US2915588A - Pressure wave generation - Google Patents

Description

Dec. 1, 1959 A. G. BOSE PRESSURE WAVE GENERATION 2 Sheets-Sheet 1 Filed Aug. 6, 1956 INVENTOR AMAR G. BOSE BY M Dec. 1, 1959 Filed Aug. 6, 1956 A. G. BOSE PRESSURE WAVE GENERATION 2 Sheets-Sheet 2 I I3 q PREAMPLIFIER SIGNAL COMPENSATION AND ,31 B '3 S RCE NETWORK POWER J J AMPLIFIER (1 (1 INVENTOR AM AR G. BOSE BYW United States Patent O PRESSURE WAVE GENERATION Amar G. Bose, Hollywood, Pa.

Application August 6, 1956, Serial No. 602,195

37 Claims. (Cl. 179-1) The present invention relates. in. general to apparatus for generating pressure waves and in particular to a transducer for generating sonic waves in an acoustlc med1um in response to an electrical input signal. Apparatus em-' bodying the inventive concepts for the reproduction of passing the entire audio spectrum without harmonic or.

intermodulation distortion, efficient conversion of, electrical to acoustical power, omnidirectional radiation characteristics, and integration of the program source;

that isto say, all the soundshould appear to come from.

a single source.

While prior art systems have attained some of thesev characteristics, the better systems have utilized a plurality of speakers, each speaker. being energized with a,

signal having a selected portion of the audio spectrum. The signals may be coupled. to. the respective speakers from a single. power amplifier through a passive crossover network, by separate power amplifiers which are in turn energized by an electronic cross-over network, or

by combinations of the two.

Although satisfactory reproduction of the .high frequency signals has been obtained with electrostatic speakers and compression tweeters, the solid angleuniformly energized with acoustic energy by such speakers is limited. In. addition, electrostatic speakers require a high voltage source to supply a biasing potential.

Greater difficulties have been experienced in reproducing the low frequencies. To improve the low frequency response, efforts have been directed toward the design of special enclosures in which the low frequency loudspeaker is housed. One such type of enclosure is known as a folded horn and is designed with a view toward providing an improved impedance match between the loudspeaker and air, the design procedure being based largely on the concept of radiation resistance. Radiation resistance is a measure of the ability of a pulsating diaphragm, such as a vibrating loudspeaker, to couple acoustic energy into air, the greater the radiation resistance, the higher the degree of coupling. Accordingly, a folded horn is designed to present a large radiation resistance to the speaker diaphragm. Although improvements in low frequency response result from the use of such structures, the better units are of complex design, bulky and expensive. Furthermore, the frequency range over which the impedance match obtains is limited.

Since the latter transducer is normally used 2,915,588 I Patented Dec. 1,. .1959

Another common approach utilizes a bass reflex cabinet having one or more ports and housing an enclosed loudspeaker supported uponv a baffle. The volume of the cabinet is designed so that at a selected frequency, the back wave from the loudspeaker emerges from the port substantially in phase with the frontal wave. Design of such enclosures involves acompromise since wavesof other frequencies emerge from the port in phase opposition to the frontal wave.

As a result of these and other difficulties, the loudspeaker has been theweakest link in high fidelity sound reproduction systems" Other approaches to strengthening this link include, placing av large plurality of closelyspaced relatively small loudspeakers in a wall, or upon a baffle comprising several intersecting planar surfaces.

. For reasons which. will be better understood from the.

discussion below, thelast-mentioned arrangements have failed to achieve a major. portion of the desirable characteristics enumerated above.

Difficulties encountered in attempting to generate low frequency sound waves will be better understood by a consideration of the. nature of a pressure wave in any acoustical medium. An acoustical medium is a com-' pressible medium capable of. responding to pressure changes with corresponding changes in density. Thus, a. pressure wave may be activated in the medium by imparting forces thereon in a cyclical manner. For example, the action of a single loudspeaker set in the wall. of a roomwhereby it imparts forces upon the air therein in response to. a low frequency electrical signal is much like a. piston moving in and out.. If a hollow hard-sur faced cylinder having the same diameter as the piston and. concentric about its axis. were placed against the;

wall, substantially equal axial forces would be imparted upon. the air column within the cylinder, and'movement. of. the pressure wave therein would be substantially axial;

that is, the particle velocity at'any point within the cyl'-- inder is substantially axial, there being negligible velocity components orthogonal to the cylinder axis.- Now if'the cylinder is removed and the piston moved in and out,- the air particles which are compressed at points of high.

wave, removal of the cylinder results in the direction of.

propagation formerly entirely axial, having components orthogonal to the axis. The pressure intensity'of' the wave along the axis is accordingly reduced. To compensate for this effect, the excursion of the. pistonmust: increase appreciably. In the case of a single loudspeaker, the excursions are often so great that operation of the.

latter is over its non-linear region, thereby introducing unwanted harmonic and intermodulation distortion.

