Patent GB-540171-A (GB)
Title: Electromechanical wave filters
IPC: H03H9/50
Filed: 1940-03-01
Abstract: 540,171. Electro-mechanical wave filters. STANDARD TELEPHONES & CABLES, Ltd. March 1, 1940, No. 3872. Convention date, March 8, 1939. [Class 13] An element of a mechanical wave filter comprises a substantially rigid body, e.g. a bar, elastically supported without friction for lateral vibratory motion, the dimensions of the body being determined by the desired values of the equivalent point masses located respectively at the input and output driving points of the body and by the desired value of the equivalent mutual mass coupling the driving points. When the driving points are chosen so that they are corresponding centres of percussion, the mutual mass of the body is zero, but in other cases the body may be regarded as two point masses coupled by a mutual mass, which transmits vibrational energy from the one driving point to the other and may be represented as a point mass located half-way between the driving points. As in the case of an electrical transformer, a rigid body considered as a mechanical transformer may be designed to transmit energy with or without a reversal of phase according to the size of the mutual mass. The rigid body may be a rectangular or cylindrical bar with adjustable loads at the ends to permit independent adjustment of the equivalent point masses and the mutual mass. Methods of eliminating undesired transmission through the frame supporting the rigid body are described. The mechanical equivalents of numerous electrical filter sections and composite filters are described including the III 1 , III 2 , IVK, VI, VII sections as classified bv Shea. In all the structures described, the rigid bars are represented by simple rectangles, fixed points of support by hatchings designated F, and reeds, leaf springs, or tensioned wires elastically supporting the rigid body by simple lines. Auxiliary stretched wires, which may also carry bars, may be used to build up composite structures and are placed in a common plane with the " stiffnesses" and bars of the main system, but with their longitudinal axes at right-angles to those of the main system. Bars designed to have no mutual mass are indicated by the mark O as in Fig. 11A which shows a unit equivalent to a III 1  section, Fig. 11B. The bar 50 having no mutual mass is supported by stretched wires 52, 53 near the driving points A, B, and is also connected by wires 51 to the frame. The inductances 81, 82, Fig. 11B, represent the equivalent point masses at the points A, B, and condensers 52<SP>1</SP>, 53<SP>1</SP> the stiffnesses 52, 53, while the shunt condenser 51<SP>1</SP> represents the stiffness 51. Fig. 13A shows a unit equivalent to a III 2  section, Fig. 13B, the bar 59 having mutual mass represented by the shunt inductance 106 coupling the two parts of the section. Constant-K confluent type sections, either alone or combined with III 1  or III 2  type sections, are described with reference to Figs. 14A, B, C, 16A, 17A, 18A, 19A, 20A, 21A, 22A, 23A (not shown), the electrical equivalents being shown in the corresponding "B-figures." An m-derived section is described with reference to Fig. 24A (not shown), fiveelement sections, either alone or combined with III 2  type sections in Figs. 26A, 27A, 28A (not shown), and six-element sections in Fig. 29A (not shown). Other complex units are described with reference to Figs. 15A, 32A, 33A (not shown). Methods of supporting one or more bars in a frame are illustrated in Figs. 34, 37 (not shown) and a complete filter coupling two moving-coil instruments in Figs. 42A ... E (not shown).