Toward the Thirtieth Anniversary of Soviet Physics

Advances in Soviet Acoustics

S. N. Rzhevkin

Classical acoustics, developed in the nineteenth century through the works of Stokes, Helmholtz, Kirchhoff, and Rayleigh, had by the beginning of the twentieth century almost come to a halt in its further development. The experimental technique of that time had exhausted its possibilities; almost all theoretical predictions had been thoroughly verified by experiment. It was difficult to hope for the discovery of new phenomena in acoustics; practical applications were limited, perhaps, only to music, short-distance telephony, and the gramophone. In this connection, theoretical work in the field of acoustics also became comparatively rare and concerned only the elaboration of details.

A fresh stream was brought into acoustics by the works of P. N. Lebedev and his school at Moscow University, carried out in the first decade of the twentieth century. P. N. Lebedev had been deeply interested in acoustics since the time of his youthful work with Kundt. Later he chose acoustic resonators as one of the objects of his classical investigations of the ponderomotive interaction of the wave field and resonators. The study of the damping decrement of acoustic resonators was the subject of a work by Lebedev’s pupil P. B. Leiberg. The pressure of sound radiation was demonstrated in the excellent work of V. Ya. Altberg, while V. D. Zernov developed extremely delicate methods for measuring the force of sound. It is important to note the significant theoretical investigation by N. P. Kasterin on the propagation of sound in a layered medium and in a medium filled with acoustic resonators. Kasterin theoretically proved the existence in such media of dispersion and absorption of sound; these conclusions were fully confirmed experimentally. Besides the general significance of Kasterin’s works for wave theory, it should undoubtedly be considered that he laid the foundations of the theory of acoustic filters, which, only in another form, were described by Stewart in America as early as the 1920s and have since become firmly established in sound engineering.

S. N. RZHEVKIN

In the search for the influence of molecular resonators on the propagation of sound, P. N. Lebedev turned to the investigation of very short acoustic waves. Lebedev’s pupil, N. P. Neklepaev, by means of a diffraction grating isolated short ultrasonic waves from the acoustic spectrum of an electric spark excited by a high-frequency discharge, and investigated their attenuation in air. P. N. Lebedev showed that Neklepaev’s data implied a significant increase in sound absorption in comparison with the classical theory. Thus Lebedev’s school stood almost on the threshold of the discovery of molecular absorption of sound. Only the inadequacy of the technical means of that time prevented Lebedev from going further and making decisive discoveries in this field.

The rapid progress in acoustics began in the 1920s of our century and was most closely connected with the successes of radiophysics that followed the invention of the electron tube, after which, within a few years, all methods of experimental acoustics were completely renewed. The generation, reception, and recording of sound, and almost all acoustic measurements, came to be carried out by electrical methods. On this basis there arose a new chapter of technical physics: the theory of electroacoustic transducers, built at the present time on a solid theoretical foundation, which developed by uniting the methods of theoretical mechanics and electrodynamics. In the first stage of the development of electroacoustics, the development of the method of electromechanical analogies, which played a very progressive role, was of great importance. However, the application of the method of analogies initially had a purely formal character and amounted to transferring into acoustics formulas and circuits known in low-current electrical engineering or radio engineering.

A great deal of work in substantiating and developing the method of electromechanical analogies was carried out by Russian scientists. Let us point first of all to a series of works by A. A. Kharkevich, who provided a foundation for the method and clarified the limits of its applicability. Further important works were done by A. I. Belov, I. S. Rabinovich, L. A. Varshavskii and V. N. Fedorovich, G. A. Gamburtsev, and K. F. Teodorchik. In A. A. Kharkevich’s works, the interpretation of the electromechanical transducer as a generalized four-terminal network, characterized by two variables at the input and two variables at the output, is systematically developed. This conception was generalized by Kharkevich to energy transducers of a more general type: thermomechanical, thermoelectric, three-terminal transducers, and other systems. The use of operator methods in Kharkevich’s works enabled him to attain great generality in the treatment both of stationary oscillatory processes and of nonstationary processes and, as a result, to provide exhaustive methods for calculating complex electroacoustic apparatus.

