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Quantum Acoustics

This article is a reworking of the scientific research of a famous scientist, Doctor of Technical Sciences, Professor of the N.N. Andreev Acoustics Institute - Lazar Davidovich Rosenberg
Quaternary acoustics is a new term. And before describing it, it is necessary to clarify the seeming contradiction in the term "quantum acoustics" itself.

Acoustics is the science of sound, which is an elastic wave that propagates in gases, liquids and solids. If the oscillation frequency of sound waves is between 40 and 15,000 hertz, our ear perceives them as audible sound. If the frequency is greater than 15.000 vibrations per second, we hear nothing, even though the physical process is the same. Such sounds which are not audible to our ear are called ultrasounds.

Acoustics considers the medium in which sound and ultrasound propagate as a continuous, uninterrupted one. Quantum theory, on the other hand, is used to describe phenomena occurring in the microcosm.

The physical bodies around us, even the same ones, are ultimately distinguishable. We often say, "they are as similar as two drops of water," even though we are sure that even two drops of water, no matter how similar they are, can be distinguished. But the word "similar" does not apply to electrons. Here we are talking about complete identity.

At first glance, it may seem that such an experiment is of purely academic interest, because repeated, very precise experiments have shown that the inertial mass of a body is exactly equal to its gravitational mass. (It must be said, however, that theorists still cannot explain why this happens.) However, when we are dealing with subatomic particles, there is reason to think that in this case the equivalence of gravitational and inertial masses may not exist. After all, if the matter is always attracted by other matter due to the gravitational interaction between them, the reaction between antiparticles and ordinary matter (not antimatter) may have the opposite direction. In other words, they can repel each other.

It cannot be discounted that somewhere in the Universe there may be distant galaxies consisting exclusively of antimatter. If this turns out to be true, and if gravitational repulsive forces do exist, then this circumstance will have very important implications for cosmogony.

Participants in an experiment at Stanford University intend to measure the gravitational force acting on electrons and compare it with the gravitational force acting on positrons. Then it will be possible to say how the mass of the Earth acts on antiparticles: attracts them or repels them.

However, as it was recently shown, there is a junction of these two sciences. As the frequency of elastic ultrasonic oscillations increases, the wavelengths decrease and finally become so small that the waves begin to "notice" the discrete structure of solids - crystal ionic lattice.

This is where quantum acoustics is born, an interesting and promising field, in which a number of previously unknown physical phenomena have already been established.
Ionic lattice
The main feature of the structure of the crystal lattice of ions are opposite electric charges, the ions themselves, which results in the formation of an electromagnetic field that determines the properties of substances with an ionic crystal lattice. These are refractoriness, hardness, density and the ability to conduct electric current. A typical example of an ionic crystal lattice is table salt.
Atomic lattice
Substances with an atomic crystal lattice usually have strong covalent bonds in their nodes, consisting of atoms themselves. The term "covalent bonding" itself comes from two Latin words: "co" - together and "vales" - having force, because it is a bond that occurs at the expense of a pair of electrons belonging simultaneously to both atoms (or in simpler terms, a bond between atoms at the expense of a pair of electrons that are common to them). The formation of covalent bonding takes place exclusively among atoms of nonmetals, and it can appear both in atoms of molecules and crystals. Covalent bonding occurs when two identical atoms share electrons with each other on a fraternal basis, thus forming a common pair of electrons for neighboring atoms. Because of this, covalent bonds strongly and evenly bind atoms in a strict order - perhaps the most characteristic feature of the atomic crystal lattice structure. Chemical elements with such bonds can boast of their hardness, high melting point.
Substance105 Joules.
Aluminum 3.8; Mercury 0.1;
Iron 2.7 Lead 0.3
Ice 3.3 Silver 0.8
Copper 1.8 Steel 0.8
Naphthalene 1.5 Zinc 1.2
Tin 0.5 Platinum 1.01
Paraffin 1.5 Gold 0.66
Chemical elements such as diamond, silicon, germanium, and boron have an atomic crystal lattice.

Molecular lattice
The molecular type of a crystal lattice is characterized by the presence of stable and closely packed molecules. They are arranged in the nodes of the crystal lattice. In these nodes they are held together by such Vanderwals forces, which are ten times weaker than the forces of ionic interaction. A prime example of a molecular crystal lattice is ice, which is a solid but tends to become liquid as the bonds between the molecules in the crystal lattice are quite weak.

Metallic lattice.
The metallic crystal lattice is more flexible and malleable than the ionic lattice, although they look very similar. Its distinctive feature is the presence of positively charged cations (metal ions) in the nodes of the lattice. Between the nodes live electrons, involved in the creation of the electric field, these electrons are also called electric gas. The presence of this structure of the metal crystal lattice explains its properties: mechanical strength, thermal and electrical conductivity, fluidity.

