Often hailed as the father of ultrasonic testing, Sergei Y. Sokolov, famed Soviet scientist at the V.I. Ulyanov (Lenin) Electrotechnical Institute, Leningrad was born in 1897 in the same Russian city. He proposed in 1928, and a few years later demonstrated a through-transmission technique for flaw detection in metals. He advanced his idea in the late 1920s, at a time when the required technology did not exist. He proposed that such technique could be used to detect irreguarities in solids such as metals. The resolution of the experimental devices which he fabricated was however poor and could not be used at a practical level.

Sokolov subsequently described a different and obviously the more important concept in ultrasonic applications. He demonstrated that sound waves could be used as a new form of microscope, basing on a reflective principle. Sokolov recognized that a 'microscope' using sound waves with a frequency of 3,000 megahertz (MHz) would have a resolution equal to that of the optical microscope. It was nevertheless not until the late 1930s that the technology for such devices was progressively developed, and the high frequencies required for Sokolov's microscope are found in microwave and ultrasonic systems used for radar and underwater navigation.

In the reflection technique, a pulsed sound wave is transmitted from one side of the sample, reflected off the far side, and returned to a receiver located at the starting point. Upon impinging on a flaw or crack in the material, the signal is reflected and its traveling time altered. The actual delay becomes a measure of the flaw's location; a map of the material can be generated to illustrate the location and geometry of the flaws. In the through-transmission method, the transmitter and receiver are located on opposite sides of the material; interruptions in the passage of sound waves are used to locate and measure flaws. Usually a water medium is employed in which transmitter, sample, and receiver are immersed.

He also described in 1937 the 'Sokolov tube' used for underwater imaging, where an acoustical image of an object comes into view as result of object "illumination" by two ultrasonic transducers. The device was also known as the Acoustic Electron-Ray Converter (AERC). It consisted of a specially developed Piezoelectric Element Matrix (PEM) lens made from a piezoceramic disk where grooves are cut into the lens surface at a depth of about 2/3 plate thickness, making about 1000 square elements with sides of 1 mm. Each element is electrically connected with a glass-metallofiber faceplate of vacuum cathode-ray tube or Cathode-Ray Converter (CRC). The electrical potential distribution on the back surface of the PEM is carried to the inner surface of the CRC. An electron beam scanning this surface converts the electrical image into a time dependent output signal that can be processed and displayed as an image. The system has being designed to have minimal geometric distortions, taking into consideration the diffraction spot diameter and piezoelements' size. (Sokolov, S.Y.- US Pat.2164125 (1937)). In 1955, the first successful experiments were carried at the Andreev`s Acoustical Institute, Moscow, Russia.

Read here the early development of metal-flaw detectors.

Some of Sokolov's early publications:

Sokolvo, S.Y. (1929) On the problem of the propagation of ultrasonic oscillations in various bodies. Elek. Nachr. Tech. 6:454-460.
Sokolvo, S.Y. (1935) Ultrasonic oscillations and their applications. Tech. Phys. 2:1-23.
Sokolov, S.Y. (1935) The practical utilization of the diffraction of light by ultrasonic waves. Z. Phys. 36:142-144.
Sokolov, S.Y. (1935) Ultrasonic methods of detecting internal flaws in metal articles. Zavodskaya Laoratoriya 4:1468-1473.
Sokolov, S.Y. (1937) Br. Pat Abstr. 477 139.
Sokolov, S.Y. (1941) Ultrasonic methods of studying the properties of quenched steel and detecting internal flaws in metallic articles. Zh. Tekh. Fiz. 11:160-169.
Sokolov, S.Y. (1946) The use of ultrasonic oscillations for observing physico-chemical processes. Zh. Tekh. Fiz. 16:784-790.
Sokolov, S.Y. (1948) Oscillations induced in a piezo-electric quartz rod by a non-uniform field. Z. Phys. 50:385-399.
Sokolvo, S.Y. (1948) The use of ultrasonics in technology and physics. Zavod. Lab. 11:1328-1335.
Sokolov, S.Y. (1949) The ultra-acoustic microscope. Zh. Tekh. Fiz. 19:271-273.
Sokolov, S.Y. (1949) The Ultrasonic microscope. Dokl. Akad. Nauk.SSSR 64:333-335.

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