Abstract
One of the basic phenomena of the ultrasonic defectoscopy is the reflection of ultrasonic energy from air enclosures which may be penetrated by ultrasonic waves only if the air layer is exceedingly thin. According to a well-known formula derived by Rayleigh which is adopted and referred to by all well-known handbooks, an ultrasonic beam of, e.g., 2.5 Mcps frequency traveling in glass should be reflected nearly completely by an air layer of thickness of 10−5 mm or more. This is not, however, in conformity with experimental results to be given below. Experiments were carried out with high-polished glass plates. The thickness of the air layer between the glass plates was determined by optical means with interference fringes. It has been found that the ultrasonic beam of 2.5 Mcps frequency penetrated without energy loss through an air layer even of 4 × 10−4 mm thickness. The investigations at several frequencies and with different thicknesses displayed a difference of two orders of magnitude between the penetrating thresholds found experimentally and those computed from the Rayleigh formula. Air layers of about 10−4 mm thickness between glass plates reflect no ultrasonic energy in the frequency range of 0.5 to 2.5 Mcps.
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