As we know, all material substances can exist in three basic states: liquid, solid, and gaseous. True, there is also the state of plasma, which scientists consider to be no less than the fourth state of matter, but our article is not about plasma. The solid state of matter because it has a special crystalline structure, the particles of which are in a certain and clearly defined order, thus creating a crystal lattice. The structure of the crystal lattice consists of repeating identical elementary cells: atoms, molecules, ions, other elementary particles, connected by different nodes.

When studying quantum acoustics phenomena we, first of all, encounter difficulties in obtaining short ultrasonic waves. The fact is that usually as an emitter and receiver of ultrasound use plates made of piezoelectric materials, i.e. such materials that have the property to "release" electrical charges under the action of mechanical stress. If you squeeze such a plate, charges of opposite signs will appear on its surfaces.

If compression is replaced by stretching, the signs of the charges will change places. This effect is reversible. If metal electrodes are placed on the plate and an electric voltage is applied to them, the plate will deform. If the voltage changes, the plate will vibrate and emit elastic waves. Thus, the piezoelectric plate can be used to emit and receive ultrasonic vibrations.

Radiation and reception of ultrasound are especially effective when the oscillating plate is tuned to resonance. This requires that its thickness corresponds to half the ultrasonic wavelength. For frequencies used in conventional ultrasonic flaw detection (several megahertz) the thickness of the radiating plate has to be about one millimeter. It is not difficult, however, to realize that in order to obtain a thousand times higher frequencies it would be necessary to work with plates, the thickness of which is only a few microns. To make such a plate, to apply metal electrodes with a thickness of a hundredth of a micron - a task that probably would not have coped with even the legendary craftsman Levsha. Therefore, nowadays, when studying high-frequency vibrations, we have to use relatively thick non-resonant plates.

This, of course, greatly reduces the efficiency of obtaining and receiving high ultrasonic frequencies. We could try to glue the piezoelectric plate to a steel bar, and then grind it until its thickness is a few microns. But the problem is that the layer of glue is tens of microns, and all the energy of ultrasound will be absorbed by it. This problem can be solved by using semiconducting piezoelectric materials, such as arsenic gallium or cadmium sulfide. If a plate cut from such material is placed against a metal surface, a so-called barrier layer, i.e., a layer depleted of electrons, is formed on its boundary with the metal. The thickness of this layer is in the microns, and its resistance is very high. Applying a constant voltage of varying magnitude to the plate's barrier layer, it is possible to change the thickness of the barrier layer within certain limits.

Now let's apply to the ends of a thick semiconductor plate an alternating voltage of high frequency, the same voltage that we want to convert into an ultrasonic wave. If the plate were homogeneous, the electric field would be uniformly distributed inside it, and under the action of this field the plate would begin to oscillate with a certain frequency. Since the thickness of the plate is far from resonant, the amplitude of these vibrations would be negligible. In the presence of a barrier layer, however, the field inside the plate is distributed unevenly: the main part of the applied voltage falls on a very thin barrier layer. Therefore, the detent layer begins to oscillate with a large amplitude, playing the role of a resonant radiator of ultrasound.

By varying the constant voltage applied to the interlocking layer, it is possible to change the thickness of the interlocking layer and, therefore, the resonant frequency of our emitter within certain limits. This allows us to work within a certain range of frequencies.

The next problem, which can be solved by methods of quantum acoustics, is direct amplification of sound waves. Used in practice and familiar to us sound waves have negligible power. To explain this, I will allow myself to give a well-known school example: if we wanted to boil a kettle of water with the energy of speech, for this purpose the entire population of Kiev would have to shout without interruption for two days. Therefore, in order to be able to do something with speech (for example, to record it on a tape recorder), it must first be amplified.

Unfortunately, until recently we did not know how to amplify directly sound vibrations, and in general we did not know how to amplify any vibrations except electromagnetic ones. Sound, light, thermal radiation we had to preliminarily transform into electric oscillations, and then amplify them to the necessary value.

For example, in order to make a recording on a magnetic tape in a tape recorder, you must first use a microphone to turn the sound into electric current fluctuations and amplify them with an electronic amplifier. Only then will the amount of energy reach the minimum value at which a "trace" remains on the magnetic tape. During playback, the tape is read from the tape, amplified again, and fed to the loudspeaker, which converts the amplified electrical oscillations into the sound of the desired volume.

Thus, we have to perform energy conversion twice - by means of a microphone and by means of a loudspeaker. Since the efficiency coefficients of these devices are very small, about 1%, the conversion is associated with large energy losses. The amplifier is required to compensate for these losses and, in addition, to give the desired gain.