Another, very fruitful, direction in the field of electroacoustics was developed by V. V. Furduyev. Proceeding from Maxwell’s conception—

...led to the possibility of the electrodynamic application of the Lagrange equations. Furduyev investigated the question of the limits of such an application, the question of the invariance of the Lagrange–Maxwell equations with respect to contact transformations, and proved a number of general and special theorems of electromechanical reciprocity that are of great importance for the calculation of acoustic apparatus. In his works Furduyev is, in essence, following the path of constructing analytical electroacoustics, analogous to the path of constructing analytical mechanics.

Over the past 30 years a large number of important works on theoretical acoustics have been carried out in the Soviet Union; I shall dwell only on the major ones. First of all, it is necessary to mention the theory of the sound of an airscrew proposed by L. Ya. Gutin (1935). Starting from the expression, given by Lamb, for the sound field arising as a result of the action on a medium of variable forces, Gutin finds the relation between the sound force of a propeller and its principal aerodynamic characteristics—thrust and torque. Gutin’s theory for the first time gave a complete solution of the difficult problem of the so-called “rotational sound” of a propeller and is invariably cited in the world literature.

V. A. Fock (1941) solved the extremely difficult problem of the attached energy and mass of a medium when an alternating flow passes through a circular aperture in a partition set across a tube. Previously only Rayleigh’s solution was known for the attached mass of elliptical and circular apertures in an infinite plane screen. Fock’s solution, presented by him in the form of a series convenient for computations, played a major role in calculations of sound ducts of complex shape.

N. N. Andreev and I. G. Rusakov in 1939 derived an approximate equation for the propagation of sound in a moving medium and considered certain particular problems.

Of great importance is the cycle of theoretical works by D. I. Blokhintsev, carried out during the period of the Patriotic War and published by him in the monograph entitled Acoustics of a Nonuniform Moving Medium (1946). The author succeeded in precisely formulating the generalized equations of acoustics and in correcting the errors of preceding works. Starting from the general equations, D. I. Blokhintsev solved several important problems of acoustics that have practical significance, in particular the problem of the propagation of sound in a turbulent flow, of the sound field of a source moving at high speed, of the excitation of sound by a flow, and of the operation of a receiver placed in a moving flow.

V. A. Krasilnikov (1940) experimentally investigated the question of the fluctuation of the phase of a sound beam during propagation in a turbulent atmosphere; the conclusions from the theoretical works of A. M. Obukhov and A. N. Kolmogorov made on this question by Krasilnikov were confirmed by him experimentally.

Of great importance in acoustics, as also in radio engineering, are the works of P. E. Krasnushkin (1943–1946) on the general theory of waves in sound ducts and in inhomogeneous media. The method of normal waves developed by him made it possible to find an approach to solving a number of the most important problems of wave acoustics and has high heuristic value. Krasnushkin was the first to give a rigorous solution for the propagation of waves in horns of a definite type, in curved tubes, and in inhomogeneous media with gradually varying parameters.

One should note the detailed work of L. M. Brekhovskikh (1945–1947), who quite fully solved the problem of the propagation of sound from a source in a layer between two media that absorb or reflect sound, and found the law of decrease of the average sound intensity with distance.

Let us also mention a number of theoretical works on acoustics connected with the calculation of electroacoustic apparatus. First of all, mention should be made here of the work of L. Ya. Gutin, who was the first to solve the difficult question of the radiation of a piston pulsating on only one side and radiating sound into free space. A. A. Kharkevich and L. Ya. Gutin carried out interesting investigations on the theory of the horn. Yu. M. Sukharevsky (1938), having investigated the sound field of horns in the open air, determined the dependence of the directivity characteristic of a horn on its design parameters. This work provided a basis for calculating the sound coverage of large areas by means of loudspeakers.

Works on the theory of the telephone were carried out by N. N. Andreev, S. N. Rzhevkin, and E. K. Kuznetsova. In Rzhevkin’s work (1935), methods were given for calculating the equivalent constants of the telephone membrane as a system with one degree of freedom; in the work of Rzhevkin and Kuznetsova (1944), a theory was given of a telephone reproducing a wide frequency band.

