US2816165A

US2816165A - Sound translating systems

Info

Publication number: US2816165A
Application number: US471951A
Authority: US
Inventors: Ross M Carrell
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-11-30
Filing date: 1954-11-30
Publication date: 1957-12-10
Anticipated expiration: 1974-12-10

Description

Dec. l0, 1957 R. M.A cARRELL SOUND TRANSLATING SYSTEMS Fgled Nov. so, 1954 Fz'y. I,

5 Sheets-Sheet l INVENTOR.

BY Z Z Dec. 10, 1957 R. M. cARRr-:LL 2,816,165

SOUND TRANSLATNG SYSTEMS Filed Nov. 30, 1954 3 Sheets-Sheet 2 maar Afrox/VH Dec. 10, 1957 R. M. CARRELL 2,816,165

souND TRANSLATING SYSTEMS 3 Sheets-Sheet 3 Filed Nov. 30, 1954 TOHNEX SOUND TRAN SLATIN G SYSTEMS Ross M. Carrell, Audubon, N. J., assigner to Radio Corporation of America, a corporation of illelaware Appiication November 30, 1954, Serial No. 471,951

8 Claims. (Cl. 179-1) This invention relates to sound translating systems, and more particularly to sound translating systems for higher order gradient operation.

Radio broadcasting in general and television broadcasting in particular, as well as motion picture recording, generally require directional microphones which discriminate against unwanted sound, such as reverberant sound or unavoidable background noises. The directivity rof a microphone is one of the main characteristics which determines the distance between the microphone and a performer in a given environment. In many cases, the directivity of some microphones is such that the microphone must be placed within a few feet of the performer, just out of camera range. In television broadcasting, these limitations of microphones constitute a problem of some proportions.

A large increase in ease and ilexibility of programing may be eiected by a substantial increase in directivity, providing that this is not accompanied by a large increase in bulk. The bulk of some types of directional microphones has limited their use to a few long range outdoor pickup applications. Other types of directional microphones require complex arrays of matched microphone elements. Such arrays, usually have low overall sensitivity.

In numerous other applications, outside of the television field, it is desirable to cut or to reduce background noises. These noises may be from operating machinery in factories or from airplane engines, for example. High order gradient systems have a marked axial noise discrimination as well as increased directivity, providing an immunity to noise fields which is superior to that obtainable from pressure or first order gradient microphones.

Much of the theory relating to higher order gradient sound translating systems has been well established. A pressure gradient responsive microphone is one in which the output is substantially proportional to a derivative of sound pressure with respect to distance from the source. Microphones of this type are classified according to the order of the pressure derivative. Thus, for example, a first order microphone has an output proportional to the first derivative. A second order microphone has an output proportional to the second derivative. An nth order microphone has an output proportional to the nth derivative. A first order pressure gradient responsive, or velocity type, microphone may comprise either two elements responsive to the pressure of a sound wave, or a single element responsive to the pressure gradient of the sound wave. A second order pressure gradient responsive microphone may include either two rst order microphones or four pressure responsive microphones. A third order microphone may include either two second order microphones or four first order microphones, or eight pressure responsive microphones. An nth order microphone may include 2n pressure responsive microphones. A fuller discussion of unidirectional and higher order-gradient microphones may be foundin Elements `of Acoustical Engineering by Harry F. Olson, second edinited States Patent tion, 1947, on pages 253 to 276. It is understood that single elements in a system may, in effect, be the equivalent of a plurality of elements.

Among the major considerations which have prevented wlde 4commercial acceptance of higher order gradient microphones is a requirement that the frequency response and the sensitivity of the microphone elements be very closely matched. Another consideration has been that the response of microphones or microphone arrays utilizlng higher order gradient operation is ordinarily not independent of frequency. A third consideration which has prevented wide use of higher order gradient microphones has been the relatively low overall sensitivity of the high order microphone array.

