US2522389A

US2522389A - Electric power source

Info

Publication number: US2522389A
Application number: US655001A
Authority: US
Inventors: Warren P Mason
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1946-03-16
Filing date: 1946-03-16
Publication date: 1950-09-12
Anticipated expiration: 1967-09-12

Description

Sept. l2, 1950 w. P. MAsoN ELECTRIF POWER SOURCE Filed March 16, 1946 i lfw. A V

l/VVE/VmR .By me MASON c. 21.4/ ATTGRNEY Patented Sept. 12, 1950 ELECTRIC POWER SOURCE Warren P. Mason, West Orange, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 16, 1946, Serial No. 655,001

17 Claims. l

'I'his invention relates to prime sources of energy and more particularly to a novel system for producing electrical energy in which an electromechanical transducer is excited by mechanical vibrations of an internal combustion device.

An object of the invention is to produce electrical energy by causing successive explosions of a combustible vapor or .powdered fuel to induce vibrations in a vibratory electrical generator.

Another object of the invention is to convert the energy of burning fuel into electrical energy with relatively little displacement of mechanical masses and without the use of rotating parts or elements of considerable mass.

Still another object of the invention is to utilize the energy of a high frequency series of explosions to excite a piezo-electric element, a magnetostrictive element or other electro-mechanical transducer.

Another object is to provide a compression wave impedance transformer which shall be capable of transforming a very low source impedance to match a very high receiver impedance, or vice versa, without introducing losses into the system.

In accordance with the invention a vessel or chamber, in which explosions of a suitable fuel take place in rapid sequence, is so proportioned as to support sustained gas pressure oscillations or standing waves therein and the vibratory mechanical energy of these oscillations is transferred, preferably by way of a novel impedancematching device, to a mechanical-electrical transducer element of appropriate type. This element, when actuated directly or indirectly by the oscillatory mechanical energy of the explosions, develops electrical energy which may be withdrawn and utilized to supply a desired load.

It is a feature of the invention that the main chamber in which the explosions take rplace is resonant or reverberant at the explosion frequency or a harmonic multiple thereof, so that pressure oscillations of large amplitude may be sustained with the addition of a comparatively small additional amount of explosive energy during each operating cycle.

It is another feature of the invention that the major portion of the oscillator pressure energy within the explosion chamber is transferred, directly or indirectly, to the mechanical-electrical transducer, means being provided to prevent its escape with exhaust combustion products.

The invention in its various aspects will be more fully understood from the following detailed description of a preferred embodiment thereof,

2 taken in conjunction with the appended drawing, the single gure of which shows a sectional diagram of the apparatus and a'schematic Vdiagram of the associated electric circuit of an engine for converting explosive mechanical energy into electrical energy of like frequency.

Referring now to the gure, an explosion cham'- ber I is provided, closed at one end by a valve 2 and at the other by a light, stiff, movable piston l. The chamber walls may be of any suitable strong, pressuresupporting material such as steel or cast iron. The chamber may be surrounded by a water jacket 4 for cooling purposes. Fuel, for example in the form of atomized liquid fuel, is supplied tothe chamber from a fuel tank 5 by way of a carburetor 6, an intake manifold 1 and the intake valve 2, being forced in by a supercharger pump 8, driven by a motor 9. These elements may all be of conventional mechanical design..

The valve 2 is preferably operated electromagnetically as by a solenoid I0 to which energy is supplied from an electric circuit whose details will be described hereinafter. Ignition of the atomized fuel may be effected by spark-gaps l I I,

likewise supplied from the electric circuit, in`

proper phase relation with the opening and closure of the intake valve 2. Combustion products may be withdrawn by way of an exhaust pipe I2 which is preferably provided with one or more filters I3 as more fully described below.