The development of experimental electroacoustics advanced very far in the direction of practical applications. Large and successful works were carried out by Soviet acousticians in creating domestic apparatus for sound cinema. The group of P. G. Tager, A. A. Shishov, and N. S. Dzhigit (VEI, 1927) successfully developed a sound-recording system using the Kerr cell. At the same time, the group of A. F. Shorin and I. I. Moshonkin (Central Laboratory of Wire Communications) developed an oscillographic recording method which, after a number of improvements, is now used as the principal one in our sound cinema. It should be mentioned here that the first recording of sound on film in Russia was made by S. Ya. Lifshits as early as 1912.

Over a number of years, the Institute of Sound Recording in Moscow, under the direction of I. E. Goron, carried out extensive work on electromechanical methods of sound recording. At the institute, the technology for manufacturing gramophone records was studied in detail, and complex apparatus for receiving, amplifying, and recording sound was developed, which made it possible to—

place the production of gramophone records on a high level. I shall also mention the successful developments, in the same institute, of a domestic system of magnetic recording on ferromagnetic film; on the basis of these developments, high-quality apparatus is now being produced on a factory scale. Much has been done in the Soviet Union in the calculation and design of loudspeakers and microphones. In this connection, one should note a series of works by V. V. Furduev, V. S. Grigor’ev, S. A. Makov (NIKFI), L. A. Varshavsky (the “Krasnaya Zarya” plant), R. L. Volkov, V. K. Iofe, and P. E. Shifman (IRPA). These works led to the creation of high-quality microphones and loudspeakers for cinema and radio broadcasting. As a very great success one should note the development at NIKFI (V. V. Furduev, A. A. Khrushchev, I. M. Bolotnikov) of loudspeakers of 20 and 40 watts for motion-picture theaters. Tests of this apparatus, carried out during the current year, showed a high quality of sound, approaching the natural. The power of the apparatus constructed is such that it makes it possible to obtain, in large halls, an intensity of up to 95 decibels. The apparatus is not inferior to the best modern specimens of foreign manufacture, surpassing them in a number of indicators.

A series of works on piezoelectric and magnetostrictive transducers, carried out in recent years, made it possible to bring to complete clarity the calculation of important hydroacoustic apparatus. We note, as the most important, the works of A. A. Kharkevich (1942–1945) and L. Ya. Gutin (1941–1945). Experimental and design work on Seignette-acoustic apparatus was conducted before 1941 in Leningrad (IRPA), where high-quality microphones and speakers were first constructed (Peshlat, Lepeshinskaya). Somewhat later, analogous work on a solid scientific basis was begun in the laboratory of V. A. Shubnikov with the participation of A. S. Shtein. During the Patriotic War, Seignette-electric apparatus was intensively developed by A. S. Shtein, who proposed a number of important devices for communication and for the detection of sound and vibrational processes. Successful types of Seignette-electric microphones and speakers were developed at the Central Institute of Communications by P. V. Anan’ev (1943–1946).

A review of the successes of electroacoustics would not be complete if I did not touch upon the methods of acoustic measuring technique developed in the Soviet Union.

The first steps in the field of electroacoustic measuring apparatus were made as early as 1919. Applying sound generators with electron tubes in acoustics, the author of these lines succeeded, at that time, in carrying out an investigation of certain important questions of the theory of hearing. Somewhat later (1923) it proved possible to realize a sound analyzer operating on the principle of a smoothly tunable electric circuit.

Of great significance for Soviet acoustics were the development, in the laboratory of the “Krasnaya Zarya” plant, by L. A. Varshavsky and subse—

S. N. RZHEVKIN

production at the measuring condenser microphone plant. This for the first time made it possible to place the measurement of sound and noise intensity on a solid footing. The development and construction of an automatic sound analyzer, carried out in 1935 by M. I. Rodman at the Leningrad Electrophysical Institute, had equally great fundamental significance. The analyzer was built on the principle of a probing tone and was equipped with an electromechanical (magnetostrictive) filter with a narrow pass band near a frequency of 50 kilohertz. The analyzer made it possible to carry out a number of important works in aeronautical and musical acoustics. A considerable success of Soviet acoustics may also be considered the development by A. I. Sokolik and M. I. Rodman (1942–1946) of electroacoustic instruments for measuring detonation in internal-combustion engines. With the aid of these instruments it is possible to count the number of detonation cycles over a definite period of motor operation. The use of the instrument on an airplane makes it possible reliably to choose an engine regime well below the detonation threshold and thus to achieve a large saving of gasoline.

