Exhibitors Herald and Moving Picture World (Apr-Jun 1930)

April 12, 1930 EXHIBITORS HERALD-WORLD 59 FIGURE 10 mitted to flow in the coil, there will be a resultant force between the coil and the magnetic field. The magnitude of this force will be a function of the rate at which the current in the coil is changing, assuming, of course, that the surrounding magnetic field remains constant. As the varying speech current is applied to the speech coil which is surrounded by the magnetic field, the coil will tend to move in and out in direct proportion to the variations of the speech current. If this coil is rigidly attached to a diaphragm, the diaphragm will likewise be set into vibration, producing an acoustic effect on the surrounding air. The receiver used with the Western Electric system is known as 555-W. It consists of a duralumin diaphragm. The diaphragm is made of a single piece of sheet aluminum alloy 0.002 inches thick. This diaphragm (A) is shown in Fig. io. To it is rigidly mounted a flat coil (B) of aluminum wire or ribbon 0.015 inches wide and 0.0002 inches thick, wound on edge. A thin coat of lacquer serves to insulate the turns from each other. It is the speech current circulating in this coil interacting with the magnetic field which forces the diaphragm in and out. The receiver has been so constructed that the diaphragm vibrates to and fro as nearly like a rigid plunger as possible. An ordinary flat piece of metal clamped around a circular edge assumes a dome shape which vibrates at low frequencies. The diaphragm can be made to vibrate with its center part essentially unflexed by adopting the shape which makes it less rigid near the edge and more rigid toward the center and then applying the force uniformly around the outside of the central portion. The coil is made mechanically selfsupporting; it is rigid and very light. The speech coil radiates heat readily, which permits a large power input without overheating. Because of the small distributed capacity and the small number of turns, the impedance of the coil is a resistance approximately constant with frequency. This involves a fundamental principle of alternating current which states that the impedance varies directly as the voltage and inversely as the current. The term impedance is the resultant of three factors— resistance, capacity and inductance. Capacity and inductance are factors encountered in alternating current circuits which are not encountered in direct current circuits. Aluminum was selected instead of copper for the speech-coil construction because of its smaller weight yielding less mass-reactance at high frequencies. The outstanding feature of this type of receiver is the high efficiency with which it converts electric energy into acoustic energy. In experimental models efficiencies as high as 50 per cent have been realized. When you consider that a receiver of 100 per cent efficiency would result in an increased sound intensity of three decibels, which is only a comfortably perceptible difference, it is not likely that much economy would be gained from higher efficiencies. This type of receiver, when used, is attached to a horn which partially isolates a column of air from the surrounding medium. This column of air affords an acoustic coupling between the receiver and the space in which the sound is to be reproduced. The horn is designed in such a way as to avoid interference between air waves as they pass through the chamber and the throat of the horn. The horn used is shown in Fig.n ; its design and construction is referred to technically as “exponential,” which qualifies its shape. MAINTAINING SYNCHRONISM Synchronization between sound record and photographic record is an inherent requirement of sound pictures. In projection this is usually accomplished by mechanically coupling the picture projection machine with the sound recorder. Synchronization in itself is not sufficient, however, for there is still the problem of speed control. Musical pitch varies directly as the rate of vibration or frequency varies. It, therefore, becomes necessary in reproducing music with fidelity of pitch that the sound record be run at identically the same speed as the record was made. To accomplish this some reproducing systems make use of a synchronous motor or certain types of induction motors whose speed characteristics are inherently constant under certain given conditions. However, load variations, frequency variations and voltage variations will produce noticeable variations in pitch of the reproduced music or speech. A trained musical ear will detect certain changes in pitch produced by one-half of one per cent speed change. To further insure against such discernible changes in pitch, the Western Electric system makes use of a device known as a motor control box which maintains a motor speed regulation of twotenths of one per cent, despite the ordinary variations of power supplies. In the design of this motor control system an entirely new principle has been devised — no known governing device being applicable. There is incorporated in this control mechanism many of the electrical principles involved in sound recording and reproducing as well as several additional principles. The motor control box is quite elaborate and need not be described in detail for the purposes of this paper. A brief statement on the bridge circuit is given below: Fig. 12 shows the governing system. Across the bridge circuit is a potential which shifts in phase 180 degrees as the speed changes from any value below 1,200 r.p.m. to any value above it. This circuit makes use of the principle of electrical resonance and is operated by the output of a 720 cycle generator which is directly connected to the shaft of the main driving motor. One section of the bridge has a fixed inductance and condenser in series and is adjusted to become resonant at 720 cycles. In the section of the circuit labelled “D” is a resistance which is equal to the impedance of the resonant circuit at 720 cycles. Across the transformer section of the circuit there is maintained an equal potential. For any speed condition other than 720 cycles or 1,200 r.p.m. there is a potential across the bridge and, as has been said before, this potentialshi fts 180 degrees as the speed changes from less than 1,200 r.p.m. to a value greater than 1,200 r.p.m. This change involves the fundamental principles of alternating current circuits, i.e., the impedance of a condenser varies inversely with the frequency and the impedance of an inductance varies directly with the frequency. These values of impedance being the reciprocal of one another, their effects will cancel each other at the frequency for which the circuit is tuned, which is in the case of this circuit 720 cycles. Fig. 12 also shows the effect of this bridge circuit variation on the remainder of the circuit which produces speed changes in the driving motor. The link between the bridge and the three legged inductance circuit is a vacuum tube circuit which causes more direct current to flow as the motor speed tends to fall. This is accomplished by means of a detector tube and two tubes which supply current from the middle winding of the magnetic impedance coil. In the circuit for the alternating current motor control box there is a rectifier tube, the function of which is to supply exciting current for the 720 cycle alternator and grid biasing voltage for all tubes in the circuit. The magnetic modulator or variable reactor which links the motor and the control circuit involves the fundamental electro-magnetic principle of saturation which permits a method of changing the impedance of the motor rotor circuit. As the direct current in the center section of the variable reactor is varied, the impedance of the rotor circuit connected to the outer sections of the reactor is varied directly. The speed of the motor varies with the impedance of the rotor. VOLUME CONTROL It is necessary to have some means of varying sound levels in theatres, first because of the variation in sound energy requirements in theatres, second because of the variation ir levels of recorded sound, third because of the variation in the size of the audiences and fourth the desirability of level control during reproduction for the purpose of emphasis. Most reproducing systems make use of two means of controlling sound levels. One is an attenuating apparatus known as a fader which is usually electrically placed between the reproducer and the gain amplifier. By varying the amount of impedance between these two elements of the circuit, the volume may be either raised or lowered. The fader is also used as a means of changing from one machine to another. Another method of controlling the volume is by means of the gain control in connection with the gain amplifier which was pointed out in the discussion on amplifiers. In a reproducing system there are many ways of changing a level, particularly in a film reproducing system, such as varying the exciting voltage of the photo-electric cell, varying the intensity of the exciting lamp, or varying the exciting current of the loud speakers. However, these methods are seldom resorted to and are not considered means of volume control. For some time there has been considerable agitation to eliminate the use of the fader as a means of volume control. However, at the present time it still seems advisable to have some ready means of controlling the volume of a reproducing system. [Article to be concluded next week]