Electronic Computers

The first electronic computers appeared in 1945. In the course of (в течение) some dozens of years mathematicians learned to solve problems of great complexity (сложность) using these machines. Electronic computers can be classified into two general types: general-purpose computers and special-purpose machines.

General-purpose computers are electronic machines that do all types of arithmetical computation - add, subtract, multiply, devide, square root (извлекать кв. корень). In addition, general-purpose computers are capable of (capable of - способный что-л. делать) performing a large number of other operations. A general-purpose computer san be regarded (to regard - считать, рассматривать) as a data-processing center. From a machine point of view, a general-purpose computer is a combination of devices. From the functional point of view, it is a system that permits the user to process data for a large number of purposes. General-purpose computers can be further classified according to size and the speed of computation.

Special-purpose electronic computers are limited either to the type of computations they can make or (either.. .or - или.. .или; либо.. .либо) as to the functions they can perform.

Electronic computers cannot replace the judgement (зд. способность мыслить) of the human brain, but they will release (to release - освобождать) it from mechanical functions.

Sound

Everything that is heard is a sound. The fact that something is heard establishes (to establish - устанавливать) the that there is sound present. However, this does not mean that when nothing is heard there is no Sound, because there are many sounds that are beyond the range of human ear.

For a sound to be heard by the human ear it should be between the frequencies of approximately (приблизительно) 20 cycles and 15,000 cycles, the former (первое) value representing the lower limit and the latter (второе) value the upper limit of the hearing limit of the average (средний) person.

However, some people are known to hear sounds up to 18,000 cycles and even as high as (зд. до...) 20,000 cycles. These people hear sounds that the average person does not notice. It also has been observed that dogs, cats, and other animals can easily hear sounds of 20,000 cycles.

For the sound to arise (to arise - возникать) it is necessary to have a sound source and a medium (среда) to travel (распространяться, проходить (о звуке, сигнале, волне)) through. Any type of a physical body may give rise to (вызывать, быть причиной (источником появления чего- л.)) sound if it vibrates back and forth (совершает колебательные движения назад и вперед) thus causing (вызывая приводя к ) a disturbance (возмущение) in a material around it, no matter (независимо от того...) whether that material is a liquid, a solid, or a gas. The speed at which the sound waves travel depends upon the material (the medium) they travel through. In air, sound waves travel at about 1,100 фа; in water, at about 4,800 fps; and in steel, at about 16,500 fps.