Projection engineering (Sept 1929-Nov 1930)

Page 12 The Condenser Reproducer The Need for Improved Sound Reproduction Fosters Improvement in Appliances. Projection Engineering, June, 1930 Departure From Fixed, Rigid Electrode Plates, Makes Practicable More Faithful Reproduction. By C. D. Peak THE condenser reproducer at its best intrigues the interest of engineers. In its recently improved form, the Edelman acoustic condenser lends itself to particularly advantageous and economical combinations with either triode amplifiers, push-pull or single tube output amplifiers, resistance or resistancedirect coupled amplifiers, and lastly, with the pentode output tube amplifier. With hum removed from the output, the last combination, the pentode with the acoustic condenser of bi-lateral flexible electrode structure, appears to be particularly effective. Previous objections to earlier and cruder forms of acoustic condensers may be briefly summarized by the statement that they were structurally imperfect in the use of a solid or rigid electrode plate, in the use of deteriorating rubber diaphragm material ; electrically imperfect in the narrowed range of frequency response and poor sensitivity, particularly below 100 cycles. Also the older form was mechanically undesirable for radio service in view of the fact that a large working area was required to obtain satisfactory volume of response. All of these objections are overcome by the new acoustic condenser, and there are many added advantages as will be pointed out. SPACED THREADS CONTROLLING AIR GAP *v METAL FOIL ELECTRODEWAXED ON CLOTH VARIABLE — -! SMALL AIR GAP DIELECTRIC CLOTH STRETCHED ON FRAME <- FACE TERMINAL "- FRAME • BACK TERMINAL POROUS METAL CLOTH STRETCHED ON FRAME SPACED — FLEXIBLE METAL STRIPS CARRIED ON METAL CLOTH FIGJ Section of acoustic condenser showing relation of parts. Uses The principal service uses of acoustic condensers may be listed as follows : 1. As an acoustic condenser reproducer, replacing dynamic or cone type speakers. In this use the reproducer may be constructed of dimensions comparable to the size required of the dynamic cone and its baffling compartment, and the new acoustic condenser may be applied in exactly the same cabinet space previously provided for such cones. However, since the bilateral electrode structure functions in much smaller volume, the cabinet may be reduced in size or arranged in new artistic forms requiring no orthodox grille effects. Especially attractive designs may be concealed from sight underneath or at the rear of a cabinet or embodied in artistic wall pictures, framed tapestries, etc. One amplifier will operate as many condenser speakers as desired without noticeable loss of volume. As many as twelve speakers can be operated from a standard 245 push-pull output. In some cases where an extra speaker is required, the condenser speaker can be shunted around the output used to feed a dynamic reproducer. This is possible because the condenser reproducer is a voltage operated device and requires no heavy operating current. Satisfactory response may be had from the new type speaker in size of only 150 square inches. The dimensions may be arranged in any convenient form, an example being 10 inches wide by 18 inches long, to conform to the general configuration of a standard chassis compartment. 2. Aside from reproducer service, the new acoustic condenser is suitable in usual operating circuits, as a supersensitive condenser microphone, adaptable for broadcasting or recording service. A feature of this use is that the condenser is readily concealed from view, avoiding "mike fright." And, because of large pickup area, can be utilized to advantage out of doors or on difficult pickup situations. Low Costs The comparatively low costs possible in the use of acoustic condensers of this type are due not only to the economy of structure, but also to the savings effected in: (a) size and cost of smaller amplifier required for equal response volume; (b) reduced cost of and size of power supply unit required of the amplifier which feeds the con denser speaker; (c) reduced cost and bulk of cabinet space and shipping weight required in the case of the acoustic reproducer. Aside from this factor, there may be mentioned the suitability for remote control installations when the acoustic condenser reproducer is employed. Replacement Uses While most suited to new radio receivers, the acoustic condenser has been developed and is adapted as a replacement item, either to replace older types of speakers in existing set installations or to supplement these by an additional speaker or two operated from the same output. It is pointed out that any intelligent serviceman can make the necessary connections to the plate output circuit. Suitable operating circuits are hereafter discussed. The most satisfactory adapter circuit is comprised simply by a stepup output transformer coupled to the regular plate output circuit by means of a coupling condenser of about two microfarads capacity. General Structure It may be noted that condenser speakers have at irregular times received the inventive consideration of many minds, but that the bulk of previous work was largely and abortively directed to the use of one or more fixed rigid electrode plates. As early as 1880, Amos Dolbear suggested the use "FRAME AIRMOVED FLEXED FACE ELECTRODE DIAPHRAGM AUDIO VOLTAGE INPUT fWfffluTTl AIRMOVED -VARIABLE AIR GAP U PHASEDiu— " FLEXED " BACK 0 ELECTRODE D (AIR" 1 PERMEABLE) (I DIAPHRAGM FIG.2 ■ III POLARIZER FRAME Acoustic condenser action.