Defying Gravity : Fascination
of levitation
Benu Chatterjee
Parts 2 : Levitation with Sound Wave
The science of magnetic field causing magnetic levitation has been discussed in Part 1 of the series on “Defying Gravity : Fascination of Levitation”. The mystery of levitation phenomenon is well known and realized in other fields namely sound and quantum physics of present interest. The idea that something intangible like sound could lift objects by defying gravity although seems unbelievable is a real phenomenon. The science of levitation with sound wave energy is discussed in the present text.
1. Introduction
In magnetic levitation, force of magnetic field from magnetic wave on an object can become concentrated enough to levitate the object by eventually overcoming the gravitational pull. The physics of magnetism, of course, demands the object to be magnetic for levitation. In contrast, acoustic levitation is more desirable because, unlike magnetic levitation, it can be used on any object – magnetic or non-magnetic. The effect of sound is all around us except in all-vacuum space atmosphere where it could not travel because of its nature as a mechanical wave. A fundamental property of sound is that it cannot be touched but only be heard. The volume of sound can range from being soft to very loud like sonic boom or window-rattling noise in discotheques. An acoustic wave can exert a force on objects immersed in the wave field. These forces although normally weak, can become large enough using high intensity waves that would counteract gravity pull for ultimate levitation of objects.
The two images at the top of this article depict the basics of present day research on acoustic levitation illustratinf nodes and antinodes of a sound wave and a demonstration (not an illusion or magic trick) of liquid droplets suspended between two speakers transmitting standing waves at 22 kilohertz (khz) frequency (just outside human's capability to hear which is in the range 20hz-20khz). The present text on acoustic levitation is based on basic concepts of physics of sound.
1.1 Terminology
In physics, sound constitutes vibration that travel through a medium such as air or water and be heard once reached the ear. It travels via particle interactions, and is thus related to a mechanical wave which makes sound incapable of travelling through vacuum such as space atmosphere. Acoustics, on the other hand, encompasses a bigger picture. Derived from the Greek word akoustos, meaning “hearing”, it is related to an interdisciplinary science of studying mechanical waves in gases, liquids and solids including topics like vibration, sound, ultra sound and infra sound. The words “sound” and “acoustics” are used interchangeably in the present text, but never as an adjective with “sound” meaning “in good or healthy condition”.
2. Physics of sound – basic considerations
Acoustic levitation is closely related to phenomena in physics namely nature and movement of sound. These aspects of sound are explained in simple terms for a better understanding of the present subject by non-physics oriented general readers.
2.1 Nature of sound
Acoustic levitation is a fascinating application of sound technology whereby the field of sound waves travelling through a medium such as air balances the force of gravity. Three important basic features affecting acoustic levitation are briefly outlined in terms of gravity, air and sound. Gravity It is a force that causes objects to attract one another. Based on Newton’s law of universal gravitation, all particles in the universe attract each other. The more massive and closer an object, stronger is the attraction for each other, the cause of which is still not known. An enormous object like the Earth thus attracts other objects easily which are close to it like fruits hanging from trees. Air Its behaviour is essentially the same as liquids. In parity with liquids, air is made of microscopic particles that move in relation to each other. Sound The intimate relation between sound and vibration is reflected when vibration form a sound wave causes disturbance in an elastic medium such as air. The vibration pattern is same when,for example, the open end of a bar fixed at the other end is tapped. The bar will vibrate with instantaneous up and down displacement from its rest position. A sine wave or sinusoidal graph is developed at any particular instant in time (positive and negative numbers denote displacements above and below respectively of bar’s rest position during vibration). The vibration pattern termed as simple harmonic motion is similar to the propagation of sound.
2.2 Mechanics of sound movement
Parameters for propagation of sound that are relevant to acoustic levitation include resonance, compression and rarefaction, interference, and standing wave. A basic knowledge of these parameters highlighted below, would be beneficial in the present context.
2.2.1. Resonance
Resonance comes from a Latin word “resonare” meaning to “return to sound”. Each object has its own natural, resonant frequency which depends on the size, shape and composition of the object. An item will vibrate strongly when it is subjected to vibrations at a frequency equal or very close to its natural frequency. This can happen when struck by another object. Thus if an object such as a rock is exposed to an external disturbance (namely struck by a metal rod) at its natural frequency, energy of the rock would continue to grow up to a very high energy level. The phenomenon is known as resonance through which a comparatively weak vibration in one object (metal rod) can cause a strong vibration and hence movement in another (rock).
A good example of resonance is the shattering of crystal glass when exposed to a musical tone from say the voice of a singer. If the singer could match the natural frequency of the glass, more energy will be put into the glass than it can handle. As the energy builds up via resonance, the glass begins to deform. A condition is soon reached beyond which packing bonds in the glass cannot handle i.e. the chemical bonds cannot hold the glass atoms together. This results in greater mobility of atoms in the glass with ultimately shattering.
2.2.2. Compression and rarefaction
Sound waves are produced by vibrating objects. In fluid mediums e.g. gases and liquids, sound waves are only longitudinal. They can be, however, either longitudinal or transverse waves in solids. A tuning fork consisting of a handle and two tines (prongs) is a good example in creating a longitudinal wave of present interest which one can hear. On striking with a rubber hammer, the tines would begin to vibrate. Sound waves would be produced which on travelling through air would cause alternate push (back) and pull (forth) vibrations of air particles adjacent to the tines. Initially, as the tine stretches outward, sound will push/compress the adjacent air molecules into a small space generating a high pressure region (compression). This is followed by a reverse inward movement of the tine when air molecules expand by being pulled apart, creating a low pressure area (rarefaction). A repeat pattern of high and low pressures of sound wave is generated. Each air molecule moves the others around it. Without this movement of air molecules, sound could not travel, and this is why sound could not exist in vacuum namely space atmosphere. It is interesting to note, from the above mechanism, how air molecules are disturbed only from their rest position without undergoing any net displacement. This would imply that propagation of a sound wave only occurs via transportation of energy and not matter from one location to another.