It is believed that many of the disadvantages inherent in higher order gradient microphones arise from an assumption that the amplifying system may only amplify the signal from the microphone, and conversely, that any special properties, such as increased directivity, must belong to the microphone alone. If the microphone and the amplifier system are considered as a cooperative unit, then effective solutions to the problems presented by higher order gradient microphone systems may be attained.

It is an object of this invention to provide a novel sound translating system for higher order gradient operation.

It is a further object of this invention to provide a sound translating system for higher order gradient operation in which the number of microphones required is minimized.

In accordance with the present invention, a sound translating system for higher order gradient operation is provided. An electric eld whose direction, magnitude and space distribution of phase angle is analogous to the corresponding quantities in a sound field is created in an electrically conductive element. This eld is attained by placing a plurality of microphones in the sound eld and applying the electrical outputs from the microphones to the electrically conductive element. Once the electric iield is created, a plurality of electrodes may be applied to various points on the electrically conductive element. The electrodes may then be connected to a utilization circuit. The electrical outputs from the electrodes will then be closely equivalent to electrical outputs which would be obtained from microphones placed at points in the sound eld which correspond to the position of the electrodes on the electrically conductive element. Thus, in effect, the electrical contacts ltake the place of microphones.

Other objects and advantages of the present invention will be apparent and suggest themselves to those skilled in the art to which the invention relates, from a reading of the following specification in connection with the accompanying drawing, in which:

Figure 1 is a curve representing a sound wave in a sound field;

Figure 2 represents a sound translating system in accordance with the present invention;

Figure 3 is a curve representing an electric eld in an electrically conductive element corresponding to a portion of the sound wave illustrated in Figure l; and

Figures 4 to 9 show embodiments of a sound translating system in accordance with the present invention.

Referring particularly to Figures l, 2 and 3, a curve 10 represents the instantaneous pressure distribution of a sound wave traveling in the direction of the arrow, as indicated. A plurality of microphones 12, M and 1.6 are represented as being disposed along a straight line in the direction ofthe sound wave. The microphones 12 and 16 may be spaced approximately one-sixth or less of the wave length 7\ of the transmitted wave. The sound pressures at the microphone positions are translated into corresponding electrical signals by the microphones. The electrical signals from the microphones 12 and 16 are assumed to be applied in some manner to an element 18 which has the physical property of presenting voltages throughout its length which correspond to instantaneous pressures at corresponding points in the sound field. The element 18, for purposes of this discussion of principles of the present invention, may be considered as a conductor. This conductor may be in the nature of a wa re transmission system composed of lumped elements.

If the microphones are of the pressure sensitive type the electrical signals applied to the ends of the electrically conductive element 18 will correspond to the instantaneous pressures in the sound wave field at the positions in said field occupied by the microphones 12 and 16. The microphone 14, placed at a position in the sound field will produce a voltage which is intermediate between that produced by the microphones 12 and 16. In a plane wave sound field this voltage will be equal in magnitude to that produced by microphones 12 and 16, but the phase angle of the voltage will be between that of the voltages produced by the microphones 12 and 16.

lf the electrically conductive element 18 is arranged so that there may exist at different places in or on the element, voltages which are intermediate in phase as well as voltage between the voltages applied to the ends of the element 18, then it can be said that there exists within the element an analogue of the external sound field existing between the microphones 12 and 16.

Consider that the conductive element 18 is one which will permit the establishment of an electric eld which is analogous to the external sound field existing on a line drawn between microphones 12 and 16. It will be seen that within the body of the electrically conductive element 18 there will exist a point at which the voltage and phase will correspond to that which would be obtained from the microphone 14. This is illustrated diagrammatically in Figure 2, where 01 is equal to 01.

Thus, it will be seen that an electrode placed in contact with the electrically conductive element corresponds and is analogous to a microphone placed in the external sound field.