Mention should also be made of a number of systems of electromusical instruments developed in the Soviet Union. In 1922 L. S. Termen invented an entirely new musical instrument, subsequently called the “Theremin,” in which the change of pitch over very wide limits was effected very impressively under the influence of the motion of the hand in free air. This was achieved by changing the beat frequency between two high-frequency generators under the influence of a change in the capacitance of the circuit of one of the generators as the hand moved. In 1925 I constructed and demonstrated at the Institute of Musical Sciences a polyphonic electronic organ equipped with a piano-type keyboard. Later, a number of musical instruments of a very perfected type were built by Gurov, Ananyev, Volodin, Rimsky-Korsakov, and Ivanov. An interesting idea for obtaining new sounds by artificially drawing a phonogram of the phonogram type on motion-picture film, the so-called “drawn sound,” was proposed and developed by Sholpo. In this way it proved possible to create interesting new timbres and rhythms.

Among the large number of questions of acoustics connected with research in adjacent fields, I should like to dwell in somewhat greater detail on physiological and architectural acoustics, since in these fields I myself had occasion to work more. In 1919, at the suggestion of Academician P. P. Lazarev, I began at the Institute of Physics and Biophysics to investigate the nature of dissonance and consonance. The application of new methods of radio engineering greatly facilitated the solution of this problem. It was possible to show that, when two sounds are listened to separately by different ears, the sensation of dissonance and consonance completely disappears, and the accuracy of establishing consonant tone combinations becomes extremely small. When two sounds are heard together by one ear, the occurrence of combination tones and beats

between overtones makes the sensation of consonance extremely distinct. The results of this work clearly indicated that the phenomena of consonance and dissonance are the responsibility of the peripheral organs, and not of the central nervous system.

In 1918 P. P. Lazarev published his book on biophysics, The Ionic Theory of Excitation, in which a number of new conceptions concerning the functioning of the organ of hearing were developed; in particular, the laws of adaptation of hearing and the laws of perception of interrupted sounds were derived theoretically, and a number of important conclusions were drawn from the theory. Verification of the principal conclusions of the ionic theory of hearing was carried out by P. N. Belikov, A. S. Akhmatov, Shustov, and others.

Electroacoustic methods were applied by the author to solve the problem of measuring sound intensity at the threshold of audibility. As a result, it proved possible to determine the threshold of hearing and to construct apparatus for professional hearing tests (1924–1926). A thermophone was used as the sound source, the intensity of which is amenable to calculation; a device was applied for reducing, by a known number of times, the voltage of currents of audio frequency (which has now come to be called an attenuator or attenuation box), and electrolytic liquid capacitors (little known at that time) were used for separating the direct-current and alternating-current circuits. With the aid of the thermophone (1928) it also proved possible to determine the dependence of the upper limit of hearing on sound intensity. Considerably later (1941), E. P. Ostrovskii and B. E. Sheivekhman investigated the influence of powerful sounds lying above the limit of hearing (more than 20,000 Hz) on the sensitivity of hearing at lower tones and discovered a new phenomenon: sensitization of hearing by ultrasound. A number of our works were devoted to the question of the applicability of the Weber–Fechner logarithmic law to the evaluation of the sensation of loudness. In agreement with a number of other investigators (Fletcher, Békésy, and others), it was established that the increase of loudness as a function of sound intensity does not obey the Weber–Fechner law.

In 1927 V. S. Kazanskii and S. N. Rzhevkin carried out an investigation of the sound spectra of bowed instruments and of the voice. In the spectrum of the singing voice, the presence of a characteristic “singing” formant was discovered in the region of 500 Hz, where strengthening of the overtones was always observed, independently of the frequency of the fundamental tone. The investigation of the singing voice was carried out in 1933 in greater detail, with far more perfect apparatus and on more extensive material. The results of these works (which I have still not published, but which were reported at the Moscow Conservatory and at FIAN) led to the discovery of a second, more important, singing formant in the region of 2500–3000 Hz, responsible for the brightness and metallic quality of the voice. The need to create a special formant in singing voices is clearly in contradiction with the clarity of the manifestation of vowel formants and worsens singing diction.

