US 3407273 A
Description (OCR text may contain errors)
Oct. 22, 1968 J. s. ARNOLD THERMOACOUSTIC L OUDSPEAKER 2 Sheets-Sheet 2 Filed Jan. 8, 1965 United States Patent ice I 3,407,273 1 THERMOACOUSTIC, LQUDSPEAKER 1 James S. Arnold, Palo Alto, .'Calil., assignor to Staiiford "Research Institute, MenlofPark,"'Calif., a corporation ofCalifornia i FiledJan'.8,1965, SenNo. 424,414
-- 3 Claims. (Cl. I79-108)' -ABSTRACT-OF THE DISCLOSURE 1 A modulated flow acoustic device is provided 'wherein the flow of acombustible gas is'mo'dulated in acedrdance with signal'inform'ationl The modulated'gas stream is burned whereby sonic energy is produced at a higher sound intensity than is obtainable in the absence of combustion.
This invention relates to modulated'ga's fiowf loudspeakers, and more particularly "0 a rnethod 'and means for increasing the'acoiistical output thereof. *rr modulated gas flow loudspeaker ijs'a device whichad V cepts a steady flow of gas and -modulates the gas fiow rate (increases and decreases) ina manner cohtinuously res' onsive tofa' control signal. The gas flow: variations produce an acousticaloutput. p In order to increase the acoustical output of a modulated flow acoustic 'devic'eg'on'e increases'themagn'itude "of the gas flow rate or the control signal. However, for any given device, electrical and mechanical; considerations limit the output which one can obtain by increasing the g'as 'flow and control signal inputs. r An object" of this invention 'is to proyideian arrange: ment whereby acoustical output of any givenrnodulated flow acoustic device can be increased'with'ut increasing the gas flow rate or control signal. I v r v g Yet another object of the present invention isthe p'roa vision ofa nov'eland improved constru'etion fora lib 7, speaker whereby it'sacousticial output isjncr'eased extem sively over that heretofore obtainable with a given loudspeakerconstruction, A 1;: 1
A further object of the preseh'tinventionisthe; vision ofa novel and improyed constfuction for a; wher by its acoustical output i s increased extensive that heretofore obtainable'withagivensire f The foregoing and other objects of this invention may be achieved .in a construction for a modulatedfiow acoustic device whereby flow of a combustible gas is modulated inv accordance with the signal information After modu theg'gas stream is burned. This adds energy fromth bustion to the sonic energy to produce ahighehs ium intensity than would be obtained in the absence of coin-' bustion. j p I The novel features that are consideredcharacteristic ofthis invention are set forthwith particularityxin-the appended claims. The invention itself both as to its organization and method of-operation,-as'well as additional objects and advantages thereof, will bestbe understood from the following description when read in connection with the accompanying drawings, in which:
FIGURE 1 is a cross-sectional view of a loudspeaker structure which is modified in accordance with this invention;
FIGURE 2 is a cross-sectional view of another embodiment of the invention; and
FIGURE 3 is a view in cross-section of a modulated flow acoustic device, such as a siren, modified in accordance with this invention.
Reference is now made to FIGURE 1 which comprises a cross-sectional view of a loudspeaker construction in accordance with this invention. A typical loudspeaker 10, which may be of the electrodynamic type, is mounted in 3,407,273 Batented Oct. 22, 1968 the usual rear closure 12, in a manner well known in the art. The rear closure 12 is preferably made of metal. An
7 opening is provided therein so that leads 14 may extend I the closure 12. The closure member 20 is attached to the rear closure 12 as by bolts22'. I
The portion of the rear surface 24 which is opposite to the diaphragm 26 of the loudspeaker isgiven the shape of a frustrum of a cone'so that it substantially parallels the diaphragm, and also is spaced therefrom. T hecenter of the cover member 20 has an opening 27therein which has the shape of two truncated cones intersecting at their smaller ends in a small straight section 28, with the smaller cone being on the side adjacent the base of the diaphragm 26 and the larger side opening out of the cover 20. A metal screen30 is placed 'in'the' opening 29 adjacent the outside edge. A passageway 32 is provided in the closure member 20 which couples the space: between the diaphragm and the back side of the closure 20 to'the outside of the container. The source of combustible gas 34 is connected by means of a pipe or hose 36 to the outside end of the opening 32. A valve 38 which is adjustable, controls the flow of gas from the source of combustible gas through the passage 32 into the space between the rear surface 24 and the diaphragm 26.
In operation, the signal source 16 provides electrical signals which drive the loudspeaker 10. Gas from the source 34 is enabled to flow in the space provided adjacent the diaphragm so that it may be modulated by the vibrations of the diaphragm in response -to the signal source.
1 thermal augmentation of the acoustic output of the loudspeaker. The amount of augmentation may be optimized by adjustment of the flow rate of the combustible gas using the valve 38. In: an embodiment of the invention which was built and successfully operated, a mixture of air and natural gas was employed as the combustible gas.
It was found that not only the flow rate adjustment, but also the air to gas ratio of the mixture affected the extent of the augmentation of the acoustic output. These adjustments may easily be monitored by using a microphone appropriately located in front of the loudspeaker, the
output 'from the microphone being applied to a voltmeter or sound level meter. In the embodiment of the invention which was built, when the optimum adjustment was obtained, the measured sound pressure level was more than 10 db higher than with no combustion but otherwise identical conditions.
