Category Science & Technology

          The sounds we hear with our two ears are known as stereophonic sound because they give the exact idea of angular and lateral position of the sound source.

          The sound signals reaching one ear are generally slightly different from those reaching the other. Their arrival times and intensities are also slightly different. Our brain is able to distinguish the differences in intensity and arrival time of sound waves at each ear. In fact, it can discriminate arrival time differences even as small as less than 1 milli second. If a pair of microphones is placed in front of a sound source, it will receive sounds with differing intensities and arrival times depending upon the position of the source relative to each microphone. When these separate, sounds are reproduced by a pair of loudspeakers, the listener’s brain is able to use the reproduced time and intensity differences to locate the original sound. Such sounds localized in space by the brain are called phantom images. The ability of the listener to perceive phantom images is called stereophonic sound. Thus with our two ears, we are able to locate exactly both the angular and lateral positions of sound. The listener feels that he is actually present at the place of performance.

          Stereophonic sound recording and reproduction requires two or more independent channels of information. It has been observed experimentally that a minimum of two sets of microphones and loudspeakers give satisfactory auditory perspective. Separate microphones are used in recording, and separate speakers in reproduction.

          At the time of a stereo-recording two microphones are used, one of which receives more sound from the left, and the other from the right. The sounds detected by each are kept entirely separate and are encoded in two completely independent channels of the programmes. Stereo-production needs two separate loudspeakers.

          There are three basic techniques for stereophonic sound pick-up; coincident, ‘spaced apart’ and ‘individual instrument’ or close miking. The coincident technique employs two microphones located very close together. In ‘spaced apart’ technique, microphones are placed several feet apart, ‘close miking’ technique involves use of several microphones, and each located close to one instrument. The outputs are recorded on tape. The reproduction loudspeakers should be identical and capable of broad-frequency response without distortion.

          The effectiveness of stereophonic reproduction was demonstrated as early as 1933. Two track stereophonic tapes for domestic use became popular in the 1950s and single groove two channel stereo-discs in 1958. In the early 1970s quadraphonic system, employing four independent channels of information, became commercially available.

          Since long, man has used clocks and watches to measure time. But those were crude watches and didn’t measure time accurately. A few years ago, scientists were able to develop a very sophisticated clock known as ‘Atomic Clock’. With its development a new era has been ushered in the field of time measurement. It is a wonder clock that remains accurate to one second for 1,700,000 years.

          Today we have mainly three types of clocks and watches: mechanical, electrical and electronic. Mechanical clocks and watches are spring driven; electric clocks are battery powered and the electronic ones are quartz based. All these clocks and watches show time quite accurately. But if they run continuously for long periods, they can get slow or fast.

          Now the smallest internationally accepted unit of time is the atomic second. It is based on atomic clock, and defined as the time interval during which exactly 9192631770 cycles of the hyperfine resonance frequency of the ground state of the caesium atom occur. Prior to this the second was the standard of time which was measured as a portion of earth’s rotation as 1/86400th of a day. 

          An atomic clock uses the frequencies produced by atoms or molecules. The time is measured by counting the number of vibrations. Most of the atomic clocks make use of frequencies in the microwave range from about 1400 to 40,000 MHz

          In 1947, an oscillator controlled by frequencies of ammonia molecule was constructed. An ammonia controlled clock was built in 1949 at the National Bureau of Standards, Washington D.C.

In 1955, a caesium-beam atomic clock of high precision was first put in operation at the National Physics Laboratory, Teddington, England. After that a number of laboratories started producing commercial models of caesium-beam atomic clocks.

          In the caesium clock, the caesium is heated in a small oven. The caesium produces a beam which is directed through an electromagnetic field. The 5 MHz output from a quartz clock is multiplied to give 9192631770 Hz that controls the electromagnetic field. Part of the 5 MHz output is used to derive a clock display unit which indicates time.

          During recent years, some other atomic clocks have also been developed which make use of ammonia maser, hydrogen maser and rubidium gas cells. Atomic clocks of 1960s were very large in size but by 1978 their sizes have been sufficiently reduced to fit in a small box.

          Atomic clocks are being used as standard of time. They are also being used in some sophisticated navigation systems and deep space communications. 

               A projector is an optical instrument that shows on screen, enlarged pictures of slides or movies. Do you know how does this instrument work?

               The projector in its simplest form consists of (i) a light source (ii) a concave reflector that focuses light (iii) a condenser lens and (iv) a projector lens. A powerful light source is needed to project images on to a screen. Most projectors use an incandescent ribbon lamp of 1000 watts. A highly polished concave reflector is placed at the back of the light source so that practically, the entire light is reflected towards the slide. The light so reflected is allowed to fall on a condenser or focusing lens. This lens is a combination of two planoconvex lenses, placed in such a position that their convex surfaces face each other. The condenser lens converge the divergent beam of the light, and throws it on the slide. The condenser lens helps to strongly illuminate the image. The concentrated rays then pass through the photographic slide or film that is placed upside down in a frame. The final or projector lens is a convex lens and is kept near the slide. It reverses and enlarges the picture of the slide and throws it on to the white opaque screen. The slide shown is systematically removed by the touch of a button and replaced by a new one. Slide projectors are also used by teachers and business people to illustrate subjects under discussion.

               Movie projectors have electrically powered reels that move the film between the bulb and projecting lens at a speed of 32 films per second, so that images appear continuous to the eyes. Sprockets in the projector pull the film into the film gate. The film then stops for a moment and light from the lamp passes through the frame. The lens projects the picture on the screen. The sprockets then turn and advance the film. As the film moves, the blade of a rotating shutter passes between the lamp and the film so that the movement of the film does not show on the screen.

               In sound film, light from the lamp passes through the sound track and strikes a light sensitive cell which produces an electric signal. It goes to an amplifier and loud speaker which provide the sound. In some cases, the sound is recorded on a magnetic strip along the film as in a video recording.