At the' end of the explosion chamber I opposite to the head end in which the intake valve 2 and the spark-gaps II are mounted there is xedly mounted a light exible piston diaphragm Il, for example, of sheet metal, Whose peripheral skirt may be xed as by welding to the inside walls of the chamber I. The piston 3 may likewise be a light, flexible diaphragm whose peripheral skirt is arranged to slide, over a short distance, within the skirt of the diaphragm I4. The separation between the piston diaphragm I4 and the piston diaphragm 3, and therefore the position of the latter relative to the head end of the chamber I may be adjusted as by a turn buckle I5. These two diaphragms together form a partition of adjustable length between the explosion chamber I and a second chamber I6, which is preferably of larger cross-section than the explosion chamber. This second chamber I6 is lled with a wave-supporting liquid such as water or oil. A partition, for example, a thin sheet I1 of rubber or the like, divides this second chamber into two parts. One part, shown in the figureto the left of the partition, is filled 'with water I8 which may be circulated through it and through the explosion chamber water jacket l' and a cooling radiator I9 of a conventional design. Circulation may be effected by a pump driven by a motor and the radiator I3 may be cooled by a fan.

The other portion of the second chamber I6, showing the ngure to the right of the partition I6, may be filled with oil 2I which is protected from the heat of the explosions in the explosion chamber l by the cooled water Il in the rst portion of the second chamber I6.

A stiff, rigid member, such as an aluminum plate 22, is movably mounted in the second chamber I6 at the end remote from the explosion chamber I. Its periphery ilts loosely between the chamber walls so that oil pressures on either side of it are equalized.

Fixed as by a suitable cement to the other side of this plate 22 are the ends of a number `of piezoelectric crystal elements 22 whose opposite vends are similarly xed to a massive backing block 24. The block 24 may be mounted on and supported by a skirt 25, movably engaging with a skirt 26 which may be an extension of the outer walls of the chamber Il. The assembly comprising the backing block 24, the crystals 23 and the aluminum plate 22 and the skirt 2i may be moved bodily toward or away from the explosion chamber I, correct positioning being effected as by turn buckles 21. With this construction the piezoelectric crystal elements are immersed in a protective bath of oil.

The piezoelectric crystal elements 23 may be cut from any suitable mother material, for example, ammonium dihydrogen phosphate (ADP) to vibrate in their fundamental longitudinal vibration mode. 'I'his material and the manner in which it should be cut are described and claimed in United States Patent to Mason 2,450,010, issued September 28, 1948. The crystal lengths may be substantially one-quarter of the wavelength oi a compression wave in the crystal material. Each one is provided on either side with a conducting plate or nlm 28, 28' which serves as an electrode in accordance with known principles of construction and operation. Corresponding electrodes 28 may be connected electrically in parallel and to one terminal of primary winding 29 of a transformer 30, the other terminal of which may be connected to the corresponding oppositely located crystal electrodes 28'. For simplicity and convenience of illustration. the electrodes which are specific to lonly a few of said crystal elements are shown so connected to the primary of transformer 30. it being understood that similar electrodes of the other crystal element may be similarly connected, as indicated. The secondary winding 3I of the transformer 30 may be connected to supply a desired load 32 and, if direct current is desired, a rectier 33 of conventional design may be interposed.

To control the timing of the ignition of the combustible mixture, a portion of the electric energy output from the crystals 23 may be fed back, by way of an adjustable phase controlling device 34, to the spark-gaps II. Similarly, to control the timing of the intake of the combustible fuel mixture, a portion of the generated energy may be fed back, for example, from the secondary winding 3| of the transformer and by way of another adjustable phase controlling device 35 to actuate the valve solenoid I0.

Any suitable starting arrangement may be employed, for example, magneto 33, energized by a manually operated switch 31 and battery 3l, which places a suitable voltage on the high tension electrodes of the spark-gaps II. This voltage must, of course, be suiliciently high to s cause the spark to jump the gaps II.

I'he dimensional arrangements of the invention will be better understood after the description of operation of the device which ensues.

The operation of the system is as follows; Assume that a charge of combustible vapor or fuel mixture has been drawn into the chamber by way of the intake valve 2 and that the latter has just closed. A spark is now caused to take place at the spark-gaps Il which ignites the fuel and sets of! an explosion, causing a high pressure to exist in the head end of the chamber, shown in the ligure as the left-hand end. This condition of high pressure travels lengthwise of the chamber at a speed equal to the velocity of sound in the hot gas, which is approximately 1200 feet per second. The condition travels in the form of a substantially plane wave because wave travel in other modes is attenuated in the manner hereinafter described.

