Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, June, 1930 Page 13 of a condenser receiver for a telephone line. Various workers introduced the essential ideas of (a) polarizing or direct super-imposed operating potential; (b) perforated electrode plate to permit access of air; (c) various operating circuits applicable to vacuum tube connections ; and ( d ) refinements in mechanical structure. Probably the most satisfactory use of acoustic condensers has heretofore been in the form of condenser microphones as rendered practical by vacuum tube amplifier circuits. The improved acoustic condenser, comprises essentially, two flexible electrode diaphragms, supported closely adjacent to each other with a definitely regulated air-gap of variable dimensions between. One of the electrode diaphragm elements is made air-permeable and usually takes the form of a metal "cloth" carrying flexible soft metal strips combined therewith. The other diaphragm element comprises an insulation treated flexible cloth to which a very thin metal foil surface is flexibly fastened. The need for flexibility is emphasized and may be attained by the patented method wherein a special wax is utilized to fasten the foil to the dielectric cloth. To define a definite range of air-gap spacing between the two electrode members, a series of spaced threads is customarily interposed between the two members. These threads are not only flexible but also serve to eliminate "back-lash" effect, previously a serious fault in condenser reproducers operated at large volume of response. Instead of permitting elasticity to return the electrode members with a rustling sound, the threads break the transient vacuum and compression of the air, and there is not a rustle left to irritate the listener's ears. A further advantage attained by this departure in structure is found in the fact that, when a polarizing voltage together with an impressed audio-frequency voltage supply is connected to the bi-lateral flexible electrode diaphragms, each electrode diaphragm is actuated in correct phase so that for all the medium and larger sizes of such condenser reproducers, no baffling of any sort is necessary in order to effectively radiate sound waves of low frequency. The air is simultaneously pushed out, for example, from both the face and the back of the acoustic condenser. FRONT BACK ACOUSTIC CONDENSER FIG.3 Plane Wave Front Propagation Instead of attenuating rapidly with distance from the source, the substantially plane wave-front radiation from this acoustic condenser reproducer travels outwardly to the listeners so that the observed intensity of sound is almost identical either nearby or as far away as fifty feet from the speaker. Due to actuation of air movement on both sides of the condenser speaker of this type, there are no actual dead sound radiating angles about the speaker, though most of the sound energy is caused to progress in a plane wave-front, bending outwardly at a narrow angle. This means that in the case of a large auditorium, efficient sound projection may be had without waste of sound energy up to the ceiling or down to the floor to cause undesired echo effects. In a small living room, the observed effect is that the sounds appear to have a non-directional source, exactly as if coming from the room itself. How Sensitive Response is Attained The fact that even a single 171 tube output affords large volume of response with this acoustic condenser when properly connected thereto is an indication of its comparative sensitivity of response. In the medium sizes, exactly as loud volume may be had as can be obtained from dynamic speakers operated from the same output, while in the larger sizes the acoustic condensers afford greater volume of response than can be had from other forms of reproducers from the same amplifier output. This is due in part to the relatively large volume of air moved at a time by the large working areas of the condenser reproducer. The sensitivity is attained by a combination of multiple plunger action and large diaphragm excitation. To comprehend this, one must first consider that it was at one time supposed that the diaphragm of a condenser speaker must be stretched very taut, not only to keep the diaphragm from touching the fixed electrode but also to avoid resonance distortions in the desired audio-frequency range of the speaker. This view led to endless mechanical difficulties, as few if any, materials will retain permanently a great radial tension without an elastic fatigue yield. Another idea was that the condenser reproducer should be arranged in the form of a multiplicity of plungers or miniature diaphragms operated in the same phase. Fairly satisfactory speakers of this type were made but were in all cases limited too narrowly in frequency response range ; further limited in respect to their relatively poor sensitivity, and were limited further by the inordinately large size required for satisfactory loudness of response. In the bi-lateral flexible electrode structure of the improved condenser, however, such limitations as well as the loss of sound energy im 5 MFD. OUTPUT (SPEAKERS) FROM OUTPUT TRANSF. OF AMPLIFIER FIG. 4 posed by the material area of the fixed rigid plate or plates are avoided. The air that is moved, is moved in phase, firstly as a multiple piston action but secondly, and more importantly, accompanied by a diaphragm action of large area extent excited by the first action. The necessity for "in phase" excitation for simultaneous movement of all areas of the diaphragms is obvious in order to avoid interferences or different rates of travel of the sound energies over different portions of each diaphragm. This combined diaphragm energization affords very marked improvement in volume of response from the same excitation energy. In effect the diaphragm is rolled or undulated over a series of flexible parallel ridge supports and moves the nearby air with high efficiency. The flexible electrode structures permit much closer normal operating separation of the electrodes than is the case when one or more of the electrodes are fixed rigid members. This close spacing of the distance between the electrodes makes a direct gain in the force obtained from a given excitation energy, but due to the simultaneous movement of both flexible electrode members, permits a relatively larger vibratory movement of each electrode. This, because both members can move oppositely in correct phase, back and forth as the excitating voltages dictate. To permit this combined piston and diaphragm action, the spacer threads serve an important function and are, further, very effective in breaking up and thwarting back-lash rustle phenomena which would otherwise be highly objectionable. So close may spacings be in practice that they are scarcely visible to the naked eye, though the soft front of one's fingers may easily feel the dual vibrations of both working diaphragms. Frequency Range Extensive audio-frequency ranges are covered by the same acoustic condenser of this type due to the structure employed. All of the necessary overtones required to characterize each human voice or each individual musical instrument are retained in propei* relative volume to the fundamental frc