2.2.3. Interference
Two types of interference patterns namely constructive and destructive types are developed when two travelling sound waves meet each other. Constructive interference occurs when two waveforms are added together namely compressions i.e. peak with peak, or rarefactions i.e. trough with trough. This would amplify the waves creating a louder sound. Destructive interference, on the other hand, occurs when two waves are out of phase i.e. peak of one wave meeting trough of the other and thereby cancelling each other’s effect. This would result in diminished waveform with practically no sound.
2.2.4. Standing wave
The concept of creating standing wave is essential to stabilise acoustic levitation. In comparison with normal waves oscillating up and down as they move through the air, a standing wave is essentially static with the waveform forced to stay in one place. The illusion of stillness of the standing waves which appear to have the same position at any time i.e. do not travel from place to place is what gives their name “standing”.
Creation of standing wave
With a simple set up of an emitter of longitudinal sound wave and a reflector, reflection of the incident sound waves bouncing off the reflector can undergo constructive or destructive interference while travelling via compressions and rarefactions. A suitable combination of reflection with interference would result in the reflected waves interfering constructively with the incident waves. For a perfect interference, standing acoustic waves can thus be generated displaying characteristic points of minimum and maximum pressure/ vibration known as nodes and antinodes respectively. This concept is illustrated as a theme image at the top of present article .
Formation of standing waves is similar to that of sound produced in string instruments. For example, when a string fixed at two points is plucked while playing a guitar, it would vibrate up and down creating the characteristics of a standing sound wave pattern. A standing wave’s nodes are at the heart of the mechanics of sound-assisted levitation/lifting of objects ranging from lightweights to megaliths.
3. Lifting of megaliths
Megaliths are massive blocks of stones/rocks some weighing several hundred tons that had been used in ancient cultures to construct a monument or a building block. Megalith sites range from ancient time such as the periods of Great Pyramid, ruined city complex of Teotihuacan in Mexico, Stonehenge to 20th century modern enigma namely the Coral Castle in Florida. In all cases, workers seemed to have mysterious powers to lift blocks of stones to construct temples, buildings, monuments etc. There are stories abound in Mayan and Greek legends.
It is important at this juncture to clarify the difference between sound-fuelled lifting and levitation. Moving megaliths can be appropriately termed as “lifting” than “levitation“. The latter term is relevant to today’s research in physics labs studying acoustic levitation of lightweight objects in air using standing sound waves.
According to archaeo-acoustics researchers, the concept of sound-assisted lifting has been known to be an anti-gravity phenomenon for thousands of years. It appears that the ancients somehow mastered the technique of lifting via sound technology or some other obscure method that would defied gravity.
3.1 Historical contribution of sound
It is apparent from the ruins of ancient civilization that apart from applying ropes, pulleys, ramps, and a massive supply of workers who strained their muscles and ingenuity to the limits, no one knows for sure as to how the unique feats of lifting huge stones of several hundred tons in weight were achieved. However, there have been suggestions that the sound technology had been used to lift big rocks. The truth about lifting megaliths with the use of sound although still remains a mystery, a few examples from a brief survey of literature around 5000 - 200 BC are presented.
1) Arab historian Al-Masudi suggested that a “magic papyrus” (paper) was placed under the stone to be moved during the construction of pyramids. On subsequent striking with a metal rod, the stone started to lift and move along a path for about 50 metres before dropping to the ground. The process was repeated for further progress. It is probable that the striking metal rod happened to create correct resonating natural frequencies of the stone which was capable of producing a massive resonance suitable for lifting/movement of stone.
2) The story goes that the foundation of Tiahuanaco in Bolivia Andes around 2000 BC involved workers who were able to lift stones off the ground and carry them through air using the sound of a trumpet. 3) According to classical Greek writers, Amphion (son of Jupiter) was able to move large stones using the sound of a lyre of harp. This action helped to construct the walls of Thebes (capital of Boeotia) around 1600-1200 BC. 4) According to Mayan legends, the temple of Uxmal in Mexico was built in 600-1000AD by a race of dwarfs who only had to whistle to move heavy rocks into place. 5) In the early 20th century, a Swedish doctor Dr. Jari reported to have witnessed stone blocks of 1 metre (m) wide, 1 m high and 1.5 m long being lifted by Tibetan monks through air to a cave at a height of 250 metres using sounds from chorus singing, drums and trumpets. In this way Tibetans have given us a direct indication of how to construct a sonically propelled anti-gravitational flying machine. 6) There are rumours surrounding the construction of Coral Castle that the builder Ed Leedskalnin secretly levitated the stones up to 30 tons by singing to them.
3.3 Thoughts on lifting megaliths
There is no doubt that the technology used by people to move massive stones with simple methods is still a mystery. However, awareness of sound technology by people in lifting massive stones is apparent from the above examples. Various sources of sound are involved namely striking with a metal rod, songs, chanted spells, clapping hands, trumpets, drums, lyres, whistles etc. By taking the stories at face values, it is possible that the vibration force of sound was likely to be a major factor in lifting megaliths. Some researchers also believe that the ancient monuments whose creation remains a mystery, were built by lifting rocks using the harmonics of sound.