As is well known in the art, at a distance, av loudspeaker excited by a low frequency electrical signal looks. like a point source which radiates spherical. waves. It

has been discovered that improved low frequency re sponse can be obtained by providing effectively a'vibrating;

surface upon which a smooth'match of the boundary conditions for generating spherical waves may be obtained; that is to say, the surface is so designed that particle velocities of the acoustic medium in the vicinity of the vibrating surface are substantially along the direction of its movement, there being negligible velocity components orthogonal to such movement. Inthis man-- ner, difiicultics, such as the above mentioned propagational phenomenon are ,believed to be virtually, elimi nated, and the vibrating surface "is capable of exciting Fig; 4 :iltusnat'esi a; hemispherical speakers-covered streiace suitabie for generating spherical, wavesaceojrdin'g to the invention when enclosing a' volume :adiacent 58. single =planar surfacegsuchzas the fioorofa room; I

. i primary biect of the present :vention: to: provide means :for :e'ifieiently generating pres- ;sure waves of relatively iong wavelength in: an acoustical mediun'iw i f I Anbther obiect iop'enwolumeg=:=; g; 6 is an xploded =view illustrating i liz'ed' inf the preferred; 2 embloime. of ithe invendspieaikr 3 y tems; to; effect fat tioniof e1 tricalrlgn transmitted: sound; and r Fig; & a schematic; circuit; diagram; of; a representaembodiment :ot' th compensation 2 network :of 2 Fig.

.A futthe obiectefiinv n oudspeaker system capable :ot'responding :to a low fr queue-y electrical signal; by; 'eflicientty exciting: a: some vave a? air a which: accurate! l ti i l? sigrl t,

-.iobject ofi the inve' as ts in s loudspeaker system which has equ I oregoi apabii oneeigata:spiteriearsurfacezl r raiity ef louds eakers like loudspeaker: 13; at Fig: the surface: :12, iisirernevied and;-

I p i s, introduces Jiegi :gth harmonic l distortion; provides an fiectsto the listener @that the sofl 26min I I I I and floor are seen to :be planar surfaces which: are: so stantia'liy: 9o? esters of, eirete Ea h of: he la t r 2v p p iseeur'dtcithef the seine or: other: suitable: mean g I er; the volurnie fineed not he air-tight; to achieve; sububs antlaally: equal 1 1: :pha'sei pressure variatiorzs are 5 ex- 40% stantia portions or: the; desired results; 2 In :fiact;,= those cited ll the =acoustic niedium' at and orthogonal to the skilled in the art may deliberately place'op'en'ings' in the i surface. volume in order to achieve different acoustical charac- In another aspect of the invention, the surface at least teristics without departing from the inventive concepts.

partially encloses a relatively small volume which is ad- As indicated above, it is desirable to place the loudjacent to a markedly larger volume having the acoustic 46 speakers as close together as practicable in order to more medium. The remainder of the small volume is bounded nearly approximate the desired vibrating spherical surface by planar surfaces which are coplanar with at least some when all the loudspeakers are excited in phase.

of the bounding surfaces of the large volume, the planar Before describing certain features and characteristics surfaces being along radii which extend from the point of the embodiment of Fig. 1, other forms which the inof origin to the sphencal surface. Where the large vol- 50 vention may take will be discussed. With reference to.

ume is rectangular and the small volume is bounded by Fig. 3, a surface 21, which is one-fourth the surface of a more than one planar surface, the latter planar surfaces sphere is illustrated covered with speakers like speaker are mutually perpendlclllar- 13 and located at the intersection of a wall and floor.