The high pressure condition reaches the stiE piston-diaphragm 3 and is there reflected without alteration of phase and returns toward the head end of the chamber. 'Ihe reflection takes place at the piston surface after the lapse of one-half of the operation cycle. At this instant the pressure at the head end of the chamber has the lowest value it reaches during a. cycle. Therefore. at or about this instant, the intake valve 2 is caused to open once more and a new charge of fuel is driven in by the pump 8 which should provide a pressure in excess of the pressure which obtains in the explosion chamber at this instant. At the instant that the pressure wave reaches the head end of the explosion chamber on its return path after reflection, i. e., after completion of a full cycle of operation, the intake valve 2 will have again closed and another spark is caused to take place at the spark-gaps II. Thereupon the full cycle is repeated.

In order that each explosion shall take place at the instant when the pressure wave due to the prior explosion shall have returned, after reflection at the piston-diaphragm 3, to the head end oi' the explosion chamber, the chamber should be substantially one-half wavelength long, or a multiple thereof. When this is the case, regions of greatest oscillatory pressure will exist at the two reflecting end faces. Because, at the high frequencies contemplated in this invention, one-half wavelength might result in dimensions which are awkwardly small, a number of half wavelengths may be preferable. Thus, with a velocity of propagation of sound of 1200 feet per second and a frequency of 5000 cycles per second, the full wavelength is about 0.24 feet or about 7.3 centimeters. With a chamber which is a full wavelength long, operation is the same as above described i'or a half wavelength chamber, except that there will exist a velocity node or high pressure region at the central plane. 'Ihe exhaust pipe I2, which should be located at a pressure node, should in this event be placed one-quarter of the chamber length from either end, as shown in Fig. l. With a half wavelength chamber the exhaust may be centrally located.

By proper dimensioning of the quarter-wavelength crystals 23 in accordance with wellknown principles they may be made resonant at u a desired operating frequency, for example, 5000 cycles per second or a multiple thereof. Precise adjustment of the length of the explosion chamber and so of the wavelength of sound in the gaseous mixture, which depends on its temperature, to match the natural frequencyof the crystals 23 may be effected in any desired manner, for example, by adjustment of the position of the reflecting piston-diaphragm 3 by the turnbuckle I5.

In order to prevent the vibration of the gas column in the explosion chamber at higher modes and restrict its energy so far as possible to the energy of plane waves, it suices properly to restrict the cross-section of the chamber. Thus, higher modal oscillations will be rapidly damped out if the chamber diameter, assuming it to be `of circular section, be less than 0.6 times the Wavelength, or in this case, 4.3 centimeters. It is advisable to introduce a small factor of safety and use a figure of about 0.5, in practice. This sets an upper limit of 3.6 centimeters to the diameter of the explosion chamber. For an explosion chamber of rectangular cross-section the corresponding criterion is that the shorter side of the cross-section should not exceed one-half wavelength. The considerations out of which this design criterion arises are fully explained in Electromechanical Transducers and Wave Filters by W. P. Mason (Van Nostrand, 1942) at pages 108 to 110. i

As stated above, the exhaust pipe I2 should be located at a pressure nodeof the vibrating gas column. This greatly reduces the amount of high frequency energy lost by way of the exhaust pipe. In order further to reduce such losses, a lter I3 may be interposed in the exhaust pipe, which filter is constructed to offer a high impedance to oscillatory energy of the operating frequency while offering only a negligible impedance to the steady flow of the exhaust combustion products. Such a filter is known as a lowpass filter. It may be simply constructed of a pair of branch pipes, each one-quarter wavelength long, and positioned one-quarter wavelength from the opening of the exhaust pipe I2 into the explosion chamber I. The theory of operation and construction of such acoustic lters is well known and is described, for example, in Elements of Acoustical Engineering" by H. F. Olson (Van Nostrand, 1940). If desired, the circulating Water pipes may likewise be provided with acoustic lters. as indicated. The position of these lters is not critical. They have the same function as the lters in the exhaust pipe and are subject to the same principles of dimensional design, so as to justify the use of the same reference numeral I3 for them, as here indicated. This principle of design would require them to be about five times as long as the filter in the exhaust pipe, hence they are shown broken at a point intermediate their ends.

