant frequency, we will achieve a standing wave. Now, if we place a small object like a water droplet or a bead between the speaker and reflector, it will levitate! The pressure variation at nodes is minimal, making them stable points where particles tend to stay. If they drift in one direction, the pressure variation on both sides of the particle will increase, and restoring forces created by the pressure will end up pushing the object back towards the node. This is similar to a ball at rest at the bottom of a bowl. If it is rolled up along the bowl, then when released it will roll back down to the bottom of the bowl.

That’s a summary of how sound can lift things, but let’s get into some details. Scientists don’t just use normal musical notes to levitate objects. In fact, we can’t even hear the experiment being done. This is because this experiment utilizes ultrasound, a very high frequency sound wave that oscillates over 20,000 times a second. This puts the nodes very close together, since frequency is inversely proportional to wavelength. What does that mean? Closer nodes help stabilize the position of the particle even more, so it’s no wonder scientists use ultrasound. Furthermore, scientists can control where the levitating object moves. By slightly adjusting the frequency of the sound wave, the locations of the nodes change as well, moving the levitating object along with it. Taking this idea one step further, a particle can be moved in three dimensions by adjusting multiple speakers.

Forces

A diagram of two equal and opposing forces acting on a box.

Here’s a quick review of how something can seemingly “float.” The force that acts on every mass on earth is known as gravitational force, which is created by the size of the Earth. It always acts towards the center of mass of the Earth (which is, unsurprisingly, at the center of the planet). In other words, the force pulls everything “down.” For something to float, it has to experience a force acting upwards that cancels out this gravitational force. In physics terms, that will result in zero net force, meaning the object will not accelerate. So, if it’s at rest, it’ll stay at rest. In order for the acoustic levitation experiment, the air pressure is what provides this upwards acting force.

This is awesome, but there are some limitations to this. Obviously, one can’t place an elephant between a speaker and a reflector and hope that the elephant will float. The forces created by these pressure differences are extremely small, usually only strong enough to support objects that weigh a few milligrams.

Applications

This might seem like a cool party trick, but there has to be a purpose for levitation, or scientists wouldn’t have dedicated so much to perfecting this method. Well, scientists use it to study liquids and chemical reactions without containers, creating a contact-free lab. When a droplet is floating in midair, it can’t interact with a wall, and that lets the researchers observe the material more accurately. In the future, acoustic levitation could be used to handle fragile samples, assemble extremely small electronic components, or study materials in ways that might be difficult with other methods.