It is also necessary to point to a major success of Soviet acoustics: the work of L. L. Myasnikov (1941), who put forward the idea of the automatic recording of speech by phonetic signs constructed on the basis of the formant structure of vowels and consonants. The idea of such an automatic stenograph was developed in detail by the author, tested experimentally, and yielded very promising results.

A large and fruitful body of work on the question of speech intelligibility under conditions of strong noise was carried out by I. G. Mamonkin and Yu. S. Bykov (1942—1944). I. G. Mamonkin succeeded in carrying out a sufficiently detailed study of the acoustic and phonetic constants for Russian speech, which until that time had been poorly known. As a result, possibilities were opened for applying the method of articulatory calculations to the design of communication systems under conditions of strong noise. This second part of the work was successfully carried out by Yu. S. Bykov.

A detailed critical review of works on the physics of hearing and speech is collected in Rzhevkin’s monograph Hearing and Speech, published in 1936. Within the scope of this review I absolutely cannot mention the numerous and important works on the physiology of hearing carried out during this period by the schools of I. P. Pavlov, L. A. Orbeli, and other physiologists and physicians. As a result of the discovery and study of the action currents of the auditory nerve, there has been enormous progress in scientific knowledge in this field.

A number of important studies on the theory and technology of musical instruments were carried out at the Institute of the Musical Industry in Leningrad; among them one should note the works of A. A. Kharkevich, A. I. Belov, A. V. Rimsky-Korsakov, B. P. Konstantinov, and others. B. P. Konstantinov, in particular, is responsible for a very valuable investigation of self-oscillatory processes in wind instruments.

I now turn to Soviet work in the field of architectural acoustics*). The first Soviet scientist to begin research in this field was S. Ya. Lifshits, who in 1926 proposed a theory of optimum reverberation. A number of acoustic designs for theaters, halls, and studios were based on his works. The further development of architectural acoustics showed the inadequacy of the concept of Sabine’s simplified statistical-geometric theory and led to the necessity of constructing rigorous theories based on the solution of the wave problem. The new formulation of the question led to a revision of all the basic principles of architectural acoustics connected with the radiation, propagation, and reception of sound in rooms, as well as with sound absorption and sound insulation. These questions were placed on a solid scientific basis in the design of the House of Sound Recording in Moscow (1933—1938) and during the process of its construction, completed only recently. I shall not dwell on the complex tech-

) For a list of the principal literature on architectural acoustics in Russian, see UFN, 32, 476 (1947). (Editor’s note.*)

technical questions that were resolved in the course of this work, such as: sound insulation from external and internal noises (in particular, from ventilation noise), the provision of variable reverberation, the technology of porous sound absorbers, the complex problems of the acoustic adjustment of studios to optimal acoustic conditions, the amplifier and electroacoustic equipment of apparatus studios, and other questions. The participants in the design and construction of the House of Sound Recording along the acoustic line (I. E. Goron, the author of these lines, G. A. Gol’dberg, S. T. Ter-Osipyants, G. D. Malyuzhinets, and others) achieved successful and important practical results, which were subsequently revealed in the course of production operation.