The forces due to the repeated reflections of the pressure waves of the gaseous medium on the piston-diaphragm 3 are transferred by the turn buckle I5 to the piston-diaphragm I4 which, in turn, transfers them into the liquid medium I8, 2|, which may be one-quarter wave long or an odd multiple thereof in the second chamber I6. The assembly consisting of the two diaphragms 3, I4 and the turn buckle I5 thus acts as a piston, driven by the pressures in the explosion chamber I and driving the liquid medium in the second chamber I6. The latter, in turn, by reason of its dimensions, acts as a quarter-wave acoustic transformer to transfer the pressure energy of the exmatch such widely diIIerent impedances presents considerable difilculties. This is especially true when it is desired to restrict the vibratory energy to the form of plane waves and avoid production of wasteful higher modal oscillations, because, as explained above in connection with the dimensions of the explosion chamber, this consideration likewise sets an upper limit of one-half wavelength to the diameter (or shortest side) of the cross-section of the second chamber. In water or oil the speed of propagation of sound is about four times its value in the hot gases of the explosion chamber, i. e., about 4800 feet per second, and the wavelength at a frequency of 5,000 per cycle is about 28 centimeters. Consequently, the shortest side of the cross-section of the second chamber should not exceed 14 centimeters. A

factor of safety is advisable so this dimension has been taken as somewhat less than I4 centimeters, e. g., 12 centimeters. Without this restriction it would be possible to match the low impedance of the gas column to the high impedance of the crystals by mere adjustment of the relative crosssections' of the ilrst and second chambers. The theory, construction and mode of operation of acoustic impedance transformers which function byreason of a change in cross-section is welld0 known in the art and is explained for example, in

Electromechanical Transducers and Wave Filters, by W. P. Mason (Van Nostrand, 1942)'. However, in order to avoid higher modal oscillations, this cross-section area ratio cannot b`e increased without limit. In the particular case selected assan example, and assuming circular cross-sections both for the explosion chamber and the transformer chamber, this cross-section area ratio ls therefore limited to the figure rial of the operating transformer medium. The

mechanical impedance looking into the end of quarter wavelength crystals has been determined to be about 0.2 of the characteristic impedance of the crystal material for an A P crystal. Taking the area of the crystal side of the plate 22 which is filled with crystal ends to be one-half the total plate area, the total impedance worked into is therefore 600,000X0.2X0.5 or 60,000 mechanical ohms per square centimeter of plate area or a total of asaassc 7 'I'he impedance at the piston diaphragm looking into the explosion chamber is 50x 42=62s mechanical ohms By reason of the cross-section change associated with the diameter change from 4 centimeters to 12 centimeters alone, the characteristic impedance looking into the liquid medium can be made =9 times as great as the impedance (50 mechanical ohms per square centimeter) of the gaseous medium or 450 mechanical ohms per square centimeter, while the total impedance looking into this medium from the the same point can be made l2 4 71- =81 times as great square centimeter will be equal to \/450 60,0l or 5200 mechanical ohms per square centimeter.

This impedance is too high for a gas and too low for a liquid. In accordance with the invention, however, it is realized by a combination of a liquid and a gas. The characteristic impedance of water or oil is about 150,000 mechanical ohms per square centimeter and that of air or gas is about 50 mechanical ohms per square centimter. I'he desired impedance, 5200 mechanical ohms per square centimeter, is about 1/so of the greater of these figures and about 100 times the lesser. It has been found that a proper admixture of two materials of diderent characteristic impedances partakes of the nature of both components. In particular, if one part by volume of finely divided air bubbles is intermixed with 99 parts by volume of water, the impedance of the mixture will be close to the desired value of 5200 ohms per square centimeter. However, it is impossible to hold the proper amount of air in the form of minute bubbles in suspension in the liquid. Therefore, in accordance with the invention, the proper amount of air by volume is retained in position in the form of minute air bubbles contained in a suitable cellular retainer such as a sheet of sponge rubber I 1. Both the material and the volume of the rubber have a negligible effect on the operation, the rubber serving for all practical purposes merely as a matrix in which the air bubbles are embedded.