This is a ridiculous situation. Let, for example, it is required to amplify sound by 100 times. During conversions initial signal power is reduced by 10,000 times (100 times from the microphone and 100 times from the loudspeaker). So you have to make an amplifier with a million times the gain in order to amplify the original signal by 100 times; the rest goes to compensate for conversion losses.

Without electronic amplifiers it is impossible to manage in radio broadcasting, in sound cinema, in television. The same situation takes place in the field of ultrasonic technology - in the field of inaudible sounds. In electronic counting and decoding machines, in radar and other similar devices, so-called delay lines are used, As the name itself shows, they are designed for the time delay of the signal. What is a signal delay for?

The point is that the counting machine works by comparison. When it counts, it compares the result with the program reference points. For such comparison the counted result must be fixed. If it is significant and will be used several times, it is recorded with the help of so called long term memory - written down on a magnetic tape. If the result is needed only once and then will be "thrown in the trash," there is no point in writing it down. It is "remembered", held in time for thousands of seconds, in order to have time to compare it with another value. This memory is called RAM. Delay lines are a type of such RAM.

The easiest way, of course, to delay a signal would be to send it through a long cable. But since the speed of signal propagation through a cable is in the tens of thousands of kilometers per second, a signal delay of one millisecond would require a cable that is tens of kilometers long. And since there are many delay lines in a machine, it would be necessary to build a building of enormous size just for them. Therefore, to delay signals, metal rods are used along which ultrasonic vibrations propagate. Their speed is known to be much less than the speed of propagation of electromagnetic oscillations.

The delay line is arranged as follows. An emitter - a piezoelectric plate - is attached to one side of the metal rod. Current pulses corresponding to the counted number are applied to it. The piezoelectric plate converts these pulses into pulses of elastic waves, which run along the rod. At the other end of the rod there is another piezoelectric plate, which converts the sound running on the rod into electrical impulses. Thus, the counted result is delayed by the time the machine needs to count the second result. The machine counts the second number, and the first number runs on the rod. The length of the rod is chosen so that both results come to the other end of the rod at the same time. Here, at the end of the rod, the impulses are compared. (The length of the rod is easy to calculate by knowing the speed of sound propagation in a given substance.)

To make the delay line work, you must put two transducers - two piezoelectric plates. Energy losses in these transducers are very high. Efficiency of one piezoelectric plate is about one percent, so on each delay line initial energy drops ten thousand times, and after each delay line, it is necessary to put a tube or semiconductor amplifier to restore original signal power.

Each counting and solving machine has hundreds and thousands of delay lines and therefore as many amplifiers. Each extra tube (or semiconductor element) requires space, energy, and maintenance in operation, and there are many such delay lines in machines. And yet, until recently, we had to put up with all this trouble: there was no other way.

Lazar Davidovich Rosenberg (05.06.1908 - 08.11.1968) - Soviet acoustics scientist, doctor of technical sciences, Stalin Prize laureate.
He was born on June 5, 1908 in Kiev in the family of a teacher.

Graduated from the Faculty of Radio of the Kiev Power Engineering Institute (1931). From 1930 he worked in the research laboratory of sound cinema at the Kiev film studio. From 1936 to 1938 he was the first head of the acoustics department of the Kiev Institute of Radioengineers. In 1938 he was head of acoustic design of the Palace of Soviets (Moscow), in the same year he was awarded the degree of candidate of technical sciences.

From 1941 to 1945 he was a senior research associate of the Physical Institute of the USSR Academy of Sciences. During the Great Patriotic War he had participated with N.N. Andreev in creation of weapon for fighting against acoustic mines. In 1942 he defended his doctoral thesis and was approved as a Doctor of Engineering.

In 1954 he was appointed the head of the Acoustic Institute of AS USSR and since 1962 he is the head of the Department of Ultrasonics and Acoustoelectronics Laboratory. He was elected the Chairman of Scientific Council on Ultrasound of the Academy of Sciences of the USSR. He was the member of the editorial board of the "Acoustic Journal".

In 1939-1950 he taught at the Moscow institute of power engineering (he supervised the training of acoustic postgraduates), in 1950-1953 he was the professor of the Department of theoretical and experimental physics in Yaroslavl institute, and since 1956 he was the professor of the Department of acoustics of the Moscow institute of Physics and Technology.

He is known as a boonist. He is the author of the essay "History is told by money" (1964), published in the journal "Nauka i zhizn" (Science and Life).

He was awarded the Stalin prize of the first degree (1951) for scientific research in the field of acoustics. He was awarded the medal "For Labor Valor" (1953).

He died on November 8, 1968 after a long and serious illness.

On topic: Basic physical concepts of electrodynamics Leningrad scientific film

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