Based on the literature, there are several descriptions and relics that testify the importance of sound in ancient ceremonies and structures. Amazingly, the concept of resonance was recognised some 30,000 years ago when cave-arts are examined which revealed a link between areas with evidence of strong resonance and the location of the art. Researches on randomly selected megalithic chambers showed that the natural primary resonance frequency to be mostly in the range 110-1102 Hz which is quite incredible. One can thus speculate that the prospect of lifting rocks could be associated with acoustic resonance created especially when a rock was struck by a metal rod.
Speculative mechanics of lifting
The big question is how does sound work in a stone. The lifting process for stones could be influenced by resonance, harmonics and movement of standing waves of sound. All this acoustic criteria are supplemented by a unique property of stones namely its piezoelectric nature which would allow stones to convert mechanical stress i.e. pressure of sound in the present case, into electricity. Thus, once a stone is struck by say a metal rod at a certain particular pitch/frequency, it would vibrate at its resonant frequency. Subsequently, a compressive standard wave sets up within the stone which is converted by it into electrical such as electromagnetic or electrogravitational energy. The piezoelectric effect in stones creates electromagnetic and hence identically gravitational potential waves with characteristic node and antinode at the centre of the stone. In this way, standing waves within the stone turn into gravitations waves. It has been suggested that by muting the opposite end of a stone (say with a non-vibrating rod), the nodes could be shifted which would unbalance stone’s centre of gravity. This shifting of nodes may make the stone lighter for subsequent easy lifting/movement. . It is speculated that Ed.Leedskalnin somehow generated a radio signal that caused the coral stones to vibrate at its resonant frequency, and then used an electromagnetic field to flip the magnetic poles of the atoms so that they were in opposition to the earth’s magnetic field.
4. Current state of affair
Researchers nowadays have managed to use sound waves to levitate and move tiny particles and liquid droplets in a very precise way.
4.1 Mechanics of acoustic levitation
For more than a century, scientists have considered the idea of using the pressure of sound waves to float objects in the air. By placing an object at a certain point within a sound wave, it is possible to have enough force exerted by the sound wave to perfectly counteract the downward pull of gravity, resulting in ultimate levitation of the object in that spot.
4.1.1 Levitation reactor Historically, it was in 1866 when standing wave levitation phenomenon was first observed in Kundt’s tube experiment with small dust particles moving toward low pressure areas i.e. nodes of a standing wave created in the tube. Although forces of ordinary standing waves are normally weak, they are powerful enough to demonstrate levitation of small items in solid or liquid. There has been demonstration of levitation of objects such as liquid droplets between a transducer emitter of sound wave and a reflector in a levitation reactor.
Principles
Based on physics of sound, the distance between the sound emitter and reflector in a laboratory levitation trial must be a multiple of half of the wavelength of the sound emitted by the transducer. This would result in creating a standing wave with stable nodes and antinodes.
The two opposing forces acting on the droplets are gravitational downward drag and an upward pull by the sound wave. Once these forces are balanced, the liquid droplets are trapped in the stable regions of the energy field namely nodes where they start to levitate in air.
Sound waves exert pressure, so when the waves are strong enough, the pressure is able to counteract the downward force of gravity. Pressure distribution of a standing wave travelling vertically upward opposing the pull of gravity, will be shared partly by a constant downward as well as upward pressures in the antinode regions with very little pressure at nodes. When the downward-moving reflected waves overlap with the upward-moving source waves, the two ‘cancel out’ in the middle at the so-called node points characterised by minimum pressure of sound waves. Any object of suitable size and mass will have more chance to float in the relatively calm regions of nodes rather than the high pressure, high vibration “turbulant” areas of antinodes. There is no net transfer of energy at all at the nodes. The upward pressure exerted by sound wave on the suface of light objects placed around the nodes of a standing wave could thus be sufficient enough to cancel the effect of gravity, allowing levitation of the object.
The weight that can be lifted by the current technology of sound wave force is about a few kilograms. For levitation of liquid droplets, their diameter must correspond to equal or less than half the wavelength of the acoustic waves. Thus the size of the liquid drops that can be levitated is limited to 4 to 5mm in diameter. Theoretically, however, there is no limit to acoustic levitation. It is even feasible to lift a human mass provided the very of energy needed is avalable from sound wave.
Operation
a) Levitation in one dimension (1–D)
The concept although known since the 1900s, the technique of acoustic levitation with precise control of motion of the objects in mid-air has been developed only recently. Levitation occurs along a fixed vertical axis in 1–D with a vertical upward force counteracting the downward gravitational pull with the acoustic axis of the ultrasound wave lying parallel with the gravitational force.
The US Department of Energy’s Argonne National Laboratory was able to demonstrate 1–D acoustic levitation in 1987 using ultrasound standing wave between two small speakers. Droplets of liquid were suspended in mid air at the sound pressure nodes. The basic unit consists of a piezoelectric transducer which converts electrical energy into the desired high frequency longitudinal standing waves (ultrasound), and a reflector. A standing sound wave is produced by optimising the distance between the transducer and reflector. Both units face each other and have concave surfaces to focus the sound waves which travel from the emitter upwards and bounce off the reflector without deviating from the straight route. Levitation was manipulated along a a fixed y-axis (1–D) by controlling the frequency of the transducer.
The theme photograph of the present article illustrates drop-by-drop suspension of a liquid at nodes of the vibrational force of a standing waves generated between the two small speakers. This is similar picture to a ping-pong ball suspended in air by a fan or water jet. The strength of the standing wave for acoustic levitation of droplets was maintained high at 160 decibels which is louder than the deafening sound of a rocket launch. In order to protect their ears, scientists used a high enough frequency of sound at about 22kilohertz (ultrasonic acoustic levitation) which is just beyond the range humans can hear.