The air and the sponge rubber in which it is contained are much more compressible than the liquid on either side of it. Therefore, this construction greatly reduces the length of the transformer chamber I8 for a given wavelength. For example, were the transformer chamber wholly filled with water or oil, and one-quarter wavelength long from the piston Il to the crystal-supporting plate 22 it would measure 7.5 centimeters at an operating frequency of 5,000 cycles per seccond. With the interposition of the sponge rubber air-supporting sheet l1, however, its length, for the same behavior, is reduced to 1.5 centime- 8 ters. the sponge rubber sheet being only about 0.25 centimeter in thickness. This reduction in length may be of considerable advantage at lower frequencies where the length of a quarter wave might be awkwardly large. In the present example, however, the length of 7.5 centimeters is not awkwardly large. On the contrary, the length of 1.5 centimeters may under some conditions be awkwardly small. If such should be the case, the transformer chamber may be made an odd multiple oi' a quarter wave in length. for example, three-quarters of a wavelength, in which case the dimension of 1.5 centimeters becomes 4.5 centimeters. The sponge rubber diaphragm II should then preferably be placed at a distance of approximately one-third the length of the transformer chamber measured from the plate 22; i. e., at the pressure node which exists onequarter wavelength from the high pressure end of the chamber I6. This arrangement has been selected for illustration in the ligure. Precise adjustment of the transformer chamber I0 to an odd number of quarter wavelengths is accomplished by movement of the assembly consisting of the crystals 2l, the plate 22 and the backing plate 24 toward or away from the explosion chamber I. This is effected by rotation of the turn buckles 21. Inward movement requires removal of some of the oil and water from the chamber I0 and outward movement requires addition of water and oil. Stop cocks Il, 4I may be provided for this purpose.

The composite impedance transformer of the invention thus permits matching of impedances over a wider range than would be possible at the high frequencies contemplated and without the excitation of higher modal oscillations and consequent energy losses, by adjustment of the cross-section ratio alone.

The quantitative relations employed in the foregoing example may be generalized, for a plurality of uuid media as follows:

Let

pi=pressure In the explosion chammcle mi br (ahead or the c ange in cross- -mammfwww pz=pressure In the transformer iangiggle velocity chamber Il (follow- I ing the change in Zh'imyse cross-section) Zs=impedance of plate 22 and crystals 2l Z=characteristic impedance of first component fluid Zs=characteristic impedance of second component fluid Za=characteristic impedance of third component iltlid e etc. Zo=characteristic impedance of composite fluid medium Then, ahead of the change in cross-section Zei-' (1) and following the change,

Zi=gj 2) But since volume velocities and pressures are continuous across the cross-section change,

p1=in (s) nA1=iiAi 4) Dividing (a) by (4) and substituting 1) and i2). Zs Ai A: l

zfz. 5)

If there be no change in cross-section. ('I) reduces to the more familiar expression for a quarter wave transmission line.

To obtain a medium having this characteristic impedance Zo by combining different fluid media of characteristic impedances Z., Zi, Ze, etc., and occupying volumes V., Vb, Cc, etc.. it is only necessary to adjust the proportions in accordance with the formula With only two component media, formula (9) reduces to It will be found by substitution that the figures of the example given above satisfy this formula.

The impedance transformer has been described in connection with its use in applicants power source, wherein the explosion chamber (source) impedance is low and the crystal (receiver) impedance is high. It is equally applicable to the inverse transformation from a high source impedance to a low receiver impedance.

It is recommended that the apparatus be operated at a fairly high energy level. For example, if the oscillating pressure within the explosion chamber I has an amplitude of one atmosphere, the total pressure then varies between =(V.+Vi)Zo (10) nero and two atmospheres. When this high value oi' alternating pressure is maintained the energy density will rbe given by the formula:

Energy density- P--:LX 10"- 1000 watts per square centimeter where Pm=peak value of alternating pressure (one atmosphere) p=density of gaseous medium v=velocity of sound in the gaseous medium Not all of this energy is transmitted to the transformer chamber I8, since if it were so transmitted, standing pressure waves could not be maintained in the explosion chamber I. However, when one-quarter of the stored energy is transferred in each operating cycle, one-quarter of the above factor or 250 watts per square centimeter becomes available for transfer. With an explosion chamber of the dimensions shown and discussed above, this energy density results in' a transfer oi about three kilowatts. Since the overall length of the entire system is not more than about two feet and its overall diameter not more than about six inches, an exceedingly compact unit. capable of delivering close to three klowatts of electric power, is thus made availa le.