Referring particularly to Figure 4, a pair of pressure operated microphones 22 and 24 are disposed in a sound field. Spacing between the microphones may be approximately M6 or less. An electric field corresponding to the sound pressure wave in the sound field is created in an electrically conductive element 26. A pair of electrodes 2S and 30 touches or is otherwise electrically connected to the electrically conductive element 26. The electrical outputs from the contacts may be subtracted in any well known manner. In the embodiment shown, subtraction is attained by connecting the electrical outputs from the contacts in phase opposition. It is seen that the output from the two contacts 28 and 30 will be substantially equivalent to a first order pressure gradient microphone having pressure responsive elements located in the sound field corresponding to the electric field within the electrically conductive element.

A pair of contacts 32 and 34 are also electrically connected to the electrically conductive element 26. The electrical outputs from the contacts are subtracted in any Well known manner, such as by connecting the outputs in phase opposition, as indicated. The electrical outputs from the contacts are substantially equivalent to electrical outputs of a pair of pressure sensitive microphones disposed at points in the sound field corresponding to the electrode positions in the electric field. The electrical outputs from each of the two pairs of contacts may be considered substantially equivalent to the output of a first order or velocity type microphone.

In order to attain second order operation within the sound system illustrated, the subtracted output from the pair of contacts 28 and 30 are subtracted from the subtracted output of the pair of contacts 32 yand 34. Again, the subtraction is attained by connecting the outputs in phase opposition. Any order of higher order gradient operation deisred may be attained by increasing the number of contacts and employing further subtracting operations.

It is thus seen that once an electric field is created which corresponds to a sound field, electrical contacts may be substituted for microphones. Thus the number of microphones required for higher order gradient operation is greatly reduced. The necessity of matching characteristics of more than two microphones is thereby eliminated.

If a unidirectional or other type of response characteristic is desired, a phase shifting network may be incorporated into the sound translating system. One such arrangement is illustrated in Figure 5. A pair of

microphones

36 and 38 are spaced approximately M 6 or less apart in a sound field. The electrical output of the microphone 38 is applied to ya phase shifting network, illustrated by a block 40. Such phase shifting networks are known in the acoustic field and, consequently, the phase shifting network 40 is not described in detail. The electrical output from the phase shifting network is applied to one end of an electrically conductive element 42. The electrical output from the microphone 36 is applied to the other end of the conductive element 42. The voltages from the microphone 36 and the phase shifting network 40 creates an electric field within the element 42. The electrical eld created corresponds substantially to the existing sound field between the

microphones

36 and 38. The phase shifting network may be designed to attain a substantially uni-directional characteristic in the sound system.

A pair of

contacts

44 and 46 are electrically connected to the conductive element 42. The electrical outputs from the contacts are connected in phase opposition, as indicated. A pair of

contacts

48 and 50 are also electrically connected to the conductive element 42 with their electrical outputs being connected in phase opposition. The electrical output from each of the contacts substantially corresponds to an electrical output of a pressure sensitive microphone disposed at 'a point in the sound field corresponding to the electric field within the conductive element 42.

The electrical outputs from the

contacts

48 and 50 are first subtracted from each other and then applied to a phase shifting network 52. The electrical output from the phase shifting network 52 is then subtracted from the combined electrical output of the

contacts

44 and 46. Again, the subtraction is achieved by connecting the electrical outputs in phase opposition'.

The combined outputs from the phase shifting network 52 and the

contacts

44 and 46 provide a second order gradient operation characteristic. The exact shape and directional response of the system shown will depend to a large extent upon the types of phase shifting networks employed. Systems for orders higher than two is attainable through the use of a larger number of contacts and additional subtracting operations.

Referring particularly to Figure 6, there is illustrated means for creating an electrical analogue corresponding to the sound pressure in a sound field. A pair of microphones 54 and 56 which may, for example, be of the pressure sensitive type, are disposed in a sound field. The spacing may be less than a wave length apart, preferably 'y/ 6 or less at the frequency to be reproduced.

In order that the system may attain an actual perform ance which is close to that theoretically obtainable, it is necessary that the outputs of the microphones 54 and 56 be equal in magnitude for equal sound pressures at every frequency within the range being considered. This may be also stated as requiring that the frequency response characteristics and the sensitivitiesv of the two microphones be as similar as possible.