FIGURE 2 illustrates in cross-section another embodi ment of this invention whereby a loudspeaker operating along the same principles as the one shown in FIGURE 1 is loaded by an acoustic horn 42. In this embodiment of the invention, there is provided the conventional magnet 44 having a circular slot therein within which the voice coil 46 is placed and supported by a voice coil support 48. The voice coil 46 is actually wound on a bobbin 50 which is attached to the voice coil support 48. The voice coil is driven by signals received from a signal source 52.
The magnet 44, voice coil and support are placed within a container 54 having a top wall 54A, the inner side form a passageway 56 therewith. An opening is placed in the center of the top wall 54A. A gas flow smoothing member 58 is placed on the top of the magnet '44 concentrically with the voice coil 46. 'The gas flow smoothing member has side walls which are substantially parallel and almost coextensive with the bobbin. Then the walls curve inwardly in-a generally conical manner to a point. The-walls of the opening in the top wall 54A-curve in a somewhat parallel manner with the walls of the gas flow smoothing member to define an exit passageway 60 therebetween. The areaaof this passageway, 'taken perpendicular to the gas flow, varies with distance along it a.
in substantially thesame manner -as' the law'of expansion ofthehorn 42.
The horn 42 is placed on the top wall 54A with the opening in its throat matching that of the exit passageway 60. An inlet opening 62 in the side wall of the container 54 admits a combustible gas into the passageway 56. The signal source provides signals to the voice coil which together with the bobbin 50 respond thereto by moving more or less into and out of the passageway 56. Thereby, the gas flow through the passageway is modulated by'the signals from the signal source 52. The flame holding screen 64 may be advantageously located within the small cross-section portion of the horn by controlling gas fiow rate and mixture ratio. As a general principle, at this location the flame velocity will approximately equal the rate of gas outflow.
The horn serves the function of increasing the efficiency and directivity of radiation.
FIGURE 3 shows in cross-section another embodiment of this invention wherein a conventional siren 70 is employed. Gas, instead of air, is fed from a source of combustible gas 72 into an opening in the case 74 of the siren 70. The gas flow is modulated by the rotating chopper wheel 76 which is within the case. The rotating chopper wheel is driven from an external drive source, represented here by a drive shaft 78. The modulated gas is urged by the rotating chopper wheel toward an outlet opening 80 in the case, over which is placed the acoustic horn 82 containing the flame supporting screen 86. The acoustic output of the modified siren is increased considerably when the gas is ignited and burned. A
Other arrangements of this invention may be provided wherein fuel and oxidant gases are kept separate and the flow of only one component is modulated. Mixture can take place after modulation, but before ignition. This may be done in any of the structures shown in FIGURES 1, -2 or 3 by feeding one of the gases into. the region between the throat of the acoustic horn and below the flame supporting screen by means of a pipe connection to a source ofsupply. i
. Because combustible gases are used, the thermoacoustic loudspeaker must employ safety devices to stop gas flow ifthe flame goes out. Flashback prevention and ignition means must also be provided. Such devices are well known in the combustion art.
'There has accordingly been shown herein a novel and useful method and means of increasing the output of an acoustic device by using it to modulate a combustible gas which is then ignited.
1. The improvement in a'loudspeaker of the type having a diaphragm which is actuated responsive to signals to modulate the air surrounding said diaphragm comprising aclosureirnember spacedfrom saiddiaphragm to defingabhamber therebetw'een, a source of. combustible gas,', .an ppenin 'gat, oneffedge of said; closure .rnember extending between said chamber and its outside-edge, a source of combustible gas, meansfor-lintroducing combustible gas through'said opening into said chamber, a second opening in said closuregnember extending outward from the region of said closure member in said chamber over'the "center of'sziid diaphragm, a 'fiame holding. screemdisposed insaid second opening near the outside edge ,of said closure member, whereby when said combustible,gaapassing through said flame holding screen ia ignited the acoustic output of said loudspeakeris increased. A
.,A .thermoacoustic loudspeaker comprising a diaphragm, means to actuate said diaphragm responsive, to signals for modulating the. air surrounding said diaphragm inaccordance with said'signals, means forming an enclosure with said diaphragm, said enclosure forming means having walls spacedopposite from and substantially parallel to said diaphragm, a source'of combustible gas rneans for introducing combustible gas from said source intosaid enclosure, a, gas escape openingrin said i encloure forming means positioned over the center of said diaphragm, whereby the vibration of said diaphragm determinesthe arnount of gas passing out of said gas escape means, and means for supporting a flame at the outside end of said gas escape opening whereby when said combustible gas is ignited the acoustic noutput of said loudspeaker is increased.
3. In combination a loudspeaker having a diaphragm and means for vibrating said diaphragm responsive to electrical: signals, a rear closure means for said loud speaker comprising a boxhavinganopening in one wall thereof the size of said, diaphragm, means mountingsaid loudspeaker in said jboxwith said diaphragm facing outwardly within said openings,' a front closure means mounted on said boxover said loudspeaker diaphragm,
said front closure means having walls opposite said dia-' phragm. which arespaced therefrom and substantially parallel thereto and .dcfining therewith a modulating en: closure a source of combustible gas, means for introducing 'said's source'of combustible gas at a location of said diaphragm displaced from the center thereof into said modulating 'chamber, and afgas escape 'DPQDF IS through said enclosure member extendingfrom the region thereof over,the center of said diaphragm outwardly, whereby 'when said combustible gas escaping from said escape opening is ignited the output of said loudspeaker s i d. p i v References Cited i UNITED STATESPATENTS 969,037 8 1934 R ieber 116-137 KATHLEEN H. CLAFFY, Primary Examiner.
A. MC QGILL', Assistant Examiner.