Ordinary standing waves can be relatively powerful. For example, a standing wave in an air duct can cause dust to collect in a pattern corresponding to the wave's nodes. A standing wave reverberating through a room can cause objects in its path to vibrate. Low-frequency standing waves can also cause people to feel nervous or disoriented -- in some cases, researchers find them in buildings people report to be haunted.
But these feats are small potatoes compared to acoustic levitation. It takes far less effort to influence where dust settles or to shatter a glass than it takes to lift objects from the ground. Ordinary sound waves are limited by their linear nature. Increasing the amplitude of the wave causes the sound to be louder, but it doesn't affect the shape of the wave form or cause it to be much more physically powerful.
However, extremely intense sounds -- like sounds that are physically painful to human ears -- are usually nonlinear. They can cause disproportionately large responses in the substances they travel through. Some nonlinear affects include:
- Distorted wave forms
- Shock waves, like sonic booms
- Acoustic streaming, or the constant flow of the fluid the wave travels through
- Acoustic saturation, or the point at which the matter can no longer absorb any more energy from the sound wave
Nonlinear acoustics is a complex field, and the physical phenomena that cause these effects can be difficult to understand. But in general, nonlinear affects can combine to make an intense sound far more powerful than a quieter one. It is because of these affects that a wave's acoustic radiation pressure can become strong enough to balance the pull of gravity. Intense sound is central to acoustic levitation -- the transducers in many levitators produce sounds in excess of 150 decibels (dB). Ordinary conversation is about 60 dB, and a loud nightclub is closer to 110 dB.
Levitating objects with sound isn't quite as simple as aiming a high-powered transducer at a reflector. Scientists also must use sounds of the correct frequency to create the desired standing wave. Any frequency can produce nonlinear effects at the right volume, but most systems use ultrasonic waves, which are too high-pitched for people to hear. In addition to the frequency and volume of the wave, researchers also must pay attention to a number of other factors:
- The distance between the transducer and the reflector must be a multiple of half of the wavelength of the sound the transducer produces. This produces a wave with stable nodes and antinodes. Some waves can produce several usable nodes, but the ones nearest the transducer and reflector usually not suitable for levitating objects. This is because the waves create a pressure zone close to the reflective surfaces.
- In a microgravity environment, such as outer space, the stable areas within the nodes must be large enough to support the floating object. On Earth, the high-pressure areas just below the node must be large enough as well. For this reason, the object being levitated should measure between one third and half of the wavelength of the sound. Objects larger than two thirds of the sound's wavelength are too large to be levitated -- the field isn't big enough to support them. The higher the frequency of the sound, the smaller the diameter of the objects it's possible to levitate.
- Objects that are the right size to levitate must also be of the right mass. In other words, scientists must evaluate the density of the object and determine whether the sound wave can produce enough pressure to counteract the pull of gravity on it.
- Drops of liquid being levitated must have a suitable Bond number, which is a ratio that describes the liquid's surface tension, density and size in the context of gravity and the surrounding fluid. If the Bond number is too low, the drop will burst.
- The intensity of the sound must not overwhelm the surface tension of liquid droplets being levitated. If the sound field is too intense, the drop will flatten into a donut and then burst.
This might sound like a lot of work required to suspend small objects a few centimeters off of a surface. Levitating small objects -- or even small animals -- a short distance might also sound like a relatively useless practice. However, acoustic levitation has several uses, both on the ground and in outer space. Here are a few:
- Manufacturing very small electronic devices and microchips often involves robots or complex machinery. Acoustic levitators can perform the same task by manipulating sound. For example, levitated molten materials will gradually cool and harden, and in a properly tuned field of sound, the resulting solid object is a perfect sphere. Similarly, a correctly shaped field can force plastics to deposit and harden only on the correct areas of a microchip.
- Some materials are corrosive or otherwise react with ordinary containers used during chemical analysis. Researchers can suspend these materials in an acoustic field to study them without the risk of contamination from or destruction of containers.
- The study of foam physics has a big obstacle -- gravity. Gravity pulls the liquid downward from foam, drying and destroying it. Researchers can contain foam with in acoustic fields to study it in space, without the interference of gravity. This can lead to a better understanding of how foam performs tasks like cleaning ocean water.
Researchers continue to develop new setups for levitation systems and new applications for acoustic levitation. To learn more about their research, sound and related topics, check out the links on the next page.