International projectionist (Oct 1931-Sept 1933)

SOUND SYSTEM NO BETTER THAN HORN AND RECEIVER UNITS IN order to obtain efficiency, a sound reproducing system as a whole must exhibit an even response to all frequencies within the audible range. In no element of the system is this need more pressing than in the speaker, or horn, units, which must meet such requirements to enable the whole system to produce the desired results. The design of receivers and horns is a matter which has received a considerable amount of attention at the Bell Laboratories, and the result of these attentions is found in the 555-W receiver and the 16 or 17-type horns. What Is a Receiver? The receiver is a unit which is employed to convert electrical energy into sound energy. The electrical energy is in the form of an alternating current, while sound is a degree of mechanical energy in the form of compressions and rarefactions in the air. It is a simple matter to produce sound energy by moving a diaphragm. It is known now that a conductor carrying a current will tend to move if placed in a magnetic field. Therefore, a coil carrying the electrical energy or speech currents is fixed to the diaphragm and situated in the magnetic field between the pole pieces of a field winding. As the speech current is alternating, the movements of the diaphragm will be of an alternating nature. In addition, the frequency of movement of the diaphragm will correspond exactly with the frequency of the speech current, and its displacement will be proportional to the value of this speech current. Thus it is possible to convert, faithfully, the electrical energy into a mechanical form. Number of Receivers A theatre installation contains one or more receivers depending upon its size. There may be one or two receivers per horn, according to whether a 17-A, 17-B, 16-A or 16-B horn is used, which, in turn, depends upon the size of the installation. The 555-W receiver has a diaphragm made of duralumin 0.002 inch thick. This diaphragm has its central portion eupped, which gives it considerable stiffness and causes it to move against the air column with a motion similar to that of a plunger. An ordinary flat diaphragm flexes throughout, and the amount of movement is much greater at the center J. Henry Schroeder than nearer the edge. The diaphragm of the 555-W receiver flexes only near its outer edge, giving considerably better results than the flat type. The speech winding comprises a coil of edgewise wound aluminum ribbon attached to the diaphragm, and situated between the pole tips of the field winding. The field winding derives its power from the "H" batteries, consuming 1.5 amps at 7 volts. The speech winding has a high carrying capacity in comparison to its weight, due to its single layer construction and small amount of insulating material, resulting in a high rate of heat dissipation. The Tone Chamber Immediately in front of the diaphragm is an air space or tone chamber which has to be very carefully designed. The sound radiating efficiency of the horn type of loud speaker may be limited due to interference between air waves as they pass through the chamber between the diaphragm and the throat of the horn. In many types of loud speaker, the dimensions of this chamber are comparable with wave lengths of sound within the audible range. To avoid this, and the resultant irregularities in the frequency response, the tone chamber in the 555-W receiver is constructed so that no serious distortion of sound can occur within the useful range of the wave length. An exceptionally high efficiency is obtained by using this specially constructed tone chamber and the plunger-like motion of the diaphragm. The efficiency of the unit, for converting power from that of electricity to that of sound, is as high as 50 per cent. When this is compared with the one per cent efficiency of the Drawing showing components of W. E. 555-r receiver [12] average loud speaker, the advances made in this field can be readily appreciated. Importance of Connections It is very important to make sure that the connections on all the receivers are made the same way. If a pair of wires are reversed on a receiver, it will mean that either the excitation current or the speech current will flow in the opposite direction in that receiver relative to the others. This will mean that the diaphragm of the wrongly wired receiver will be moving in the opposite direction to the diaphragms of the other receivers. In other words, when one diaphragm is compressing the air, the other is creating a vacuum, with the result that one effect neutralizes the other. If the two receivers of one horn are wired in opposition, theoretically, no sound will be heard from the horn. Such an occurrence, even if the receivers are on different horns, will result in the sound distribution being ruined. Some Don'ts If for any reason, such as a blown fuse, there is no field current, very faint sound will be heard. This is due to the residual magnetism left in the poles of the field. Never operate a receiver except in connection with a horn. The horn provides a long column of air which acts as a load on the diaphragm and prevents it from vibrating so violently as to burst, as it would do if used without a horn. In tliis respect, never raise the fader all out in order to play an exit march through the curtains, or increase the volume for gun shots, etc. Such practice will sooner or later — usually sooner — result in broken diaphragms. Purpose of the Horn The purpose of the horn is to afford a coupling between the receiver unit and the outside air, causing the sound energy, produced at the unit, to be properly radiated into the theatre. The diaphragm of the receiver will vibrate to the point of self-destruction without delivering any real amount of sound energy, and it is only by loading it with an air column that we can obtain the desired results. The air column must be small in area at the input end to place an appreciable pressure load on the diaphragm, and it must be large at the other end to radiate