A weak force is normally exerted by an acoustic wave on objects immersed in a linear wave field which can be, however, appreciably strengthened using nonlinear sound wave. Nonlonear acoustics is a branch of physics that deals with sound waves of sufficiently large amplitudes where nonlinear rather than traditional linear equations would aplply. There has been considerable progress in developing high intensity sound waves with nonlinear charateristics.The present day practice of acoustic levitation would not be possible without considering the complex nonlinear acoustic phenomena. The “louder” acoustic waves would allow levitation of denser materials like glass, ceramic, aluminium and steel.The concept of nonlinear acoustics was first introduced by Rayleigh in as early as 1902 as an acoustic counterpart of electromagnetic wave. Maths involved is highly complex and unlikely of any interest to general readership.
b) Levitation in two dimensions (2–D)
Object has so far been allowed to float in one direction (1–D) without being able to have any lateral/sideways movement. The latest technique from ETH, in Zurich transforms traditional motionless suspended droplets to a controlled movement so that the sound waves could not only levitate lightweight objects but also move them around in 2–D in mid-air.
It uses multiple emitter-reflector modules working in parallel next to each other. The device looks like a chess board with penny-size square emitting its own sound, and a clear plastic plate placed a small distance away above the board to reflect back the sound. The forces of acoustic wave are varied from module to module in order to transfer objects or droplets of liquid from one module to the next. Movement of objects needs a careful balance of sound wave force. The sound intensity of the “giving” square from where the object is leaving is lowered, while it is ramped up in the “receiving” square where the object is expected to move to.
Acoustic waves are thus kept varying from module to module along with transfer of liquid particles or droplets from one module to the next – essentially surfing the droplets on a wave of sound. The tricky part is to figure out a balanced sound level to move objects from square to square without damaging them. If pushed too hard, the sound waves may cause the water droplets to explode. On the other hand, if not pushed enough, the droplets will fall because gravity wins. Drops of water, hydrocarbons and various solvents have been successfully levitated. An important application is in the pharmaceutical industry where it would be now possible to mix molecules in a frictionless way without any possible contamination.
c) Levitation in three dimensions (3–D)
At Tokyo University, the science of acoustic levitation has been further progressed with addition of more speakers, and controlling the focal points of the generated standard sound waves. Two speakers were used for levitation, while two more were used to move the focal point. With this 3–D acoustic manipulation, levitation was achieved by trapping the objects in nodes of standard ultrasonic waves. The technique has demonstrated floating of foam balls, fine electrical points, piece of wood, metal nuts and screws. Since 3–D means the objects can be driven in x, y and z axes, they can be moved around to wherever one wants.
5. Advantages of acoustic levitation
It is well known that the contactless methods of handling matters are typically based on electromagnetic principles but, of course, limited by the inherent material properties. Acoustic levitation, on the other hand, is both contact-free and material- independent method and requires minimum effort to prepare samples. Unlike magnetic levitation, acoustic method would apply to any materials, not necessarily magnetic. The contactless method of moving objects by acoustic levitation could have useful implications in chemical engineering and biotechnology where any contact with surfaces can spoil the chemical substances and interfere with the reaction processes. However, a basic limitation of the technique lies with the size of the object which has to be half the wavelength of the sound wave used.
6. Commercial feasibility
Acoustic levitation has considerable commercial potential. The technology would allow studies of various chemical reactions and biological processes and development and production of pharmaceuticals and electronics. As a result, multiple droplets can be mixed in mid-air. Some special chemical and biological experiments can be carried out in this contact-less method that require particles or droplets to be processed and analyzed without worrying about any chemical changes that can occur due to contact with the container surface.
7. Final comments
To sum up, nodes in standing longitudinal waves of nonlinear characteristics are essential for acoustic levitation of objects against gravitational downward pull. Application of the technique is independent of materials. As a contact-free process, acoustic levitation is useful in modern day’s usages of small size and microchips. There is apparently no known theoretical limit to what acoustic levitation can lift given enough vibratory sound. Current technology, however, limits the amount that can be lifted by the sound force to about a few kilograms.
In 1987, NASA performed for the first time an anti-gravity experiment at Argonne laboratory by suspending droplets of liquid in mid air between two small speakers. The drops were, however, motionless i.e. 1-D with no lateral movement. About 25 years later, a new technology from ETH in Zurich provided 2-D mobility to the motionless levitation. Acoustic levitation continued its progress from being motionless 1-D in 1987, mobile 2-D in about 2012 to a controllable hovering of objects in 3-D by 2013. A controlled movement of suspended droplets and objects (magnetic or non-magnetic) allow multiple droplets to be mixed or transported in mid air without any potential contamination or interference from container surface. The new technology would thus have wider implications allowing pharmaceutical industry to mix solutions for drugs in midair without the potential contamination of the containers.
Unlike researches on acoustic levitation in physics lab, piezoelectric property of megaliths and resonance phenomenon from vibration physics of sound seem to play a major role in lifting large items such as rocks and stones in ancient times. Compared to the incredible feats of acoustic lifting of megaliths by our ancestors, the present scientific demonstration looks primitive. But it is certainly a step in the right direction demonstrating the potential of acoustic levitation which will improve as more scientific research is being carried out. One could say that the successful application of acoustic levitation to droplets, will hopefully one day lead to lifting larger objects. In fact, given enough high power from vibratory sound which is still beyond today’s capabilities, it is theoretically feasible that a human mass can be levitated by the acoustic method provided a human could survive the applied immense acoustic force and the availability of necessary protective equipment.