In a particular form which the invention takes for The embodiment of Fig. 4 is a hemispherical speakergcnerat ng some waves in air 1n response to an electrical covered surface 21 which is illustrated enclosing a relamput s1gnal, the surface 1s covered with a plurality of tively small volume adjacent to the single planar surface closely-spaced loudspeakers connected together so that all. formed by the floor of a room. The remaining repreflle P move In Phase in response to e np t sentative embodiment, illustrated in Fig. 5, is a. speakereleciflca] Signal covered spherical surface 23 which is represented as sus- Other features, ob ects and advantages of the invenpended from the ceiling of a room. i tron Will become apparent from the following specifica- Each of the embodiments described above is capable t1on when read with reference to the accompanying drawof achieving the objects of the invention. By exciting mg whlghi all the speakers on a surface in phase with a low-fre 1 Illustrates, iii-Preferred embodimentof a loudquency electrical signal, substantially equal radial forces speaker Syster? acfordmg t0 the lllvenilon p ly Sllitare imparted upon the air adjacent the surface to launch locatlon the come! of a room; a sperical wave in the air, substantially free of the low Fig. 2 shows the speaker system of Fig. 1 with the frequency propagational effects described above. More-. front spherical speaker-covered surface removed in or-. over, at higher frequencies the relatively small loudder to illustrate the preferred means for enclosing the speakers act like many tweeters, each oriented in a differvolume along the walls and floor of the corner; cut direction to produce a substantially omnidirectional Fig. 3 lllustrates a surface which is one-fourth the surradiation pattern over the listening areas in the room. face of a sphere and covered with loudspeakers, especailly Furthermore, since each speaker diaphragm need only suitable for generating spherical waves when enclosing move a fraction of its normal displacement when used.

a: volume adjacent two orthogonal planar surfaces, such alone to produce a given volume level in the room, sub-g as one wall and the floor; i

, stantially linear operation of each speaker results, theres 5. by reducingspeaker harmonic and intermodulation distortionto a remarkably low level, even at low frequencies.

By combining the speakers in' the manner described, the. loudspeakers employed may be inexpensive and have relatively poor low frequency characteristics when singly excited by an electrical signal, yet yield an improved response when co-acting according to the invention. Moreover, since the diaphragm displacement of each speaker is so small and the propagational effect virtually eliminated by arranging the speakers on the spherical surface according to the invention, electrical compensation may be easily provided to yield the desired frequency response characteristics while maintaining harmonic and intermodulation distortion levels low. Such an electrical compensation network is described below.

Witl1.reference to Fig. 6, there is illustrated a typical suitable loudspeaker 13 of a conventional small type and the details of securing it to the sperical surface. The speaker utilizes a permanent magnet 35 to set up a magnetic field in which voice coil 31, secured to diaphragm 32, vibrates in response to an electrical signal applied at terminals 33 and 34. Portion 25 is a fragmentary drawing of any of. the surfaces 12, 21, -22,,or 23 viewed from the inside of the enclosed volume. Speaker 13 is secured to' portion 25 by screws and nuts like screw 27 and nut 28 respectively with rubber washer 26 interposed be tween the surface and speaker rim to effect a substantially air-tight seal. Other means for securing the speakers to the surfacewill be apparent to those skilled in the art.

Before discussing the electrical meansfor driving the n'gvelf loudspeaker systems, characteristics of the various illustrated embodiments will be described. The speakercoveredsperical surface 23 of Fig. is designed to radiate info a solid angle of 411-; the hemispherical surface 22 of Fig.4, into a solid angle of 21r; the one-fourth spherical surface of. Fig. 3,.into a solid angle of 11-; and the oneei'glrthlspherical surface l2 of Fig. 1, into a solid angle of Thus, if the planar surfaces adjacent the enclosed volumes aiefpejrfe'ct acoustical reflectors, and the-surfaces have tlije'san' e radiusv of curvature, the number of speakers r'equiredjfon each embodiment to generate apressure wave ofgiverr intensity at a. given radial. distance from the respective surfaces is. in the ratio of 1 2 4 i 8 for the surfaceslLi l, 22 and 23 in Figs. 1,. 3 4 and 5 respecma For example, the twenty-two speakers upon surr5651; located in a corner' generate sound intensities equivalent to 176' speakers upon" surface 23 suspended in space'. It is thus seen thatthe system 11 is the preferred embodiment for use in most rooms.

