Patent GB-388269-A (GB)
Title: Improvements in or relating to sound producing devices
IPC: H04R1/42
Filed: 1932-06-10
Abstract: 388,269. Loud-speakers. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.-(Assignees of High, J. S. ; 179, Kendall Boulevard, Oaklyn Manor, New Jersey, U.S.A.) June 10, 1932, No. 16477. Convention date, June 10, 1931. [Class 40 (iv).] An air-flow sound-reproducing device comprises in combination a laminated valve seat structure in which inlet and outlet passages are formed in and between the laminations and adapted to cause air supplied thereto under pressure to flow through the passages in a plurality of thin sheets, a valve head so positioned as to reverse'the direction of flow of the currents in the sheets and to control the flow of air from the inlet to the outlet passages, the valve head being located substantially at the loci of current reversal, means for byepassing air under pressure to the rear face of the valve head in such manner that the bias pressure so produced cannot be applied to the front face thereof, whereby the valve head is suitably biased or balanced over a wide range of applied air pressures, and means for controlling the throttling action of the valve upon the flow of air therethrough in accordance with the sound it is desired to reproduce. The figures show an air-flow loud-speaker comprising a valve chest 1 which merges into the small end of an exponential horn 3 and which includes a valve seat 5 and valve head 7, the latter being connected to an actuating device 9 by a rod 11 extending through a stuffing-box 13 provided with a washer 15 for preventing air leakage. The device 9, valve chest 1 and horn 3 are mounted on a base-plate 17 so as to permit adjustment of the armature clearance in the device 9. As shown in Fig. 1, this is effected by mounting the device on a plate 19 having a flexible portion 21 near one end which is permanently secured to the base-plate, the other end of the plate 19 being movable with respect to the base-plate, mounted on a resilient washer 23 and held in adjusted position by a screw 25. The adjustment of the device 9 may also be kept constant by a bracket 26. Alternative forms of armature clearance adjusting means are also described which consist in providing an adjustable link in the rod 11 or in arranging the valve chest and horn or the device 9 so as to be bodily movable in a direction parallel to the base-plate. In place of an electromagnetic actuating device the rod 11 may, if desired, be directly or indirectly connected to the stylus-carrying arm of a phonograph. The moving element of the device 9 and the valve head 7 are so proportioned that the system has a high natural frequency. An air current is led into the valve chest through an inlet port 37 and is there divided up into a number of thin filaments or sheets by means of laminµ 27, 29, 31, 33 arranged as shown in Fig. 3, so as to cause the air current to flow initially towards and into a common throttling chamber 35' one side of which is constituted by the valve head 7. The direction of the current is reversed in the chamber 35, the air flowing back through further thin passageways between the laminµ 27, 29, 31, 33 into the common outlet conduit 3, the outer ends of the laminµ 33 being streamlined, as shown in Fig. 2. Movements of the valve head 7 corresponding to the vibrations imparted to rod 11 serve to throttle the air flow in the chamber 35 in accordance with the sound it is desired to produce. The head 7 is balanced pneumatically by the provision of an outlet conduit 39 extending to free air and through which, under control of an adjustable valve 41, air is permitted to leak to an extent sufficient to maintain the average pressure behind the head slightly less than the average pressure on the face thereof. Alternatively, the biasing pressure may be derived directly from the main air supply through a separate reducing valve, if necessary. The inlet air pressure can be observed in a conduit 43 communicating with the inlet port 37. In order to prevent the valve head from self-oscillation, a sponge rubber layer 45 is pressed against the rear face thereof by an adjustable plate 47 held in position by the inner end of a member 49 which is screwed into an opening in the end 51 of the valve chest and is provided with an operating knob 53. This arrangement also serves as a mechanical means for adjusting the valve head clearance. Self-oscillation may also be suppressed by feeding into the valve . chamber, rearwardly of the valve head, air pulsations derived from the input conduit, the phase of the pulsations being reversed 180‹ through suitable devices. The surfaces of the head 7 and seat 5 may be modified as to contour to give any desired type of response. Thus the head may be provided with a plurality of rectilinear or circular channels of any desired contour and the ends of the laminµ between the inlet and outlet openings in seat 5 may be shaped to correspond. The head 7 may also be faced with a thin sheet of leather &c. to obviate chatter.