Patent GB-344506-A (GB)
Title: Improvements in apparatus for measuring acoustical impedances
IPC: H04R29/00
Filed: 1929-12-06
Abstract: 344,506. Measuring acoustic impedances. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A.-(Assignees of Flanders, P. B. ; 28, Prospect Street, East Orange, New Jersey, U.S.A.) Dec. 6, 1929, No. 37499. Convention date, Feb. 19. [Class 40 (iv).] Relates to apparatus for measuring the acoustic impedance of devices such as horns, speaking tubes, or other elements of a sound-wave transmission system, such impedance being defined as the ratio between the sound-wave pressure and the volumetric displacement at a given point. The apparatus comprises means for measuring the ratio of the pressures at the input and output ends of a sound conduit when the latter is terminated successively by the device to be tested, and by two devices of different, known, acoustic impedance. These various pressures are utilized to produce electromotive forces which are then balanced against each other to obtain the values of the pressure-ratios. The apparatus shown comprises a steady source of sound 11 (such as a telephone receiver) connected to one end of a conduit 10 the other end of which is adapted to receive a pressure responsive instrument 12 (such as a telephone transmitter), the device to be tested (such as a horn 18, Fig. 4) and the known or standard impedances. The latter may consist of (1) a plug or the like adapted to close this end of the conduit to constitute an impedance of infinite value, and (2) an adjustable impedance comprising a tube 20 with a close-fitting adjustable plunger 21. As shown in Figs. 5 and 6, the conduit 10 and the device 11 are mounted on a base 48 pivoted on a stud 49 whereby the output-end of the conduit may be caused to traverse a curved face-plate 47 on a fixed bracket 46. By this means, the conduit may be aligned successively with (a) a full part of the plate constituting a closure-element for the conduit, (b) an opening 59 communicating with the variable impedance device, or (c) an opening 61 adapted to receive the device to be tested. Fig. 4 shows the electrical supply and balancing circuits, the pressure-ratios being measured in the latter. The supply-circuit comprises a wave-source 23 connected through a variable resistance 24 wavefilter 25, and transformer 26 to the leads 14, 15 of the receiver 11. The transformer 26 has two equal secondary windings between which equal high resistances 27, 28 are interposed. The balancing-circuit comprises a main circuit 29 including a telephone receiver 30 and branch circuits by means of which e.m.fs. proportional to the sound-wave pressures are introduced. The e.m.f. proportional to the input sound-pressure is introduced through a circuit comprising resistances 31, 32 in series with the resistances 27, 28, a reversing-switch 33, shuntresistance 34, and shielded transformer 35. The e.m.f. proportional to the output sound-wave pressure is introduced through a circuit comprising an amplifier 36 connected to the detecting-transmitter 12, a shielded transformer 37, and resistances 38, 39 in series with a variable coupling impedance comprising a variable resistance 40 and a vario-coupler 41. The telephone receiver 30 is coupled to circuit 29 through a shielded transformer 42 and tuning- means comprising an inductance 43 in series with a variable condenser 44.