Q However', the other embodiments are not without utility. For example, the embodiment in Fig. 3 is useful where a corner is notavailable in a room or whereitis desired to place the system against but a singleiwall and tlie floor or ceiling. The system in Fig. 4 is especially useful when it is' desired to direct sound 360 about a source such as'rnight be requiredat an outdoor meeting id-ala-rge amphitheater, g

The embodiment of Fig. 5 may be advantageously utilized where-it is desired to obtain wide coverage in the vertical'as well asho'riz'ontal planes. For example, such a system mightbe 'suspended in the center of a large arena to cover theentire volume. with the sound. Not only, are the advantages enumerated. above realized, but the objectionable echos prevalent in spaced speaker systems-.whieh make understanding of the announcements difficult are virtually eliminated by the present system.

With reference to Fig. ;7, there is illustrated a block diagrampf-an electricalsystem for driving the novel speaker systems in a mariner which provides high fidelity reproduction of the entire. audio spectrum. A signal some 41', such as a ph'onograplh, microphone, tuner'or spectrum to compensate for the fall off in response of the individual loudspeakers at these ends. The latter network then energizes preamplifiers and power amplifier 43 which in turn drives the speaker voice coils 31. of the loudspeakers 13. The loudspeakers may be connected in parallel, series or series-parallel, as long as they are. connected so that electrical signals of like polarity actu ate each speaker diaphragm in the same radial direction, that is to say, the speakers are driven in phase. Normally, the type of connection is arranged to present the proper load impedance to the driving power amplifier.

Certain additional advantages result when the speakers are connected in series to present a relatively'high im-- pedance to the power amplifier. The latter may then be of the transformerless type, thereby markedly reducing the cost of the system while at the same time eliminating a component subject to distortion causing non-linearities. Furthermore, such power amplifiers may be similar to the cathode follower type, capable of exceptionally high linearity and virtually free from oscillation.

With reference to Fig. 8, there is illustrated a schematic circuit diagram of an example of a compensation network which provides the desired low frequency and high frequency boosts while uniformly passing the middle range of frequencies. 'Tubes V1 and V2 together with the associated components comprise the low frequency compensation circuit. The In terminal is coupled to the grid of V1 by capacitor 45, both the grids of V1 and V2 being supplied with a biasing potential from battery 47 through identical resistance capacitance networks comprising re-.

sistor 52 shunted by capacitor 50 serially connected to resistor 52. Resistor 51 is much larger than resistor 52; hence, at low frequencies capacitor 50 presents a high impedance which in turn results in a high elfective plate load impedance, thereby providing a high gain at low frequencies. At higher frequencies, capacitor 50 acts as a short circuit around resistor 51 and the plate load impedance is then effectively'resistor 52 and the gain is then relatively low. A capacitor 45 couples the plate of tube V1 to the grid of tube V2. The plate of the latter tube is coupled by coupling capacitor 53 to the grid of tube V3, tubes V3 and V4 and associated circuitry comprising the high frequency compensation network. The grids of tubes V3 and V4 are supplied with a biasing potential from battery 61 through resistors 54 and 66 respectively. Each plate is coupled to the B+ terminal by relatively high value resistors 57. Each cathode is connected to ground by resistor 56 shunted by capacitor 55, the latter parallel network supplying a portion of the high frequency compensation in the following manner.

At lower frequencies capacitor 55 is effectively an open circuit and resistor 56 introduces degeneration into the circuit, thereby effecting a relatively low gain. At higher frequencies, capacitor 55 presents a low impedance across resistor ,56 and increased gain of the triode amplifier is then realized. Additional high frequency compensation results from the characteristics of capacitor 63, resistor 64- and resistor 66, capacitor 62 serving as a coupling capacitor. The compensated signal is provided at the Out terminal by coupling the latter to the plate of tube V4 through coupling capacitor 65. The Out terminal may be coupled to a cathode follower stage when the distance between compensation network 42 and the following stage is an appreciable distance. 7

While the compensation network is indicated as driving the preamplifier, it may be placed between the preamplifier and the power amplifier. art may also introduce the compensation directly .into the power amplifier and/or the pre-amplifier without the addition of extra tubes.

Those skilled in the,

Other compensation networksmay be designed by those skilled in the art, the circuit 7 described above serving only as an example of'such nct works. y

The desired frequency response of the compensation network may becletermined in the following manner. The speaker system is placed in a corner setup in an anechoic chamber and a calibrated microphone positioned to face the speaker system. A power amplifier energizes the speaker system with a signal originating from an audio oscillator, the amplifier output being adjusted so that the calibrated microphone responds to a pressure wave of constant intensity over the audio spectrum' The output voltage from the power amplifier is measured and plotted as a function of frequency to yield the desired frequency response characteristic of the compensation network.