I shall dwell in somewhat greater detail only on the question of the new system of sound absorption, in whose creation I and a number of colleagues who worked with me had to do a great deal. I have in mind resonant sound absorbers, the development of which is of considerable interest both as a purely physical problem connected with the creation of an “acoustic black body,” and at the same time is highly relevant for solving the basic tasks of architectural acoustics. The use of resonators in theaters and churches was known as far back as the time of ancient Greece and Rome; however, the essence of the matter had not been clarified until recently. In a 1936 work I established the conditions under which resonators increase and under which they decrease the reverberation of a room. M. S. Anitsiferov experimentally confirmed the principal theoretical conclusions of this work. G. D. Malyuzhinets (1936) pointed out the fundamental possibility of total absorption of sound by a perforated (flat) structure, behind which lies a layer of porous material that creates friction. In 1938 I indicated what relations must obtain between the frictional resistance in the throat of the resonator, its dimensions, and the number of resonators per unit area in order for total absorption of sound to occur when it is incident on such a system. Systems of this kind were implemented and studied in detail experimentally by S. T. Ter-Osipyants, as a result of which the tentative theoretical considerations developed during this period, and which had raised a number of doubts, received substantiation, and this gave an impetus to the further development of the work. After the study of the simplest systems of sound absorbers with only one type of resonator (monoresonant), more complex systems were developed that strongly absorb sound over a broader frequency range. As a result of a number of works by our group (in particular by V. S. Nesterov and others), methods were developed for calculating and designing resonant-type sound absorbers, and the practical possibility was demonstrated of absorbing more than 90% of all incident sound energy at a frequency of 100 c/s and more than 98% at high frequencies (300–4000 c/s). Questions of the technical application and technology of resonant sound absorbers were also worked out in detail.

Resonant sound absorbers proved highly useful for solving a number of acoustic problems in studios and theaters, for absorbing ventilation noise, and for special purposes. In the studios built for the House of Sound Recording, resonant sound absorbers of various types were widely used.

In designing the Palace of Soviets, the very difficult task arose of eliminating the echo from the immense dome of the large hall, 100 m high; the use of resonant sound absorbers offered one of the possible ways of solving this problem, especially convenient for low frequencies, which are not absorbed at all by ordinary absorbers.

Great successes in developing the theory of sound absorbers consisting of a series of layers of porous materials were achieved by G. D. Malyuzhinets (1938–1941), who showed that, with an appropriate choice of layers, this system could solve the problem of strong absorption of sound over a wide frequency range. Significant successes in the development of porous sound absorbers were achieved by V. A. Andreevsky. He proposed a number of new types of absorbers (ceramic, foam-fireclay, and others) and provided the technology for their manufacture.

In designing the hall of the Palace of Soviets, great difficulties arose in solving acoustic problems. A number of major studies and investigations in this direction were carried out in the course of the scientific development of the acoustics project for the Palace of Soviets under the direction of the Acoustic Commission of the Academy of Sciences, which worked for a number of years under the chairmanship of N. N. Andreev (1937–1941). From this cycle of work we shall note the investigations of Yu. M. Sukharevsky—on the theory of acoustic feedback; of A. V. Rabinovich—on the study of echo; and of V. S. Grigoriev and S. A. Makov—on the design of a system of powerful loudspeakers and on the experimental study of sound fields, for which a special sound-measuring test range was created near Moscow. Among the group of acousticians there arose the interesting idea of transmitting sound in the large hall by means of a system of small radiators distributed over the entire area of the hall. The detailed theoretical development of this direction, undertaken by L. D. Rozenberg, showed the feasibility of the proposed system; it is very interesting that a system of this kind also made it possible automatically to create artificial reverberation and thereby eliminate a substantial shortcoming of a heavily damped hall. A number of works on the question of reflection and diffraction of sound in the large halls of the Palace of Soviets were carried out by G. D. Malyuzhinets and G. A. Goldberg.

The cycle of work on the acoustics of the Palace of Soviets, interrupted by the outbreak of the Patriotic War in 1941, undoubtedly represents a major event in the development of Soviet science, and it would be no exaggeration to say that the successes achieved in this field made it possible to lay the scientific foundations for the development of architectural acoustics and, beyond doubt, in a number of questions anticipated foreign science and technology.

I turn to research in ultrasonics. This field of acoustics developed in interaction with other branches of the physical sciences—molecular physics, colloid and physical chemistry, optics, and others—as well as with technology.

