Patent GB-361976-A (GB)
Title: Improvements in or relating to electro-acoustic converting devices such as loud speakers
IPC: H04R1/24
Filed: 1930-08-29
Abstract: 361,976. Loud-speakers. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Bostwick, L. G. ; 174, North Maple Avenue, East Orange, New Jersey, U.S.A.) Aug. 29, 1930, No. 25905. Convention date, Sept. 28, 1929. [Class 40 (iv).] Relates to an electro-acoustic converting device, such as a loud-speaker, comprising a substantially rigid moving part including a diaphragm, elastically supported and co-operating with an air chamber connected to a born, and aims at obtaining an adequate response over a wide range of frequencies, and particularly at the higher frequencies. Accordingly, the moving part, the elastic support thereof, the size of the air chamber and the area of the opening of the air chamber into the horn are so mutually proportioned that the acoustic load imposed by the horn is substantially equal numerically to the mass reactance of the moving part, at a frequency (f2) equal to the resonant frequency of an undamped system consisting of a mass equal to the effective mass of the moving part coupled with a stiffness equal to that of the elastic support of the moving part and a stiffness equal to that of the volume of air in the air chamber, multiplied by the difference between unity and the square of the ratio of a frequency (f1) equal to the resonant frequency of an undamped system consisting of a mass equel to the effective mass of the moving part coupled with a stiffness equal to that of the elastic support of the moving part to the frequency (f2), all quantities being expressed in a consistent system of units. Alternatively, the components referred to above may be so mutually proportioned that the acoustic load imposed by the horn is substantially equal numerically to the elastic reactance of the volume of air in the air chamber at the frequency (f2). The diaphragm is preferably actuated by a moving coil in which case the coil, magnetic field intensity, the moving part, the elastic support thereof and the size of the air chamber are all so mutually proportioned that the mechanical damping resistance introduced by the magneto-electric effect of movement of the moving part is substantially equal numerically to the mass reactance of the moving part at a frequency equal to the difference between the resonant frequencies (f2) and (f1). Alternatively, such components are so mutually proportioned that the square of the force factor (force applied to moving part per unit electric current flowing through the coil) is substantially equal numerically to twice the electrical ohmic resistance of the coil multiplied by the mass reactance of the moving part at a frequency equal to the difference between the resonant frequencies (f2) and (f1). The pure ohmic resistance of the coil is made substantially equal to the total impedance of the external circuit connected to the coil. The diaphragm preferably has a natural frequency above 3000 cycles per second and the horn associated therewith provides a load resistance therefor substantially equal to the mass reactance of the diaphragm in the frequency range above 3000 cycles per second, the air chamber between the diaphragm and horn having a stiffness reactance substantially equal to the horn load resistance at some frequency in this range. The Specification contains a theoretical discussion including mathematical formulµ and actual values and dimensions of the various parts of the speaker are given. Such a loud-speaker having a high and substantially uniform efficiency over a range of relatively high frequencies may be combined with a speaker having a high efficiency over a range of relatively low frequencies. Fig. 1 shows a construction of moving-coil loud-speaker comprising a centrally domed diaphragm 16 driven by a coil 17 located in the air gap of a pot magnet 10, 11, 12. The flat edge of the diaphragm is clamped to the polepiece 12 between a washer 18 and a ring 19 which is internally threaded to receive the flanged end of a short exponential horn 20. A projecting ring 24 on the flanged end serves to clamp the diaphragm further and to space the horn at the proper distance therefrom. A plug 21 is secured by pins 22, 23 in the throat of the horn. The coil is preferably composed of a flat ribbon conductor wound edgewise, the turns being held together by a suitable lacquer. Fig. 4 shows a high-frequency loud-speaker similar to that shown in Fig. 1 combined with a large diaphragm loud-speaker comprising a frusto-conical diaphragm 38 and coil 39. A single electromagnet provides the fields for both the coils 17, 39 and comprises a cylindrical casing 34 having a central core 11 and a cylindrical polepiece 35 spaced from the casing 34 by a ring 36 and provided with an inwardly directed flange at its lower end which co-operates with core 11 to form the gap for coil 17. The upper end of member 35 is surrounded by a face-plate 37 to form a gap for coil 39. The magnetizing winding (not shown), occupies the space 43 and, if necessary, an additional winding may be placed in the space 44. With this construction, the response of the two speakers may be equalized by varying the relative values of the flux densities. Fig. 5 shows a modified construction in which the high-frequency horn is formed directly in the field magnet structure, the diaphragm 16 being inverted and protected by a cover 50. The coil 39 is preferably wound with copper conductor thereby making very large damping resistances possible. Fig. 6 shows a construction in which the high-frequency speaker 59 is mounted centrally in the mouth of the large exponential horn 52 of a low-frequency receiver 51. This Figure also shows a form of input circuit whereby each speaker is furnished with frequencies corresponding to its own particular range and whereby a constant resistance load is maintained on the wave source. This circuit comprises an inductance 55 and capacity 56 connected in series between the terminals A, B, the speaker 59 being connected to the ends of inductance 55 through a high-impedance auto-transformer 57 and the speaker 51 to the terminals of capacity 56 through an autotransformer 58.