In practical manufacture it is simpler to match the frequency responses of two microphones than it is to match both the. frequency response and the sensitivity. Considering that in a practical case that the microphones 54 and 56 have identical frequency response characteristics but different sensitivities, this may be compensated for by a proper adjustment of the gains of the amplifiers 58 and 6i), which may be done manually or automatically.

The amplified signals from the amplifier 60 arel applied to a phase shifting network 64. The phase shifting network may be employed to provide the desired directional characteristic within the sound translating system. The signal voltages from the amplifier 5S and the phase shifting network 64 are applied across opposite ends of a resistance network 66.

The resistance network 66 may be considered as a phase divider. The output voltage from the amplifier 58 is applied between an input tap 68 and a point of reference potential 70, hereinafter referred to as ground. The signal output from the phase shifting network 64 is applied between a tap 72 and ground.

Consider that the amplifier 58 and the phase shifting network 64 have equal and low output impedances (compared to the tapped resistance 76). Consider also that voltages of equal magnitudes but unequal phase appear at the terminals of the amplifier 58 and phase shifting network 64. lt will be shown that voltages taken between intermediate points on the resistor 76 and ground will have phase angles between the phase angles of the amplifier 5S and the phase shifting network 64.

Consider, first the voltage appearing between the output terminal 63 of the amplifier 58 and ground. This is essentially the output of the amplifier 58 alone, for it has been stated that the resistance 74 is much greater than the output impedance of the amplifier 58.

Similarly, it will be seen that voltage between the terminal 72 and ground will be essentially that due to the phase shifting network 64 alone.

At an intermediate point on the resistor 76, the voltage to ground will be the vector sum of the voltages contributed to that point by the amplifier 53 and the phase shifting network 64. The relativev contribution of each is determined by the point where the voltage to ground is taken.

`lf negligible current is drawn from a tap on` the resistor, and if the voltages appearing at terminals 68 and 72 are equal in magnitude, and if these voltages are less than )t/ 6 apart, it will be found that the phase angle of the voltage between the tap and ground varies nearly linearly with the ratio of the resistance between thetap and one end to the total resistance. Also the magnitude of the voltage will be constant within i2 db as the tap is moved from one end to the other. The variation in phase and magnitude of the voltage is symmetrical about the midpoint of the resistor 76.

It is apparent that this phase .division network satisfies the conditions of an analogue of the sound field between the microphones Sland 56. The usefulness of this analogue is limited to total phase shifts of roughly 100 degrees or less; but this is not a limitation in the present system which is limited by other considerations to phase shifts less than 90.

It is seen that the phase relationship of the electrical voltage at different points on the resistor 76 may correspond to the phase of a corresponding acoustical wave in a sound field within which the microphones 54 and 56 are disposed. Consequently, electrical outputs from electrical contacts placed at various points along the resistor 76 will be substantially the same iu form as electrical outputs taken from microphonesdisposed in a corresponding sound eld between the microphones 54 and 56. The principles described herein and employed in the present invention may be employed to attain higher order gradient operation in a sound translating system without 6 the necessity of a large number of microphones with close ly matched characteristics.

The voltage across a tap 7f3 and ground is applied to an amplifier 77, illustrated here as being a triode electron discharge device comprising a grid 79, a cathode and an anode 82. Suitable operating potential for the amplifier is supplied by a source indicated by B+. The amplified electrical signal from the amplifier 77 is applied across one half of a center tapped output transformer 84.

The voltage output across a tap 36 and ground is applied to an amplifier 88, which comprises a triode electron discharge device having a grid 9), a cathode 92 and an anode 94. The amplified elctrical signals from the amplifier 33 is then applied to one half of the output transformer 34. It is seen that the electrical outputs from the

amplifiers

77 and 88 are connected in phase opposition across the transformer 84 to provide means for subtracting the outputs. This provides an arrangement equivalent to a first order gradient sound system.

It is seen that the voltages appearing at the

taps

70 and 86 will be of different phase relationships depending upon the phase relationship of the sound field within which the

microphones

58 and 60 are disposed.