In the direction of obtaining short ultrasonic waves, S. Ya. Sokolov succeeded in setting a record; by optical means he detected waves in quartz at frequencies of the order of \(2.75\cdot 10^8\) Hz (1932). In studying the radiation of large quartz plates, he also discovered, by measuring the amplitude at individual points, the important fact that crystalline quartz is for the most part an inhomogeneous material, and that the amplitude of oscillations varies irregularly along the surface. S. Ya. Sokolov also found that the action of ultrasound on molten metal during its solidification has a very strong influence on the crystalline structure. N. N. Andreev (1928) developed the theory of oscillations of quartz plates and showed what kind of deformations they undergo. Detailed theoretical studies of the operation of plates made of quartz and Rochelle salt, taking into account the electrical and mechanical load, were carried out by I. G. Rusakov (1944), A. A. Kharkevich (1943–1945), and L. Ya. Gutin (1941–1945). Powerful ultrasonic oscillations were obtained by E. P. Ostrovskii (1936) with the aid of the magnetostriction effect; S. N. Rzhevkin and E. P. Ostrovskii (1935) showed that ultrasound is capable of producing erosion and dispersion of solid bodies, and they gave an explanation of this effect. The methods developed for obtaining ultrasound and the techniques of dispersion found further application in the chemical-pharmaceutical industry. The interesting phenomenon of the luminescence of a liquid under the action of powerful ultrasound was studied in detail by V. L. Levshin and S. N. Rzhevkin (1937). Kornfeld (1940) studied the phenomenon of cavitation of a liquid under the action of ultrasound.

A number of biological effects were studied in the high-frequency laboratory of the State X-Ray Institute, organized by me (1930–1937). From this cycle we shall note only the interesting phenomenon, discovered by E. P. Ostrovskii and O. N. Istomina, of stimulation in the development of seeds and plants. It was found that the “sonication” of pea seeds and seed potatoes two months before planting gives a very strong acceleration of development, an increase in growth, and an increase in yield as compared with the control by 50–60%, and in some cases by 200%.

In 1937 S. N. Rzhevkin established by an optical method that, owing to the formation on the surface of pulsating piezoquartz of flexural waves, it emits, in addition to the principal plane wave, additional waves in lateral directions. Theoretical and experimental works by P. E. Krasnushkin (1939–1944) clarified in detail the significance of this inhomogeneity of quartz radiation in measurements by the Pierce interferometer method and made it possible to make this method precise. In a number of works by A. S. Predvoditelev

and his co-workers, using the method of the ultrasonic interferometer, studied important molecular properties of liquids, vapors, and gases.

L. I. Mandelstam and M. A. Leontovich gave a theory of the absorption of ultrasound and thus connected the so-called second (volume) coefficient of viscosity with the Kneser coefficient of molecular absorption of sound due to the transfer of energy to the internal degrees of freedom of molecules.

A large number of works have been devoted to the study of the diffraction of light by ultrasound. This phenomenon, theoretically predicted by L. Brillouin in 1922 and experimentally discovered by Debye and Sears (1932), was studied in detail theoretically and experimentally in a series of works by S. M. Rytov (1933–1938); in particular, he theoretically predicted and experimentally proved that, when light is incident obliquely on the fronts of plane ultrasonic waves, instead of symmetric spectra of plus–minus first order, an asymmetric process arises, similar to Bragg reflection from a crystal lattice. L. I. Mandelstam, N. D. Papaleksi, and G. S. Landsberg proposed an ultrasonic light modulator. P. A. Bazhulin (1940) investigated the attenuation of ultrasound in various media.

S. N. Rzhevkin and S. I. Kreimer studied, by an optical method, a number of wave processes in ultrasound; in particular, they studied the phenomena of diffraction by a single cylinder and by a grating, the phenomenon of waves in a second medium under total internal reflection, the passage of sound through a plate, and resolved on models a number of questions of reflection and diffraction that are important for architectural acoustics.

Special emphasis should be placed on the important significance of the method of ultrasonic flaw detection of opaque bodies, invented by S. Ya. Sokolov (1928) and brought by him by 1937 to a technically perfected form. With this method it became possible to detect the smallest cavities and cracks in metal parts having thicknesses of tens of centimeters. The fundamental significance of this method of “seeing” inside opaque bodies by means of ultrasound should also be regarded as very great.

From the brief survey presented above of the advances in acoustics, it can be seen that the scientists of our country have done much for the development of science and technology in their field. Soviet acousticians may be proud of their scientific discoveries, inventions, and technically developed designs.