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(1) In acoustics, a region (acoustical shadowed zone) in which the sound of remote powerful sources, such as artillery fire and explosions, cannot be heard, although it is heard again at greater distances (in the “anomalous audibility zone”). On the surface of the earth blind spots usually have the shape of an irregular ring surrounding the sound source. Two or even three blind spots separated by anomalous audibility zones are sometimes observed. The inner radius of the first blind spot is usually 20–80 km and occasionally reaches 150 km; the outer radius may be as great as 150–400 km.
The refraction of sound in the atmosphere is the reason for the formation of blind spots. Since the temperature in the lower layers of the atmosphere decreases with altitude (to minus 50°-75°C at an altitude of 15–20 km), sound beams are deflected upward, moving away from the surface of the earth. At an altitude of 40–60 km the temperature rises once again (to 0°-30°C), the beams are bent downward and, passing over the blind spot, return to the surface of the earth, thus forming an anomalous audibility zone. The second and third anomalous audibility zones occur as a result of the first and second reflections of the sound beams from the surface of the earth. The wind changes the shape of sound beams, which may lead to significant distortion of the annular shape of the blind spot and may even break the ring. The study of the anomalous propagation of sound is one method of determining temperatures in the middle atmosphere. A similar phenomenon is often observed during the propagation of sound or ultrasound at sea.
REFERENCESArabadzhi, V. I. “Zvuk zondiruet atmosferu.” Priroda, 1968, no. 5, pp. 78–82.
Khrgian, A. Kh. Fizika atmosfery. Leningrad, 1969. Section 74.
Tolstoi, I., and K. S. Klei. Akustika okeana. Moscow, 1969. Chapter 5.
the site of exit on the optic nerve from the retina. The blind spot is situated in the fundus of the eye below the region of maximum visual acuity. Individual nerve fibers join to form the optic nerve near the blind spot, which does not have photosensitive elements and therefore does not perceive light stimuli. A circular ridge, called the optic disk, is formed where the fibers converge in the fundus of the eye. The optic disk surrounds a depression from whose temporal side retinal blood vessels grow into the eye. Optic-nerve bundles exit from the eye through perforations in the optic foramen of sclera, that is, the portion of sclera in the region of the blind spot.
blind spot[′blīnd ‚spät]
ii. An area on the airport not visible from the control tower.
iii. Any part of the aircraft that obstructs the sight of the pilot or crew.