The voltage appearing across a tap 96 and ground is applied to an amplifier 98 comprising a triode electron discharge device having a grid lili), a cathode 102 and an anode 1912-. The amplied signal voltage is applied across one half of a center-tapped output transformer 106.

The voltage appearing across a tap 163 and ground is applied to the amplifier comprising a triode having a grid 112, a cathode 114i and an anode 116. The amplified signal voltage from the amplifier 11@ is applied across one half of the center-tapped output transformer 106. It is seen that the output signal voltages from the amplifiers 93 and 110 are connected in phase opposition thereby providing a subtracting operation. This arrangement provides means for attaining first order gradient operation in the sound system. Suitable operating potentials for the amplifiers 93 and 11@ are provided by a source designated as B+.

In order to attain second order gradient operation, the signal voltages developed across the

secondary windings

115 and 117 of the

transformers

84 and 106, respectively, are subtracted from each other. The signal Voltage from the secondary winding 117 of the transformer 106 is applied to a phase shifting network 118. The signal output from the phase shifting network 113 is applied to an amplifier 120. The signal output from the secondary winding 115 of the transformer 84 is also applied to the amplifier 120. The

secondary windings

115 and 117 are connected so that the developed signal voltages thereacross are in phase opposition to provide a subtracting operation. The subtracting operation provides an `arrangement for second order gradient operatron.

After the output signal voltage from the secondary Winding 115 of the transformer tid is subtracted from the signal voltage of the phase shifting network 118, the subtracted or combined voltage is amplified by the amplifier 120 and then applied to an equalizing network 122. This equalizing network compensates for the rising frequency response inherent in a higher order gradient system. The reasons for this rising response are well known to the art and may be found in any standard reference work on the subject. The essential function of the equalizing network 122 is to provide a substantially flat overall frequency response characteristic from the entire system.

This arrangement illustrating one embodiment of the present invention has provided an electrical analogue corresponding to a sound field in a resistor '76, and may 1n some respects be considered as one dimensional.

Referring to Figure 7, there is illustrated another means for creating an electrical analogue corresponding to the sound pressure within a sound field. A plurality of

microphones

124, 126, 128 and 130 are disposed in a sound field. The acoustic signals picked up by the microphones are translated into corresponding electrical signals. The electrical signals from the microphones are applied across a plurality of

plates

132, 134, 136, and 138, respectively, and ground. The plates electrically Contact a sheet of electrically conductive material 14) and are suitably spaced at convenient points. The electrical signal outputs from the microphones create an electric field within the material 140. The electric field created may be made to correspond to the instantaneous pressure of the sound waves within the sound field.

A plurality of

electrodes

142, 144, 146 and 14S are suitably mounted to a block 149, which may be an electrically insulated material. The electrodes may be disposed at various points on the surface of the material 140. The placement of the electrodes will depend upon the particular directional characteristic desired.

Leads

141, 143, 145 and 147 are connected to the

electrodes

142, 1M, 146 and 14S, respectively. The leads maybe connected to utilization circuits, not shown.

To attain higher order operation, the electrical outputs from the electrodes 146 and 14S may be subtracted from each other. Likewise, the electrical outputs from the

electrodes

142 and 144 may also be subtracted from each other. The subtracted outputs from the two sets of

electrodes

142, 144 and 146, 148 may be subtracted from each other to attain second order operation. This subtraction operation may be continued to attain third, or even higher order gradient operation. The number of electrodes must be increased as the order of operation increases. Means for amplifying and subtracting the voltages may be similar to those illustrated in Figure 6, and consequently are not again described in detail.

Itis noted that the contacts on the conductive material 149 provide the same functions as microphones disposed in a sound field. Consequently it is seen that the `directional or other output characteristics of the sound illustrated may be varied by varying the positions of the contacts on the material 14). Thus it is not necessary to move a plurality of microphones to attain certain desired outputs when, for example, a person in front of a television or movie camera moves to different positions.

