GB778033A - Method of and apparatus for acoustic silencing - Google Patents

Abstract

778,033. Silencing air &c. currents. BOLT BERANEK & NEWMAN, Inc. Nov. 1, 1955. No. 31223/55. Class 7 (2). [Also in Groups XXI and XXVIII] In a method for eliminating sound energy produced by pressure variations at a predetermined region in a flowing fluid medium the amount of flow of which varies in accordance with some time function and at which region a predetermined impedance is presented to the flow, the impedance is increased and decreased substantially in accordance with the said function and synchronously with, though free from, the said pressure variations but within limits such that the impedance never becomes so great as to prevent substantially all fluid flow at the region. The invention is applicable to the silencing of refrigeration circulating systems, hydraulic drive systems, vacuum pumps, jack hammers, and I.C. engines. As shown in Fig. 1, a planar rotor 13 having a scalloped or smoothly varying curved periphery A. B is inserted between the upper and lower sections 9, 11 of an air intake conduit 5 connecting a carburetter 7 with an air inlet 3 in a manifold 1 of an automobile engine. The sections 9, 11 are sealed to the upper and lower portions of a housing 25 enclosing the rotor and of ellipsoidal configuration, the shaft 19 of the rotor being supported in bearings 31, 33 within the housing and driven by a flexible shaft 21. In operation, the pistons of the engine in successively drawing air through the conduits 9, 11 produce undesirable pulsations in the air flow, and rotation of the rotor 13 in synchronism with the operation of the distributer 17 and ignition of the combustible mixture in the cylinders of the engine causes, when the air flow is maximum, the scallops A of the peripheral edge of the rotor to pass between the conduits 9, 11 in order to reduce the available area for the passage of the air and therefore offer a maximum impedance to the flow of air so as to permit only a predetermined volume of air to pass through the conduit 9; during periods when air is not drawn through the carburetter to any appreciable degree the valleys B in the periphery of the rotor coincide with the flow path through the conduits to expose a large flow area thereby maintaining substantially the same predetermined volume of air flow through the conduit 9. The periphery of the rotor 13 is so shaped that the area available for the flow of air past the rotor continuously varies in. accordance with a sinusoidal function corresponding to, for example, the sinusoidal fundamental frequency of the sound emanating from the engine ; an increase or decrease in the velocity of air flow into the manifold opening 3 in response to an increase or decrease in pressure drop, respectively, produced by operation of the engine, will be offset by a corresponding decrease or increase, respectively, in the effective flow area between the conduits 9, 11 such that the product of that area and the air velocity therethrough will be substantially constant. In a modification, the rotor 13<SP>1</SP>, Fig. 2, is provided with peripherally disposed apertures 35, e.g. circular apertures, and is located in a cylindrical housing 25<SP>1</SP> divided into upper and lower compartments 41, 43 by a partition 39 apertured at 37 in a manner similar to the rotor; the conduits 9, 11 are integral with the housing. When the apertures 35, 37 are not in alignment a by-pass air flow path is provided between the conduits 9, 11 the resistance to the flow of air being controlled by a valve 45 pivoted about a shaft 47 located at the inner edge of the partition 39, the shaft being actuated by links 49 connected to the throttle control arm 51 of the engine. In a further modification, Fig. 3 (not shown), a cylindrical housing mounted above the carburetter is provided with a rotor having apertures which become successively aligned with corresponding apertures in the housing, the rotor being driven by a flexible shaft and located in any position relative to the upper wall of the housing by a screw-threaded adjusting device. A by-pass air flow path is provided by side apertures in the housing, the area afforded by those apertures being controlled by a rotatable sleeve valve having openings and actuated by links associated with the throttle control of the engine. Alternatively, the positions of the valve or rotor are altered by governors, Fig. 4 (not shown), or use is made of the intake manifold pressure, or the spark advance control. Further apertures may be provided in the upper wall of the housing, such apertures being normally covered by the periphery of the rotor or an extension thereof. In other modifications (a), Fig. 5 (not shown), the rotor housing is mounted within a toroidal air filter, (b), Fig. 6 (not shown), a fixed screen, provided with a number of horizontal elongated apertures, together with a movable grid having corresponding apertures, forms a valve mechanism located in a conduit through which a fluid flows, the grid being oscillated in a vertical direction so that the apertures in the screen and grid become more or less aligned and thus provide an increase or decrease in resistance to the flow of fluid; a microphone responds to the variation in pressure in the conduit to produce a signal fed through an amplifier to an electro-magnet by which the grid is synchronously moved in accordance with pressure variations, and (c) a plunger 20, Fig. 7, resiliently mounted by springs 28, 30 within a conduit 24 has circumferential slots 22 corresponding with slots 26 of the conduit, the plunger being moved up and down in response to pressure variations in the flow of the fluid through the device; a by-pass path is provided between openings 32 of the conduit 24 and corresponding openings 34 in a main conduit 36 within which the conduit 24 is resiliently mounted by a spring 38.