Benu Chatterjee
Parts 2 : Levitation with Sound Wave
The science of magnetic field causing magnetic levitation has been discussed in Part 1 of the series on “Defying Gravity : Fascination of Levitation”. The mystery of levitation phenomenon is well known and realized in other fields namely sound and quantum physics of present interest. The idea that something intangible like sound could lift objects by defying gravity although seems unbelievable is a real phenomenon. The science of levitation with sound wave energy is discussed in the present text.
1. Introduction
In magnetic levitation, force of magnetic field from magnetic wave on an object can become concentrated enough to levitate the object by eventually overcoming the gravitational pull. The physics of magnetism, of course, demands the object to be magnetic for levitation. In contrast, acoustic levitation is more desirable because, unlike magnetic levitation, it can be used on any object – magnetic or non-magnetic. The effect of sound is all around us except in all-vacuum space atmosphere where it could not travel because of its nature as a mechanical wave. A fundamental property of sound is that it cannot be touched but only be heard. The volume of sound can range from being soft to very loud like sonic boom or window-rattling noise in discotheques. An acoustic wave can exert a force on objects immersed in the wave field. These forces although normally weak, can become large enough using high intensity waves that would counteract gravity pull for ultimate levitation of objects.
The two images at the top of this article depict the basics of present day research on acoustic levitation illustratinf nodes and antinodes of a sound wave and a demonstration (not an illusion or magic trick) of liquid droplets suspended between two speakers transmitting standing waves at 22 kilohertz (khz) frequency (just outside human's capability to hear which is in the range 20hz-20khz). The present text on acoustic levitation is based on basic concepts of physics of sound.
1.1 Terminology
In physics, sound constitutes vibration that travel through a medium such as air or water and be heard once reached the ear. It travels via particle interactions, and is thus related to a mechanical wave which makes sound incapable of travelling through vacuum such as space atmosphere. Acoustics, on the other hand, encompasses a bigger picture. Derived from the Greek word akoustos, meaning “hearing”, it is related to an interdisciplinary science of studying mechanical waves in gases, liquids and solids including topics like vibration, sound, ultra sound and infra sound. The words “sound” and “acoustics” are used interchangeably in the present text, but never as an adjective with “sound” meaning “in good or healthy condition”.
2. Physics of sound – basic considerations
Acoustic levitation is closely related to phenomena in physics namely nature and movement of sound. These aspects of sound are explained in simple terms for a better understanding of the present subject by non-physics oriented general readers.
2.1 Nature of sound
Acoustic levitation is a fascinating application of sound technology whereby the field of sound waves travelling through a medium such as air balances the force of gravity. Three important basic features affecting acoustic levitation are briefly outlined in terms of gravity, air and sound. Gravity It is a force that causes objects to attract one another. Based on Newton’s law of universal gravitation, all particles in the universe attract each other. The more massive and closer an object, stronger is the attraction for each other, the cause of which is still not known. An enormous object like the Earth thus attracts other objects easily which are close to it like fruits hanging from trees. Air Its behaviour is essentially the same as liquids. In parity with liquids, air is made of microscopic particles that move in relation to each other. Sound The intimate relation between sound and vibration is reflected when vibration form a sound wave causes disturbance in an elastic medium such as air. The vibration pattern is same when,for example, the open end of a bar fixed at the other end is tapped. The bar will vibrate with instantaneous up and down displacement from its rest position. A sine wave or sinusoidal graph is developed at any particular instant in time (positive and negative numbers denote displacements above and below respectively of bar’s rest position during vibration). The vibration pattern termed as simple harmonic motion is similar to the propagation of sound.
2.2 Mechanics of sound movement
Parameters for propagation of sound that are relevant to acoustic levitation include resonance, compression and rarefaction, interference, and standing wave. A basic knowledge of these parameters highlighted below, would be beneficial in the present context.
2.2.1. Resonance
Resonance comes from a Latin word “resonare” meaning to “return to sound”. Each object has its own natural, resonant frequency which depends on the size, shape and composition of the object. An item will vibrate strongly when it is subjected to vibrations at a frequency equal or very close to its natural frequency. This can happen when struck by another object. Thus if an object such as a rock is exposed to an external disturbance (namely struck by a metal rod) at its natural frequency, energy of the rock would continue to grow up to a very high energy level. The phenomenon is known as resonance through which a comparatively weak vibration in one object (metal rod) can cause a strong vibration and hence movement in another (rock).
A good example of resonance is the shattering of crystal glass when exposed to a musical tone from say the voice of a singer. If the singer could match the natural frequency of the glass, more energy will be put into the glass than it can handle. As the energy builds up via resonance, the glass begins to deform. A condition is soon reached beyond which packing bonds in the glass cannot handle i.e. the chemical bonds cannot hold the glass atoms together. This results in greater mobility of atoms in the glass with ultimately shattering.
2.2.2. Compression and rarefaction
Sound waves are produced by vibrating objects. In fluid mediums e.g. gases and liquids, sound waves are only longitudinal. They can be, however, either longitudinal or transverse waves in solids. A tuning fork consisting of a handle and two tines (prongs) is a good example in creating a longitudinal wave of present interest which one can hear. On striking with a rubber hammer, the tines would begin to vibrate. Sound waves would be produced which on travelling through air would cause alternate push (back) and pull (forth) vibrations of air particles adjacent to the tines. Initially, as the tine stretches outward, sound will push/compress the adjacent air molecules into a small space generating a high pressure region (compression). This is followed by a reverse inward movement of the tine when air molecules expand by being pulled apart, creating a low pressure area (rarefaction). A repeat pattern of high and low pressures of sound wave is generated. Each air molecule moves the others around it. Without this movement of air molecules, sound could not travel, and this is why sound could not exist in vacuum namely space atmosphere. It is interesting to note, from the above mechanism, how air molecules are disturbed only from their rest position without undergoing any net displacement. This would imply that propagation of a sound wave only occurs via transportation of energy and not matter from one location to another.
