Patent GB-284467-A (GB)
Title: Improvement in loud speakers
IPC: H04R1/26
Filed: 1927-02-01
Abstract: 284,467. James, R. W., (Premier Labora tory Co.). Feb. 1, 1927. Loud - speakers; magnetic instruments; receivers utilizing arc - discharge; diaphragms. -An acoustic device comprises a series of sound - producing and/or amplifying instrumentalities, all operated from a common source and each possessing or being tuned to the particular natural frequency of a note of the musical chromatic scale to which it most readily responds. An instrument may be provided for each note of the audible musical scale, but in the preferred form, twelve horn-type loudspeaker unite corresponding to the notes of one of the lower middle octaves are associated with a large aperiodic diaphragm or sounding board responsive only to the notes of lower register. The units may either be operated electromagnetically or by means of an electric arc. Fig. 9 shows an electromagnetic unit in which the diaphragm 1 is held in a casing 2 between compressed cork washers and threaded rings 3, 4. The rings are provided with conical mating surfaces so that the diaphragm can be tuned bv flexing it away from the magnet poles. The permanent magnets 12, with pole pieces 13 having their ends inclined towards one another, are mounted in a casing 11 adapted to screw into the casing 2 so as to vary the gap between the polepieces and diaphragm, a ring 17 screwing on the casing 11 serving as a lock nut to secure the parts in adjustment. The operating coils 15 of each unit are preferably tuned so that, when connected in circuit with the other coils, they will resonate at the frequency to which the associated diaphragm is tuned. The diaphragm 1 is associated with a resonant chamber composed of the adjustable sound chamber formed between the diaphragm and the front wall 19 of the casing 2 and a horn 20 of a length equal to the wave length of the tone desired. A second adjustable resonating chamber is also preferably provided on the other side of the diaphragm, this chamber being formed by closing the outer end of the casing 11 by a cap 23, the flanges 24 of which fit smoothly on the casing. The two chambers may either be tuned sympathetically to the same note or the horn may be tuned to the fundamental of the diaphragm and the second to a higher harmonic, in order to increase the responsiveness of the unit to harmonic overtones. The horn 20 may also be made adjustable by forming it in two telescopic parts. Fig. 14 shows a method by which a number of such telescopic horns can be adjusted simultaneously by means of a shaft 27 carrying pinions 28<a>, 28<b>, 28<c> of different diameters which engage, respectively, with racks 26 on the outer portions 25b of the horns. Instead of an electromagnetic unit, a unit employing an electric arc, Fig. 12, may be used. The electrodes 30<a>. 30<b> pass through insulators 31 in a casing 29 which terminates in the resonant chamber. 32. In the complete instrument shown in Fig. 1, the twelve horn type units and the large diaphragm 34 are arranged in a single casing 33 having a hinged adjustable lid 45. The diaphragm 34 is mounted at the bottom of the casing with its edge clamped between compressed cork washers 36 by angle brackets 35. The horn type units are arranged in four columns, the three units of each column being connected in series and the four columns in multiple. This arrangement is then placed in series with the large diaphragm unit 38, as shown in Fig. 16. A condenser 43 in shunt with the unit 38 serves to by-pass the high-frequency currents. A variable resistance 44 is also preferably placed in shunt with the unit 38 in order to vary the volume of sound produced by the diaphragm 34. This resistance takes the form of a rheostat 57, Fig. 1. A series of simultaneously operable pivoted louvres 46 are arranged in the upper portion of the casing 33 so as to control the volume of sound emitted. In a modified construction of the arrangement shown in Fig. 1, the large diaphragm, instead of being mounted near the bottom of the, casing, is placed opposite to and substantially closes an opening in one of the side walls of the casing. In a. further modification, Fig. 7, the cabinet 80, in which the horns are placed, encloses only the mouths 81 of the horns.