International projectionist (Jan 1943-Dec 1944)

Review Of Projection Fundamentals VI. — Vibrations New technical problems will unavoidably be imposed on the projectionist by war conditions. At the same time, he will want to prepare himself for the technical surprises sure to appear when the ivar ends. In the conviction that our readers will consider the present an ideal time to review their knowledge of fundamentals, IP here presents the sixth of a series of articles dealing with the bases of electricity, optics, sound and other foundations of projection room technique. VIBRATIONS OF many kinds, including sound, electrical and light vibrations, are among the most important business of the projectionist. A vibration is a periodic or oscillatory motion. It is a motion that repeats itself over and over through the same path. Thus, if a projector head vibrates because it is worn or out of adjustment, each single particle of material in that mechanism may move upward, for example, l/100th inch, then downward l/100th inch, then upward 1/ 100th inch, etc. But the vibration in the projector head may not be only up and down — the head and each separate particle of matter in it may simultaneously be vibrating forward and back also, and from side to side also. In that case the head, and each particle of its matter, would follow a vibratory path constituting a complex closed curve, not simply an up-anddown straight line — none the less, if the path were followed regularly so many times per second the motion would be a steady vibration. It is convenient and helpful, and simplifies matters to think of and treat vibrations in terms of waves. A wave in water is simply a vibration; it consists (as a cork floating on the water will show) of a regular, periodic up-and-down motion of the water molecules. Although the water may appear to move laterally also, this is an optical illusion ; the cork shows that while the wave or vibration moves latterly, the water does not but simply heaves up and down; in fact, if there is a current opposed to the waves the cork — and therefore the surface of the water — may be moving in one direction while the waves — the vibration, not the water — are moving in a different direction. Similarly with sound waves, which are vibrations in air — the air does not move throughout the theatre from the sound source at the loudspeaker. If it did, operation of the loudspeakers at high volume would create a considerable draft. It is the vibration that moves through the air, spreading from the speakers to every part of the auditorium. This particular vibration needs a little further attention, for it is different in kind from the vibration represented by a common wave in water, or by a projector head out of adjustment. There are two different kinds of waves — transverse and compressional. An ordinary wave in water is of the transverse type. Consider the bobbing cork, or a single molecule of water, in relation to the wave as a whole. The cork, the water molecule, move up and down — the wave moves or spreads laterally. That is, the direction in which the vibration moves is at right angles to the direction of motion of the individual particles of the matter undergoing vibration. The transverse wave can be represented quite neatly by tying one end of a rope to a wall; and, holding the other end in one hand without stretching the rope too tight, by moving the wrist up and down. A wave will move along the rope from hand to wall. The particles of the rope are moving up and down. The wave or vibration is progressing at right angles to that direction, along the rope to the wall. To represent a compressional wave, substitute a long spiral spring for the rope, and move the hand forward and back. A wave or region of compression, followed by a region of expansion, can be made to travel along the spring from hand to wall. In this case, however, the individual particles of the spring are moving in the same path as the wave. They are not moving at right angles to it. Polarized Waves Many of the vibrations the projectionist encounters in his work are in a sense combinations of compressional and transverse waves. Return again to consideration of the loudspeaker. The diaphragm drives forward, compressing or pushing together the molecules of air immediately in front of it. These, in turn, compress or push against the molecules in front of them. These molecules repeat the process and thus a series is set up which eventually pushes against the eardrum of the listener. But meanwhile, each molecule, having collided with the molecule just ahead of it, rebounds; thus each individual molecule moves through the same path as the wave as a whole, and to that extent the vibration is compressional. But that is not all there is to it. You don't have to stand in front of the loudspeaker to hear its sound. You can stand alongside of it and hear it. Obviously all the molecules don't push straight ahead and rebound straight back. Some strike or are struck glancing blows, an i are driven, or rebound, sideways. A compressional wave, in a fluid such as air or water, spreads more or less in all directions (less, if it is of high frequency). Since the vibration spreads in all directions, all ihe molecules of the air are not moving solely in the same path as the vibration. Some are moving at right angles to it, or at various angles to it. In fact, many of the molecules are somewhat like the individual particles in the badly vibrating projector head — simultaneously moving up-anddown, forward-and-back, and from leftto-right: following a small but complex curved path. Light, Heat, Electricity Light, although some of its details are still a mystery, presents many evidences of being vibratory in nature, with the vibration normally proceeding in all directions at once. Of course this does not mean that a beam of light cannot be concentrated or focussed. Sound, which is a similar kind of vibration, can be concentrated into beams by suitable trumpets. One of the faults of oldertype loudspeakers was that they concentrated too much sound into too narrow beams. But in light, as in sound, individual particles of the "ether" or whatever it is that vibrates, vibrate in all directions, with the direction of the wave and at all angles to it. Polaroid filters illustrate this fact beautifully, for they hold back and refuse to pass all vibratory motion except that proceeding in certain directions. This can be understood by further reference to the sound wave. The sound molecules vibrate in three general directions— up and down, right and left, back and forth. Suppose the mean free path of a molecule— the distance it can move in any direction before it collides with the next molecule and rebounds — to be l/l,000th of an inch — to take an arbitrary figure. Then if you were to place in the path of the sound a large baffle board containing a slit l/2,000th inch high, the left-toright and forward-and-back vibrations could still proceed, but up-and-down vibration, or any molecular motion that included a large component of up-anddown vibration, would be stopped. A 16 INTERNATIONAL PROJECTIONIST