2.2.3. Interference
Two types of interference patterns namely constructive and destructive types are developed when two travelling sound waves meet each other. Constructive interference occurs when two waveforms are added together namely compressions i.e. peak with peak, or rarefactions i.e. trough with trough. This would amplify the waves creating a louder sound. Destructive interference, on the other hand, occurs when two waves are out of phase i.e. peak of one wave meeting trough of the other and thereby cancelling each other’s effect. This would result in diminished waveform with practically no sound.
2.2.4. Standing wave
The concept of creating standing wave is essential to stabilise acoustic levitation. In comparison with normal waves oscillating up and down as they move through the air, a standing wave is essentially static with the waveform forced to stay in one place. The illusion of stillness of the standing waves which appear to have the same position at any time i.e. do not travel from place to place is what gives their name “standing”.
Creation of standing wave
With a simple set up of an emitter of longitudinal sound wave and a reflector, reflection of the incident sound waves bouncing off the reflector can undergo constructive or destructive interference while travelling via compressions and rarefactions. A suitable combination of reflection with interference would result in the reflected waves interfering constructively with the incident waves. For a perfect interference, standing acoustic waves can thus be generated displaying characteristic points of minimum and maximum pressure/ vibration known as nodes and antinodes respectively. This concept is illustrated as a theme image at the top of present article .
Formation of standing waves is similar to that of sound produced in string instruments. For example, when a string fixed at two points is plucked while playing a guitar, it would vibrate up and down creating the characteristics of a standing sound wave pattern. A standing wave’s nodes are at the heart of the mechanics of sound-assisted levitation/lifting of objects ranging from lightweights to megaliths.
3. Lifting of megaliths
Megaliths are massive blocks of stones/rocks some weighing several hundred tons that had been used in ancient cultures to construct a monument or a building block. Megalith sites range from ancient time such as the periods of Great Pyramid, ruined city complex of Teotihuacan in Mexico, Stonehenge to 20th century modern enigma namely the Coral Castle in Florida. In all cases, workers seemed to have mysterious powers to lift blocks of stones to construct temples, buildings, monuments etc. There are stories abound in Mayan and Greek legends.
It is important at this juncture to clarify the difference between sound-fuelled lifting and levitation. Moving megaliths can be appropriately termed as “lifting” than “levitation“. The latter term is relevant to today’s research in physics labs studying acoustic levitation of lightweight objects in air using standing sound waves.
According to archaeo-acoustics researchers, the concept of sound-assisted lifting has been known to be an anti-gravity phenomenon for thousands of years. It appears that the ancients somehow mastered the technique of lifting via sound technology or some other obscure method that would defied gravity.
3.1 Historical contribution of sound
It is apparent from the ruins of ancient civilization that apart from applying ropes, pulleys, ramps, and a massive supply of workers who strained their muscles and ingenuity to the limits, no one knows for sure as to how the unique feats of lifting huge stones of several hundred tons in weight were achieved. However, there have been suggestions that the sound technology had been used to lift big rocks. The truth about lifting megaliths with the use of sound although still remains a mystery, a few examples from a brief survey of literature around 5000 - 200 BC are presented.
1) Arab historian Al-Masudi suggested that a “magic papyrus” (paper) was placed under the stone to be moved during the construction of pyramids. On subsequent striking with a metal rod, the stone started to lift and move along a path for about 50 metres before dropping to the ground. The process was repeated for further progress. It is probable that the striking metal rod happened to create correct resonating natural frequencies of the stone which was capable of producing a massive resonance suitable for lifting/movement of stone.
2) The story goes that the foundation of Tiahuanaco in Bolivia Andes around 2000 BC involved workers who were able to lift stones off the ground and carry them through air using the sound of a trumpet. 3) According to classical Greek writers, Amphion (son of Jupiter) was able to move large stones using the sound of a lyre of harp. This action helped to construct the walls of Thebes (capital of Boeotia) around 1600-1200 BC. 4) According to Mayan legends, the temple of Uxmal in Mexico was built in 600-1000AD by a race of dwarfs who only had to whistle to move heavy rocks into place. 5) In the early 20th century, a Swedish doctor Dr. Jari reported to have witnessed stone blocks of 1 metre (m) wide, 1 m high and 1.5 m long being lifted by Tibetan monks through air to a cave at a height of 250 metres using sounds from chorus singing, drums and trumpets. In this way Tibetans have given us a direct indication of how to construct a sonically propelled anti-gravitational flying machine. 6) There are rumours surrounding the construction of Coral Castle that the builder Ed Leedskalnin secretly levitated the stones up to 30 tons by singing to them.
3.3 Thoughts on lifting megaliths
There is no doubt that the technology used by people to move massive stones with simple methods is still a mystery. However, awareness of sound technology by people in lifting massive stones is apparent from the above examples. Various sources of sound are involved namely striking with a metal rod, songs, chanted spells, clapping hands, trumpets, drums, lyres, whistles etc. By taking the stories at face values, it is possible that the vibration force of sound was likely to be a major factor in lifting megaliths. Some researchers also believe that the ancient monuments whose creation remains a mystery, were built by lifting rocks using the harmonics of sound.
