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J. Perelman
"Entertaining physics". Book 2.
Chapter 10. Sound. Wave motion


Physics and technology of our days have the means to create a "silent sounds" much greater frequency than those which we are told: the number of oscillations can be achieved in these "Versuch" ("ultrasound") up to 10 000 000 000 in the second.

One way of obtaining ultrasonic vibrations is based on the property records in a certain way carved from quartz crystal, when compressed to elektrizuetsja on their surfaces (this property of crystals is called piezoelectricity.); if, on the contrary, charge periodically the surface of the plate, under the action of electric charges it is alternately compressed and expanded, i.e. ranges: obtained ultrasonic vibrations. Charge the same plate using commonly used in radio tube oscillator, whose frequency is selected in accordance with the so-called "own" the period of oscillation of the plate *.

Although ultrasounds are silent to us, they find its effect in other, very tangible manifestations. If, for example, an oscillating plate immersed in a container of oil on the surface of the liquid, covered by ultrasonic vibrations, bubbled slide 10 cm height, and oil droplets razbrasivayutsya to a height of 40 cm Plunging into such an oil bath the end of a glass tube about a meter long, we feel in the hand holding the other end, burned, leaving marks on your skin. Contact with the tree, the end of the tube in a state of fluctuation, burns a hole; the ultrasonic energy is converted into heat.

Ultrasounds are scrutinized Soviet and foreign researchers. These fluctuations have a strong effect on a living organism: thread algae torn, animal cells burst, blood cells are destroyed; small fish and frogs wordplays sverkhzvukovye in 1 - 2 minutes, the body temperature of animals increases, the mouse, for example, up to 45°C. Ultrasonic vibrations are applied in medicine; inaudible ultrasounds share the fate of the invisible ultraviolet rays, coming to the aid healing.

Especially successfully applied ultrasounds in metallurgy for the detection of discontinuities, holes, cracks and other defects in the bulk of the metal. The method of "scanning" metal ultrasound is that the tested metal moisten with oil and subjected to the action of ultrasonic vibrations. The sound is dispersed heterogeneous metal parts that are cast as if the sound shadow; outline inhomogeneities so clearly emerges on the background of the uniform ripples covering the oil layer, the resulting picture can even shoot the **.

"Enlighten" ultrasound can metal thickness in a meter or more, which is completely inaccessible to x-rays; thus detected heterogeneity is very small - one millimeter. There is no doubt that before the supersonic vibrations greater prospect ***.

* Quartz crystals are expensive and low-power source of ultrasound and are used frequently in the laboratory. Technical application found artificial synthetic materials, such as ceramics of barium titanate. (Approx. as amended).
** Method of ultrasonic flaw detection (detection of defects) was proposed in 1928, the Soviet scientist S. I. Sokolov. Now apply special receivers of ultrasonic vibrations, which can replace oil and make measuring easier. (Approx. as amended).
*** It is interesting to note that ultrasound is found in nature. In the noise of the wind and the surf has a frequency corresponding to the field of ultrasound. The ability to emit and receive ultrasound have many living beings (butterflies, cicadas and other). Bats use ultrasound in flight, learning reflected signals about the obstacles in its path. (Approx. as amended).

Entertaining physics J. Perelman


System Orphus


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