A speaker system constructed as illustrated in Fig. 1, and utilizing twenty-two standard Carbonneau four-inch loudspeakers on a rcsinated fiberglass surface, which is one-eighth the surface of a sphere, with fiberglass insula tion lining the inner planar surfaces of the enclosed volume, reproduced sounds recorded on a high-fidelity test record and encompassing substantially the entire audio spectrum with unusual realism, virtually free of distortion, when energized with a power amplifier coupled to the preamplifier by the compensation network of Fig. 8. Two serially-connected ll-speaker chains were con nected in parallel to provide an impedance of twenty-two ohms. This combination was connected to the l6-ohm output of the power amplifier, the latter output being shunted by a 58-ohm resistor to provide the correct impedance to the amplifier.

The compactness of the novel system will be better appreciated when it is realized that the exemplary system described in the preceding paragraph utilizes a surface which is one-eighth the surface of a sphere having a radius of but nineteen inches. Yet, remarkable reproduction of the entire audio spectrum is attained, even at the low frequencies where wavelengths in air are beyond twenty feet, and much larger than the relatively small dimensions of the novel speaker system.

Typical circuit parameters for the network of Fig. 8

are:

Capacitors 53, 62 and 65 microfarad 0.1 Capacitor 45 do 0.5 Capacitors 50 do 1.0 Capacitors 55 ..micromicrofarads 220 Capacitor 63 do Resistors 52 ohms.... 560 Resistors 51 do 33,000 Resistors 57 and 66 do 100,000 Resistor 54 do 150,000 Resistor 46 do 510,000 Tubes V1, V2, V3 and C4 A 12AX7 Battery 47 potential volt 1.0 Grid-cathode potential tubes V3 and V4 volts..- 1.5 B+ potential ..do 275 The novel loudspeaker systems have been illustrated as being positioned on the floor of a room in Figs. 1-4; however, it is to be understood that the planar surfaces may be oriented in any manner. For example, the embodiment of Fig. 1 may be positioned in a corner of the room formed by the ceiling and two walls. This is especially advantageous in a room with a rug on the floor, the rug serving to subdue reflections and reduce the effects of standing waves.

While the particular embodiment described herein has been directed to spherical surfaces with a plurality of loudspeakers thereon, the inventive concepts are not to be construed as limited thereby. For example, those skilled in the art may construct electrostatic loudspeakers having vibrating spherical surfaces according to the invention. Other loudspeaker covered surfaces may be employed to generate the desired spherical waves by properly phasing the energy applied to the loudspeakers to create a constant phase front equidistant from a point of origin. Furthermore, some ofthe advantages of the in-' vention may be derived by enclosing a volume with three orthogonal planar surfaces and a speaker covered surface without departing from the inventive concepts.

Although the examples described in detail herein have concerned loudspeaker systems, the principles of the in-f vention are equally applicable wherever it is desired to generate spherical pressure waves in an acoustic medium. For example, the inventive concepts may be employed for generating pressure waves in water, such as might be] utilized in connection with fathometer or sonar systems. Another use of the invention concepts maybe found in the delay line art wherein it is sought to generate pressure waves in acoustic mediums such as mercury.

It is apparent that those skilled in the artmay make numerous modifications of and departures from the particular embodiments described herein without departing um, apparatus comprising a surface defined by points substantially equidistant from a point of origin, means including said surface for enclosing a substantially fluid-l tight volume, and means responsive to an electrical signal for imparting in substantial time coincidence substant ial-' ly equal forces upon said medium at and substantially orthogonal to said surface along nearly the entire area of said surface to propagate a substantially spherical wave through said medium whereby a substantially smooth matching of the boundary conditions along said surface is obtained.

2. For generating pressure waves in an acoustic medi-. um, apparatus comprising a surface defined by points,

substantially equidistant from a point of origin, means including said surface for enclosing a substantially fluidtight volume, and means for cyclically imparting in substantial time coincidence substantially equal forces upon said medium at and substantially orthogonal to, said sur-- face along nearly the entire area of said surface to propagate a substantially spherical wave through said medium,

whereby a substantially smooth matching of the boundary conditions along said surface is obtained.