Based on the literature, there are several descriptions and relics that testify the importance of sound in ancient ceremonies and structures. Amazingly, the concept of resonance was recognised some 30,000 years ago when cave-arts are examined which revealed a link between areas with evidence of strong resonance and the location of the art. Researches on randomly selected megalithic chambers showed that the natural primary resonance frequency to be mostly in the range 110-1102 Hz which is quite incredible. One can thus speculate that the prospect of lifting rocks could be associated with acoustic resonance created especially when a rock was struck by a metal rod.
Speculative mechanics of lifting
The big question is how does sound work in a stone. The lifting process for stones could be influenced by resonance, harmonics and movement of standing waves of sound. All this acoustic criteria are supplemented by a unique property of stones namely its piezoelectric nature which would allow stones to convert mechanical stress i.e. pressure of sound in the present case, into electricity. Thus, once a stone is struck by say a metal rod at a certain particular pitch/frequency, it would vibrate at its resonant frequency. Subsequently, a compressive standard wave sets up within the stone which is converted by it into electrical such as electromagnetic or electrogravitational energy. The piezoelectric effect in stones creates electromagnetic and hence identically gravitational potential waves with characteristic node and antinode at the centre of the stone. In this way, standing waves within the stone turn into gravitations waves. It has been suggested that by muting the opposite end of a stone (say with a non-vibrating rod), the nodes could be shifted which would unbalance stone’s centre of gravity. This shifting of nodes may make the stone lighter for subsequent easy lifting/movement. . It is speculated that Ed.Leedskalnin somehow generated a radio signal that caused the coral stones to vibrate at its resonant frequency, and then used an electromagnetic field to flip the magnetic poles of the atoms so that they were in opposition to the earth’s magnetic field.
4. Current state of affair
Researchers nowadays have managed to use sound waves to levitate and move tiny particles and liquid droplets in a very precise way.
4.1 Mechanics of acoustic levitation
For more than a century, scientists have considered the idea of using the pressure of sound waves to float objects in the air. By placing an object at a certain point within a sound wave, it is possible to have enough force exerted by the sound wave to perfectly counteract the downward pull of gravity, resulting in ultimate levitation of the object in that spot.
4.1.1 Levitation reactor Historically, it was in 1866 when standing wave levitation phenomenon was first observed in Kundt’s tube experiment with small dust particles moving toward low pressure areas i.e. nodes of a standing wave created in the tube. Although forces of ordinary standing waves are normally weak, they are powerful enough to demonstrate levitation of small items in solid or liquid. There has been demonstration of levitation of objects such as liquid droplets between a transducer emitter of sound wave and a reflector in a levitation reactor.
Principles
Based on physics of sound, the distance between the sound emitter and reflector in a laboratory levitation trial must be a multiple of half of the wavelength of the sound emitted by the transducer. This would result in creating a standing wave with stable nodes and antinodes.
The two opposing forces acting on the droplets are gravitational downward drag and an upward pull by the sound wave. Once these forces are balanced, the liquid droplets are trapped in the stable regions of the energy field namely nodes where they start to levitate in air.
Sound waves exert pressure, so when the waves are strong enough, the pressure is able to counteract the downward force of gravity. Pressure distribution of a standing wave travelling vertically upward opposing the pull of gravity, will be shared partly by a constant downward as well as upward pressures in the antinode regions with very little pressure at nodes. When the downward-moving reflected waves overlap with the upward-moving source waves, the two ‘cancel out’ in the middle at the so-called node points characterised by minimum pressure of sound waves. Any object of suitable size and mass will have more chance to float in the relatively calm regions of nodes rather than the high pressure, high vibration “turbulant” areas of antinodes. There is no net transfer of energy at all at the nodes. The upward pressure exerted by sound wave on the suface of light objects placed around the nodes of a standing wave could thus be sufficient enough to cancel the effect of gravity, allowing levitation of the object.
The weight that can be lifted by the current technology of sound wave force is about a few kilograms. For levitation of liquid droplets, their diameter must correspond to equal or less than half the wavelength of the acoustic waves. Thus the size of the liquid drops that can be levitated is limited to 4 to 5mm in diameter. Theoretically, however, there is no limit to acoustic levitation. It is even feasible to lift a human mass provided the very of energy needed is avalable from sound wave.
Operation
a) Levitation in one dimension (1–D)
The concept although known since the 1900s, the technique of acoustic levitation with precise control of motion of the objects in mid-air has been developed only recently. Levitation occurs along a fixed vertical axis in 1–D with a vertical upward force counteracting the downward gravitational pull with the acoustic axis of the ultrasound wave lying parallel with the gravitational force.
The US Department of Energy’s Argonne National Laboratory was able to demonstrate 1–D acoustic levitation in 1987 using ultrasound standing wave between two small speakers. Droplets of liquid were suspended in mid air at the sound pressure nodes. The basic unit consists of a piezoelectric transducer which converts electrical energy into the desired high frequency longitudinal standing waves (ultrasound), and a reflector. A standing sound wave is produced by optimising the distance between the transducer and reflector. Both units face each other and have concave surfaces to focus the sound waves which travel from the emitter upwards and bounce off the reflector without deviating from the straight route. Levitation was manipulated along a a fixed y-axis (1–D) by controlling the frequency of the transducer.
The theme photograph of the present article illustrates drop-by-drop suspension of a liquid at nodes of the vibrational force of a standing waves generated between the two small speakers. This is similar picture to a ping-pong ball suspended in air by a fan or water jet. The strength of the standing wave for acoustic levitation of droplets was maintained high at 160 decibels which is louder than the deafening sound of a rocket launch. In order to protect their ears, scientists used a high enough frequency of sound at about 22kilohertz (ultrasonic acoustic levitation) which is just beyond the range humans can hear.