Operation at high energy levels oiers the further advantage that the fuel pump il,l which injects the atomized fuel at the low pressure instants of the combustion cycle, need work only against very low pressures and therefore need require only a small amount of power.

What is claimed is:

l. A source of oscillatory energy which comprises a substantially closed acoustic resonant chamber, thermal means for setting up standing pressure waves of a gas within said chamber at an orderly high frequency and a pressureresponsive piezoelectric element coupled to said chamber in such fashion as to be actuated by the pressures of said waves.

2. A source of oscillatory energy which comprises a pressure-supporting vessel, an elastic fluid within said vessel, an elastic member coupled to said fluid, thermal means for producing pressure changes of said fluid in ordered time sequence, and means for deriving oscillatory electric energy from strains in said elastic member.

3. A source of oscillatory energy which comprises a pressure-supporting vessel, an elastic means for producing pressure changes of said fluid in ordered time sequence, and means for deriving oscillatory electric energy from strains in said elastic member.

5. A source of oscillatory energy which comprises a closed vessel, a wave-supporting fluid within said vessel, a mechanical-electrical transducer coupled to said fluid, said transducer beassassin l in'g adapted w deliver electric energy in response to mechanical pressures thereon, and means for developing standing waves of compression in said fluid of a wavelength such that said transducer is coupled to a high pressure region of said fluid.

6. A source of oscillatory energy which comprises a closed vessel, a fluid within said vessel capable of supporting standing waves of a natural frequency and wavelength determined by the nature of the fluid and the dimensions of the vessel. means for developing standing waves in said fluid. and a mechanical-electrical transducer element coupled to said fluid, said element having a natural oscillation frequency substantially like the natural frequency of said standing waves.

7. A source of oscillatory energy which comprises a closed vessel, a fluid within said vessel capable of supporting standing waves of a natural frequency and wavelength determined by the nature of the fluid and the dimensions of the vessel, means for developing standing waves in said fluid, a mechanical-electrical transducer element coupled to said fluid, said element having a natural oscillation frequency substantially like the natural frequency of said standing waves, a load circuit, and electric connections from said transducer element to said load circuit.

8. A source of oscillatory energy which comprises a closed vessel, a fluid within said vessel capable of supporting standing waves of a natural frequency and wavelength determined by the nature of the fluid and the dimensions of the vessel, means for developing standing waves in said iluid, a mechanical-electrical transducer element coupled to said duid, said element having a natural oscillation frequency substantially like the natural frequency of said standing waves. a load circuit, means for supplying electric energy derived from said transducer element to said load circuit, and means for feeding back a portion of said electric energy to maintain said standing waves.

9. A source of oscillatory energy which comprises a pressure-supporting vessel, a fluid within said vessel capable of supporting standing waves of a natural frequency and wavelength determined by the nature of the fluid and the dimensions of the vessel, means for developing standing waves in said fluid, said vessel having a. length substantially equal to an integral number of half wavelengths of sound in said uid and having a cross-section whose shortest dimension is less than said half wavelength, a movable piston for adjusting the length of said vessel and for responding to the changing pressures in said vessel, a vibratory mechanical-electrical transducer whose natural frequency accords with the frequency of said standing waves and impedance transformer means for transferring the energy of pressures exerted upon said piston to said mechanical-electrical transducer.

l0. A source of oscillatory energy which comprises a pressure-supporting vessel, a iluid within said vessel capable of supporting standing waves of a natural frequency and wavelength determined by the nature of the fluid and the dimensions of the vessel, means for developing standing waves in said fluid, said vessel having a length substantially equal to an integral number of half wavelengths of sound in said iluid, and having 'a cross-section whose shortest dimension is less than said half wavelength, a movable piston for adjusting the length of said vessel and for responding to the changes in pressure in laid vessel, a vibratory mechanical-electrical transducer whose natural frequency accords with the frequency of said standing waves, impedance transformer means for transferring the energy of pressures exerted upon said piston to said mechanical-electrical transducer. and means for feeding back electrical energy developed by said transducer to control the operation of said standing wave developing means whereby to control the frequency of said waves.