Referring particularly to Figure S, there is illustrated another means for creating an electrical analogue for a sound eld. A plurality of

microphones

15d, 152, 154 and 156 are disposed in a sound field. The acoustical signals picked up by the microphones are translated into corresponding electrical signals. The voltages from the

microphones

150, 152, 154 and 156 are applied to a plurality of plates or

contacts

157, 158, 159 and 169, respectively. The plates are electrically connected to a mass of electrically conductive material 162. The signal voltages from the plates produce an electric field within the mass of conductive material 162. A plurality of

electrodes

164, 166, 168 and 171i are disposed to electrically contact the mass 162. The electrodes may be suitably mounted on a block 172, which maintains a constant desired spacing between the contacts. The block 17.2 may be suitably attached to a shaft 174. The shaft may be designed to rotate within a mounting 176 about the axis of the mass of the conductive material 162. The mounting 176 is attached to the material 162 in any suitable manner.

The electrical signal voltages from 'the electrodes 164, 16o', 16d and 171i are suitably applied to

leads

163, 15, 167 `and 169. The leads may be connected to utiliza tion circuits (not shown) and may be utilized in `any desired manner, such as previously indicated. in effect, the electrodes applied to the electric field takes the place of microphones disposed in corresponding positions in the sound field. Thus variable directional characteristics are attainable from the sound system by rot-ating the positions of the electrodes rather than moving a plurality o microphones.

Referring particularly to Figure 9, there is illustrated means for creating a three-dimensional electrical analogue corresponding to a sound field. A plurality of

microphones

178, 180, 182, 184, 18.6 and 188 are disposed within a sound field. The. electrical outputs from the microphones are connected to

plates

190, 192, 194, 196, 198 and 200, respectively. The plates electrically contact a container 202. The container includes an electrolytic liquid 204 therein. An electric eld corresponding substantially to the sound field within which the microphones are disposed is created within the electrolytic liquid 204. A plurality of

electrodes

206, 298, 210 and 212 are mechanically mounted to a block 213 and disposed within the liquid 264. The block 213 is attached to one end of a shaft 14, the other end of which is rotatably mounted to the top of the container 202. The block 213 is pivotally mounted to the shaft 214 on the pivot 215. A bent rod 2.17 is attached to the free end of the block and extends through the inside of the shaft. The shaft has a slot opening (not shown) to permit movement of the rod up or down. Movement of the rod changes the angular position of the block 213, with respect to the shaft 214. The angular positions of the electrodes 206, 2&18, 210 and 212 will consequently also be changed when the rod is moved.

Leads

216, 218, 220 and 222 are connected to the

electrodes

206, 20S, 210 and 212, respectively. The leads may be connected to utilization circuits, not shown. The voltages applied to the contacts from the electrolytic liquid 204 may then be used in any desired manner, such as, for example, in the manner previously indicated to attain higher order gradient operation within a sound system.

Numerous other means and arrangements may be employed for creating an electric field corresponding to a sound field. Once the field is created, it is seen that it may be utilized in many ways for creating sound systems involving higher order gradient operation. Such sound translating systems eliminate the necessity of using a large number of microphones which must be critically matched and substitutes therefor simple electrical contacts. The critical matching of microphone characteristics is thereby eliminated. ,Since the number and matching requirements of microphones are minimized, it is seen that the cost of higher order gradient operation sound systems is greatly reduced.

For some purposes it may be desired to create special directivity patterns for the systems by warping the electric field in the analogue. This can be done by using electrically conductive :material of special shapes, or of non-uniform resistivity per unit area.

Special effects may also be created by using a field material which possesses an interval time delay. Such a material might be a high dielectric constant material, such as barium titallate, coated by a lm of resistive material and backed by a conductive base.

What is claimed is:

l. A sound ,translating system comprising means for converting acoustical signals into corresponding electrical signals, an electric wave transmission element to provide yan electric field therein analogous to the sound field producing said electrical signals, means for applying said electrical signals to said element, means for electrically contacting a plurality of lpoints on said element, a utilization circuit, and means for electrically connecting said plurality of points to said utilization circuit.