A weak force is normally exerted by an acoustic wave on objects immersed in a linear wave field which can be, however, appreciably strengthened using nonlinear sound wave. Nonlonear acoustics is a branch of physics that deals with sound waves of sufficiently large amplitudes where nonlinear rather than traditional linear equations would aplply. There has been considerable progress in developing high intensity sound waves with nonlinear charateristics.The present day practice of acoustic levitation would not be possible without considering the complex nonlinear acoustic phenomena. The “louder” acoustic waves would allow levitation of denser materials like glass, ceramic, aluminium and steel.The concept of nonlinear acoustics was first introduced by Rayleigh in as early as 1902 as an acoustic counterpart of electromagnetic wave. Maths involved is highly complex and unlikely of any interest to general readership.
b) Levitation in two dimensions (2–D)
Object has so far been allowed to float in one direction (1–D) without being able to have any lateral/sideways movement. The latest technique from ETH, in Zurich transforms traditional motionless suspended droplets to a controlled movement so that the sound waves could not only levitate lightweight objects but also move them around in 2–D in mid-air.
It uses multiple emitter-reflector modules working in parallel next to each other. The device looks like a chess board with penny-size square emitting its own sound, and a clear plastic plate placed a small distance away above the board to reflect back the sound. The forces of acoustic wave are varied from module to module in order to transfer objects or droplets of liquid from one module to the next. Movement of objects needs a careful balance of sound wave force. The sound intensity of the “giving” square from where the object is leaving is lowered, while it is ramped up in the “receiving” square where the object is expected to move to.
Acoustic waves are thus kept varying from module to module along with transfer of liquid particles or droplets from one module to the next – essentially surfing the droplets on a wave of sound. The tricky part is to figure out a balanced sound level to move objects from square to square without damaging them. If pushed too hard, the sound waves may cause the water droplets to explode. On the other hand, if not pushed enough, the droplets will fall because gravity wins. Drops of water, hydrocarbons and various solvents have been successfully levitated. An important application is in the pharmaceutical industry where it would be now possible to mix molecules in a frictionless way without any possible contamination.
c) Levitation in three dimensions (3–D)
At Tokyo University, the science of acoustic levitation has been further progressed with addition of more speakers, and controlling the focal points of the generated standard sound waves. Two speakers were used for levitation, while two more were used to move the focal point. With this 3–D acoustic manipulation, levitation was achieved by trapping the objects in nodes of standard ultrasonic waves. The technique has demonstrated floating of foam balls, fine electrical points, piece of wood, metal nuts and screws. Since 3–D means the objects can be driven in x, y and z axes, they can be moved around to wherever one wants.
5. Advantages of acoustic levitation
It is well known that the contactless methods of handling matters are typically based on electromagnetic principles but, of course, limited by the inherent material properties. Acoustic levitation, on the other hand, is both contact-free and material- independent method and requires minimum effort to prepare samples. Unlike magnetic levitation, acoustic method would apply to any materials, not necessarily magnetic. The contactless method of moving objects by acoustic levitation could have useful implications in chemical engineering and biotechnology where any contact with surfaces can spoil the chemical substances and interfere with the reaction processes. However, a basic limitation of the technique lies with the size of the object which has to be half the wavelength of the sound wave used.
6. Commercial feasibility
Acoustic levitation has considerable commercial potential. The technology would allow studies of various chemical reactions and biological processes and development and production of pharmaceuticals and electronics. As a result, multiple droplets can be mixed in mid-air. Some special chemical and biological experiments can be carried out in this contact-less method that require particles or droplets to be processed and analyzed without worrying about any chemical changes that can occur due to contact with the container surface.
7. Final comments
To sum up, nodes in standing longitudinal waves of nonlinear characteristics are essential for acoustic levitation of objects against gravitational downward pull. Application of the technique is independent of materials. As a contact-free process, acoustic levitation is useful in modern day’s usages of small size and microchips. There is apparently no known theoretical limit to what acoustic levitation can lift given enough vibratory sound. Current technology, however, limits the amount that can be lifted by the sound force to about a few kilograms.
In 1987, NASA performed for the first time an anti-gravity experiment at Argonne laboratory by suspending droplets of liquid in mid air between two small speakers. The drops were, however, motionless i.e. 1-D with no lateral movement. About 25 years later, a new technology from ETH in Zurich provided 2-D mobility to the motionless levitation. Acoustic levitation continued its progress from being motionless 1-D in 1987, mobile 2-D in about 2012 to a controllable hovering of objects in 3-D by 2013. A controlled movement of suspended droplets and objects (magnetic or non-magnetic) allow multiple droplets to be mixed or transported in mid air without any potential contamination or interference from container surface. The new technology would thus have wider implications allowing pharmaceutical industry to mix solutions for drugs in midair without the potential contamination of the containers.
Unlike researches on acoustic levitation in physics lab, piezoelectric property of megaliths and resonance phenomenon from vibration physics of sound seem to play a major role in lifting large items such as rocks and stones in ancient times. Compared to the incredible feats of acoustic lifting of megaliths by our ancestors, the present scientific demonstration looks primitive. But it is certainly a step in the right direction demonstrating the potential of acoustic levitation which will improve as more scientific research is being carried out. One could say that the successful application of acoustic levitation to droplets, will hopefully one day lead to lifting larger objects. In fact, given enough high power from vibratory sound which is still beyond today’s capabilities, it is theoretically feasible that a human mass can be levitated by the acoustic method provided a human could survive the applied immense acoustic force and the availability of necessary protective equipment.