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The science of sound, or acoustics, as it is often called, has been made over radically within a comparatively short space of time. Not so long ago the lectures on sound in colleges and high schools dealt chiefly with the vibrations of such things as the air columns in organ pipes. Nowadays, however, thanks chiefly to a number of electronic instruments engineers can study sounds as effectively 1 . The result has been a new approach to research in sound. Scientists have been able to make far-reaching discoveries in many fields of acoustics 2 .
Foremost among the instruments that have revolutionized the study of acoustics are electronic sound-level meters also known as sound meters and sound-intensity meters. These are effective devices that first convert sound waves into weak electric signals, then amplify the signals through electronic means 3 . The intensity of a sound is measured in units called decibels. “Zero" sound is the faintest sound 4 . The decibel measures the ratio of the intensity of a given sound to the standard "zero" sound. The decibel scale ranges from 0 to 130. An intensity of 130 decibels is perceived not only as a sound, but also 5 . The normal range of painlessly audible sounds for the average human ear is about 120 decibels. For forms of life other than ourselves, the range can be quite different.
The ordinary sound meter measures the intensity of a given sound, rather than its actual loudness. Under most conditions, however, it is a quite good indicator of loudness. Probably the loudest known noise ever heard by human ears was that of the explosive eruption in August, 1883, of the volcano of Krakatoa in the East Indies. No electronic sound meters, of course, were in existence then, but physicists estimate that the sound at its source must have had an intensity of 190 decibels, 6 .

A. and finally measure them.
B. since it was heard 3,000 miles away.
C. and they have been able to put many of these discoveries to practical use.
D. that loud sound is of high intensity.
E. as they study mechanical forces.
F. as a painful sensation in the ear.
G. that the unaided human ear can detect.



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