2. A sound reproduction system comprising a plurality of sound translating devices disposed in a sound field for converting acoustical signals into corresponding electrical signals, an electric wave transmission element, means for applying said electrical signals to said element to create an electrical field therein, said electrical eld substantially 4corresponding in phase -to said acoustical signals in said sound field, mean-s for electrically contacting va plurality .of points on said element, a utilization 9 circuit, and means for electrically connecting said plurality of points to said utilization circuit.

3. A sound translating system comprising two microphones for converting acoustical signals into corresponding electrical signals, an impedance element having resistive phase shift characteristics and thereby being of a character to transmit said electrical signals without shifting the phase thereof, means for applying said electrical signals from each of said two microphones to opposite ends of said element, means for electrically contacting a plurality of points on said impedance element, a ultilization circuit, and means for electrically connecting said plurality of points to said utilization circuit.

4. A sound reproduction lsystem comprising a plurality of sound translating devices disposed in a sound field for converting acoustical signals into electrical signals, an impedance element, means for applying said electrical signals to said element -to create an electrical field therein, said electrical eld substantially corresponding in phase to said acoustical signals in said sound eld, a pair of electrodes for electrically contacting a pair of points on said element whereby an electrical voltage appears in each of said electrodes, means for subtracting the voltage of one of said electrodes from the other of said electrodes, a utilization circuit, and means for electrically connecting the substracted voltage from said electrodes to said utilization circuit.

5. A sound reproduction system for higher order gradient operation comprising a plurality of sound translating devices for converting acoustical signals into corresponding electrical signals, an impedance element, means for applying said electrical signals from said sound translating devices to said element, a plurality of pairs of electrodes for electrically contacting a plurality of points on said element whereby electrical voltages having different phase relationships appear in each of said electrodes, means for producing a voltage representing successive subtraction of the voltages of each of said pairs of electrodes, a utilization circuit, and means for lapplying the subtracted voltage from said electrodes to said utilization circuit.

6. A sound reproduction system comprising a plurality of sound translating devices disposed in a sound eld for converting acoustical signals into corresponding electrical signals, a sheet of electrically conductive material, means for applying said electrical signals to said sheet to create an electrical eld therein, said electrical eld substantially corresponding in phase to said acoustical signals in said sound field, electrodes for electrically contacting a plurality of points on said sheet, said signal applying means being l adapted to be moved to dilerent points on the surface of said sheet, a utilization circuit, and means for electrically connecting said plurality of points to said utilization circuit whereby the voltages at said electrodes correspond in phase to the electrical signals proudced by sound translatlicilig devices disposed in the corresponding acoustic 7. A sound reproduction system comprising a plurality of microphones disposed in a sound field for converting acoustical signals into corresponding electrical signals, a three dimensional electrically conductive element, means for applying said electrical signals to said element to create an electrical field therein, said electrical field substantially corresponding in phase to said acoustical signals in said sound eld, a plurality of electrodes for electrically contacting a plurality of points on said element, a member for maintaining said electrodes in a lxed spaced relationship, a mounting, means for pivotally attaching said member within said mounting whereby the positions of said electrodes on said three dimensional element may .be varied, a utilization circuit, and means for electrically connecting said plurality of electrodes to said utilization circuit.

8. A sound reproduction system comprising a plurality of sound translating devices disposed in a sound eld for converting acoustical signals into corresponding electrical signals, an electrically conductive element, said element including a container having an electrically conductive liquid contained therein, means for applying said electrical signals to said container to create an electric eld within said electrically conductive liquid, said electric field substantially corresponding in phase to said acoustical signals in said sound field, electrodes for electrically contacting a plurality of points within said liquid, means for moving said electrodes to dilerent points within said liquid, a utilization circuit, and means for electrically connecting said electrodes to said utilization circuit.

References Cited in the le of this patent UNITED STATES PATENTS 2,305,597 Bauer Dec. 22, 1942