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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,
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
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