My Science School

Dolby mentioned in tape recorders and other sound system actually refers to a noise reduction circuit used in them. Over the past 30 years, sound recording has been revolutionized by technical developments by Dolby labs founded by Ray. M. Dolby, a sound engineer. His first invention was a circuit that eliminates the noise, usually heard as a hiss, inherent in most tape recordings. Unlike the earlier antihiss technologies which distorted the quality of sound, Dolby’s method involved separating the acoustic components of a given sound into different electronic channels based on their frequency and amplitude, eliminating those signals that contributed most to noise and then recombining the other components. 

Sound is nothing but a form of energy. It travels in the form of waves. When these waves strike the wall the molecules or the atoms vibrate due to the sound energy.

The vibration passes from one atom onto its adjacent atom thus travelling vibration travels through the whole wall and when the wall ends the vibration in turn takes place in the air molecules.

Thus the sound travels through the wall and further into the air. Due to these sound created on one side of the wall can be heard on the other side. This phenomena proves that sound needs medium to travel. In vacuum the sound cannot travel because there are no atoms or molecules present to vibrate. In solids the atoms are packed together tightly than those of liquids and gases.

The vibrations need much less gap between the atoms in order to travel faster. Sound travels more intensely in solids. That is why audible sound passes through hard materials and be audible on the other side.

            Yes, Gravel used on railway tracks is known as ballast. It is used to dissipate the vibration produced by trains travelling at high speeds. In effect, the gravel layer acts as a cushion and damps the vibrations so that they do not travel long distances. If the rails are laid on a solid base, these vibrations can travel long distances and lead to cracks on the base as well as on nearby buildings.

            The ballast provides a foundation for the sleepers distributes the pressure on to the formation and helps in diverting the rain water. In typical ballast, voids constitute 20 to 30 percent of the volume.

 

  In radio sets, the problem of not receiving signals in certain directions is felt in the medium wave band (530-1620 kHz). This is because for this band they use a coil wound on a ferrite rod as the antenna.

  Ferrite rod is directional in its sensitivity. It picks up all signals that come in a direction perpendicular to its axis and rejects them if they come parallel to its axis. Therefore the reception is entirely based on the direction in which they reach the radio set. 

 Radio receivers use an inbuilt loop antenna for receiving radio frequency (RF) signals, which are nothing but electromagnetic signals, broadcast by transmitting stations. Some sets use telescopic antenna to receive frequency modulated (FM) signals.

Normally the dimensions of the loop are chosen to be comparable to the wavelength of the signals that will be received by the set. The dimensions of the loop are such that the current generated by the signal in the loop are of same magnitude and phase throughout the loop. If the phase of the loop is set at right angles (perpendicular) to the direction from which the vertically polarized waves are coming, then currents are induced in each of the vertical side antennas.

But these two antennas send currents around the loop in the opposite directions; hence, they cancel out each other as both the currents are equal in magnitude and opposite in phase. It is because during normal position of the loop antenna plane with respect to the incoming waves, both the sides are equidistant from the transmitter signal and there is no induced EMF. If the loop is rotated by 90 degrees, the plane of the loop will be along the direction of incoming radio waves, and so the voltages induced in the vertical side of the loop are not cancelled.

Because the distance involved between the vertical sides is equal to the width of the loop the voltage is maximum when the plane of the loop is along the direction of the incoming radio waves. External antennae help reduce this directional effect to a certain extent. 

         Many compact cameras and some SLR cameras have built-in-flash. The flash cannot be detached from these cameras and may result in “red-eye” that is, the pupils of a subject’s eyes appear red-in colour photographs.

            Red-eye occurs when light from the flash is reflected off the back of the retina. (Subjects pupils are dilated and the On-camera flash actually illuminates the blood vessels in the eye). The only way to avoid this is to close down the pupils (contract the iris) by raising the room lights, or using “red-eye reduction flash mode”.

            The feature is available in sophisticated compacts. This mode fires a pre-flash or a bright lamp prior to exposure, which makes the iris contract and close down the pupils, thereby reducing the effect. In compacts where there is no red-eye reduction feature, the following technique may be employed to reduce the red-eye effect. The flash light may be bounced off the ceiling or wall, before it reaches the subject.

            Engineers have exploited the human eye’s ‘persistence of vision’ to display pictures on television screens.

            The sets flash every frame at a rate of 50 times a second. (This is the frame or refresher rate).That is, each frame appears on the screen for about 20 milliseconds making us feel that the picture is continuous. To see the picture being refreshed on the screen, just wink your eye rapidly and look at a TV screen. A black bar dividing the picture into two should be visible. (This is the simple stroboscopic effect.)

            Cameras have typically a shutter speed of 1/100th of a second, that is, the shutter stays open for only 10 milli seconds. As this time interval is much smaller than the TV frame rate, the shutter does not give enough time for the picture to be displayed next time.

            Hence, the camera freezes the TV picture including the dividing bar and captures it in the film. If you carefully observe the scenes in movies, you will notice that any TV set in them displays the picture with a bar black slowly moving across the picture. To photograph from a TV, use an SLR camera and set the shutter speed to 1/8th of a second (125 ms) for flicker free pictures. (Theoretically, the shutter speed should be just below the frame rate). Use a tripod to avoid hand shake.