Sound surrounds us in our everyday existence, yet we rarely perceive it as a tangible force. After all, we often associate sound with what we hear rather than what we can touch.
However, the notion that sound—an intangible entity—can lift and suspend objects in the air might strike one as implausible initially. Surprisingly, this concept is entirely feasible. This intriguing phenomenon is referred to as acoustic levitation, and it uses sound properties to cause objects to float in environments with either regular or reduced gravity.
How Does Acoustic Levitation Work?
Understanding this seemingly magical feat requires understanding its key components: gravity, sound, and air.
As a refresher, gravity is the influential force emanating from any object possessing mass, drawing nearby entities toward it. It’s worth noting that the potency of gravity increases with an object’s mass.
Next, sound is a vibration that travels through a medium like solid objects, liquids, or gas whose source is an object that changes shape or moves quickly.
The sound wave from, say, striking a bell, creates a repeating series of high and low pressure in the region of air around the bell by pushing and pulling the air molecules around it through its movement.
Each repetition counts as one wavelength of the sound wave, which then travels as the movement of molecules pushes and pulls the molecules around them.
Lastly, air is essentially a fluid since it behaves and moves in the same way liquids do, and it is also made up of microscopic particles that move in relation to each other.
This similarity is why scientists often conduct their aerodynamic tests underwater. Air is “lighter” because particles in gasses move faster and are much farther apart than the particles in liquids.
Now, acoustic levitation uses sound travelling through the air to balance the force of gravity. It works differently depending on the gravity in the environment, like causing objects to hover in the air here on Earth or holding them steady in space so they do not drift away. Achieving this relies on the property of sound waves, particularly those of higher intensity.
The Physics Behind Acoustic Levitation
Achieving sound levitation requires two components: a transducer and a reflector. The transducer is a vibrating surface that generates sound while the reflector bounces it back. These two surfaces are often concave to better focus the sound. Three properties of this travelling and reflecting sound enable it to suspend objects in the air.
- Sound Waves
The first of these properties is the sound wave itself, which, as you may recall from your O-level physics tuition, is a longitudinal pressure wave whose points move parallel to the direction the wave travels. This motion resembles pushing and pulling one end of a stretched slinky toy.
- Bouncing of Sound Waves
The second is the bouncing of the wave, which follows the law of reflection, which simply states that something leaves or bounces off a surface at the angle it strikes it in the first place.
To understand wave reflection, let us return to the slinky example once more, but this time it is attached to something on one end. When you pick up the other free end and move it quickly up and down, it would cause a wave to travel down the length of the spring, bounce off the surface at the other end, and travel back to you.
- Interference and Standing Waves
Lastly, when sound waves reflect off a surface, the compressions and rarefactions interact and cause interference.
In other words, a compression that meets another compression will amplify one another, but when it meets a rarefaction, they balance each other out. A combination of reflection and interference results in standing waves, which are at the heart of acoustic levitation.
These standing sound waves feature specific low and high-pressure points, known as nodes and antinodes, respectively. To better understand how these work, think of a river with rapids and rocks where some sections are calm while others are turbulent. Foam and floating debris collect in the calmer parts of the river, while a floating object must be propelled or anchored against the flow of the water in the faster-moving sections. This is essentially how an acoustic levitator works by using sound moving through a gassy medium like air instead of water.
An acoustic levitator creates standing waves when its reflector and transducer are at the right distance away from each other. And once the orientation of the waves is parallel to gravity’s pull, parts of the standing wave will have constant upward and downward pressures while its nodes will have very little pressure.
Depending on gravity, objects hover in slightly different areas within a sound field. In space, floating particles will collect in the calm and still nodes of the standing waves. Meanwhile, on Earth, the same objects will collect just below the nodes where acoustic radiation pressure or the pressure that sound waves exert on surfaces will balance out the pull of gravity on the object. This makes the particle appear as if it were levitating in the air.
While levitation may seem like a superpower only possible in works of fiction, the contents above prove that it can take place in reality through the power of sound. But, while acoustic levitation is yet to have significant applications at present, it may be a stepping stone to greater discoveries down the line.
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