11. A source of oscillatory energy which comprises a pressure-supporting vessel, a fluid within said vessel capable of supporting standing waves of a natural frequency and wavelength determined by the nature of the fluid and the dimensions of the vessel, means for developing standing waves in said fluid, said vessel having a length substantially equal to an integral number of half wavelengths of sound in said fluid and having a cross-section whose shortest dimension is less than said half wavelength, a movable piston for adjusting the length of said vessel and for responding to the changes in pressure in said vessel, a vibratory mechanical-electric transducer whose natural frequency accords with the frequency of said standing waves, impedance transformer means for transferring the energy of pressures exerted upon said piston to said mechanical-electrical transducer, an exhaust pipe connecting with said vessel, and an acoustic lter connected to said exhaust pipe, adapted to block high frequency pressure energy while permitting release of pressures not directly associated with said standing waves.

l2. A source of oscillatory energy which comprises a pressure-supporting vessel, an elastic fluid within said vessel, means for setting up standing pressure waves of said fluid within said vessel, a mechanical-electrical transducer element of high input impedance, and means for transforming the standing wave pressure energy of said chamber to match said high input impedance which comprises a second chamber containing a liquid and a layer of trapped bubbles of gas.

13. A source of oscillatory energy which comprises a pressure-supporting vessel, a gaseous medium of low impedance within said vessel, means for setting up standing pressure waves of 'said medium within said vessel. a mechanicalelectrical transducer element of high input impedance, and means for transforming the standing wave pressure energy of said chamber to match said high input impedance which comprises a second chamber containing a liquid and a layer of a material comprising a multitude of separate cells, each cell containing a gas.

14. A source of oscillatory energy which comprises a pressure-supporting vessel, an elastic fluid within said vessel, means for setting up standing pressure waves of said fluid within said vessel, a mechanical-electrical transducer element of high input impedance, and means for transforming the standing wave pressure energy of said chamber to match said hlgh input impedance which comprises a layer of a soft. solid material comprising a. multitude of separate cells, each cell containing a gas.

15. In a system for developing electrical energy from mechanical energy, and in combination with a low characteristic impedance source of oscillatory pressures and a high impedance mechanical-electrical transducer adapted to deliver oscillatory electric current when actuated by 13 oscillatory pressures, an impedance-matching transformer which comprises a chamber coupled to said source and to said transducer, a liquid mass in said chamber, and a plurality of minute gas bubbles suspended in said liquid.

16. In a system for developing electrical energy from mechanical energy, and in combination with a low characteristic impedance source of oscillatory pressures and a high impedance mechanical-electrical transducer adapted to deliver oscillatory electric current when actuated by oscillatory pressures, an impedance-matching transformer which comprises a chamber coupled to said source and to said transducer, a liquid mass in said chamber, and a sheet of soft, liquid-tight cellular material suspended in said liquid, the cells of said material containing gas.

17. A confined column of gas, a mechanicalelectrical transducer, a liquid acting as a coupling medium between said gas column and said zo 14 to maintain said standing waves, said transducer, coupling medium, and gas column being tuned to the same natural frequency of vibration.

WARREN P. MASQN.

REFERENCES CITED The following references lare of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,213,611 Fessenden Jan. 23, 1917 1,348,828 Fessenden Aug. 3, 1920 1,378,420 Merritt May 17, 1921 1,493,340 Hahnemann et al. May 6, 1924 1,510,476 Hammond Oct. 7, 1924 1,544,010 Jordan June 30, 1925 1,563,626 Hecht et al. Dec. 1, 1925 1,677,632 Harden July 17, 1928 1,738,565 Claypoole Dec. 10, 1929 2,190,713 Hintze Feb. 20, 1940 2,215,895 Wippel Sept. 24, 1940 2,362,151 Ostenberg Nov. 7. 1944 2,384,465 Harrison Sept. 11, 1945 2,421,026 Hall et al May 27, 1947