The motion picture projectionist (Nov 1931-Jan 1933)

20 Motion Picture Projectionist November, 1931 Some Aspects of Loudspeaker Development By W. L. WooLFf The third installment of Mr. Woolf's treatise on the subject of loudspeaker development is devoted to the tone chamber and its influence on the quality of the reproduced sound. A number of different types of tone chambers are considered and some interesting conclusions are drawn concerning them. The Motion Picture Projectionist is deeply indebted to Mr. Woolf for the masterly way in which he has presented his subject. — The Editor. Part III IN loud speakers of either the cone or horn type, a vibrator is actuated by a motor. The first article of tnis series dealt with motors and the second article with vibrators. This article deals with the construe z-\ Hw y9 # Fig. 1 tion of those parts of a loud speaker which affect the quality of sound after it leaves the diaphragm or vibrator and before it reaches the ear. It is entirely possible to select a motor capable of driving the diaphragm with satisfactorily uniform efficiency over the desired audible frequency range of say, from 85 to 8500 cycles per second and to employ this motor to drive a piston diaphragm of light weight and flexible periphery without receiving the desired results when interpreted as sound. It is necessary to employ such designs as will effect a satisfactory transfer of energy from the diaphragm to the surrounding air. Cone speakers, due to the size of the cone, in the process of vibration, obtain sufficient grip on the surrounding air to cause the surrounding air to vibrate in unison with the vibration of the cone. This is due to the large size of the cone itself. The disadvantage of the cone speaker lies in the excessive weight of the vibrating parts which absorb a great deal of energy and in the fact that with an increase in the size of any vibrating part, it becomes increasingly difficult to maintain the vibration of that part as a whole, and to avoid vibrations within the material itself which set up undesirable sound frequencies. Advantages of the Small Diaphragm The diaphragm of three or less inches in diameter can be made both much lighter than a cone and comparatively more rigid. The light weight of the diaphragm permits it to vibrate as a whole at much higher frequencies than the cone and its rigidity permits it to create a purer wave form. Inasmuch, however, as the diaphragm is small in size, it does not grip enough of the surrounding air to permit it to become a satisfactory loud speaker when vibrating in free air. It is therefore necessary to contribute to the diaphragm some means of loading it, in order that the kinetic energy which it possesses by virtue of its rapid vibration, may be transferred into the surrounding air, causing it also to vibrate in unison with the diaphragm. The most common and perhaps the oldest devices consisted of placing the diaphragm in an air tight tone chamber supplied with a horn. The oldest diaphragms were flat and the inner surface of the tone chamber was parallel to the diaphragm. Many investigators saw the necessity of an improved type of diaphragm without giving thought to the necessity of altering the tone chamber above the diaphragm in order to achieve the best results from the diaphragm and motor at hand. Tone Chamber Construction In Fig. 1, there is shown the usual fThe Amplion Products Corp. diaphragm in a tone chamber which leads out of the unit and into the throat of the horn. The horn is not shown. Vibrations of the lever 3 are transmitted to the diaphragm through the reed 4. This simple type of tone chamber possessed a number of disadvantages which require correction before the best results from loud speakers could be obtained. The length of the wave path from the center of the diaphragm to the throat of the horn is much shorter than the length of the path from the periphery of the diaphragm to the throat. An impulse traveling from the center of the diaphragm therefore reaches the throat of the horn at 2, before the impulse from the periphery at 7. Inasmuch as the wave length of the higher desired frequencies is approximately from one to two inches, a diaphragm with a one or two-inch radius could produce a train of waves which would reach the throat with variations in wave phase through a complete cycle, that is, an impulse from the periphery may reach the position 2, at the same time that the next succeeding impulse from the center of the diaphragm reaches the position 2, thus building up the strength of that particular frequency which results in the arrival of an impulse from the periphery and the center of the diaphragm in the same phase at the same time. A different wave length may be such that the arrival of the impulse from the center and the periphery would be exactly out of phase so that the impulse from one part of the diaphragm practically nullifies that from another part. In order to obviate this phenomenon, investigators have sought to balance out the lengths of the paths from the various elements of the diaphragm to the throat, in order that impulses from all parts of the diaphragm will arrive at the throat at the same instant and in the same phase. Another disadvantage of the type of tone chamber shown in Fig. 1, is the fact that a pressure wave emanat