3. For generating pressure waves in an acoustic medium, apparatus comprising, a surface defined by points substantially equidistant from a point of origin, means including said surfacefor enclosing a substantially fluidtight volume, and means responsive to an electrical signal for setting up pressure variations of substantially equal magnitude in said medium at and substantially orthogonal to said surface along nearly the entirearea of said surface.

4. Apparatus for generating pressure waves in an acoustic medium comprising, an outward surface which at least partially encloses a volume having a point of,

medium, said wave having a substantially inphase pressure front along a reference surface within said medium, outside said volume and defined by points substantially equidistant from said point of origin, at least one of said points lying upon said outward surface, whereby a substantially smooth matching of the boundary conditions along said outward surface is obtained.

5. The apparatus of claim 4, wherein said outward" surface is the surface of a sphere.

, 6. The apparatus of claim 4 wherein said outward face is the curved surface of a hemisphere.

volume having a point of origin and enclosed'substantiallyfluid-tight by a surface, at least a portion .of said .surface beingTsubstantially curved and defined by points which lie substantially equidistant from said point of or1g1n,a-plurality of closely-spaced lou dspeakers -dis"- PQSfid upon substantially thev entire curved surface and coupling the voice coils of said loudspeakers to. said input terminals whereby the diaphragms move'in the same direction relative to said curved surface in response to the application of 'an electrical signal to said input terminals. i

The apparatus of claim 18 wherein there aretwo planar surfaces and said curved surface is the curved sur-' face of one-fourth of a sphere. a

20. ,The apparatus of claim 18 wherein there are three planar surfaces and said curved surface is the curved surface of one-eighth of a sphere.

orientedto emit sonic waves outward relative to, said volume in response to an electrical signal, a. voice'coil and diaphragm associatedwith eachflloudspeaker, input .terminals', and means for coupling the voice coils of said loudspeakers "to said input terminals {whereby the diaphragrns of said loudspeakers move in the'same direction relative to said curved surface in response to the application of an electrical signal to said input terminals.

11. Theapparatus of claim 10 wherein the curved surface is the surface of a sphere. v V

12. The apparatus of claim lowh'ere in the curved surface isthe'ic'urved surface of a hemisphere.

13. .T he apparatus of claim '10 wherein the curved surface is the curved surface of one-fourth of a sphere;

14. The apparatus of claim 10 wherein the curved surface is the curved surface of one-eighth of a sphere.

15. Apparatus for generating sonic waves comprising, a volume enclosed substantially fluid-tight by a surface having a substantially curved portion andv at least one planar portion, and a plurality of closely-spaced loudspeakers disposed upon nearly the entire surface of said curved portion and responsive in phase to an electrical input signal.

16. Apparatus for generating sonic waves comprising, a volume enclosed substantially fluid-tight by a surface having a substantially curved portion and at least one planar portion, a plurality of closely-spaced loudspeakers disposed upon substantially the entire curved portion and oriented to emit sonic waves outward relative to said volume in response to an electrical signal, input terminals, a voice coil and diaphragm associated with each loudspeaker, and means for coupling the voice coils of said loudspeakers to said input terminals whereby the curved portion in response to the application of an electrical signal to said input terminals.

17. Apparatus for generating sonic waves comprising, a volume enclosed substantially fluid-tight by a substantially hemispherical surface, a plurality of closely-spaced loudspeakers disposed upon substantially the entire curved portion of said surface and oriented to emit sonic waves outward relative to said volume in response to an electrical signal, input terminals, a voice coil and diaphragm associated with each speaker, and means for coupling the voice coils of said loudspeakers to said input terminals whereby the diaphragms move in the same direction relative to said curved portion in response to the application of an electrical signal to said input terminals.

18. Apparatus for generating sonic waves comprising, a volume enclosed substantially fluid-tight by a plurality of intersecting orthogonal planar surfaces and a curved surface which is a portion of a spherical surface, a plurality of'closely-spaced loudspeakers disposed upon substantially the entire curved surface and oriented to emit sonic waves outward relative to said volume in response toan electrical signal, input terminals, a voice coil and diaphragm associated with each speaker, and means for diaphragms move in the same direction relative to the v 21. Apparatus for generating pressure waves in an acoustic medium within a relatively large volume, cornprisiug a relatively small substantially fluid-tight volume bounded by a curved "surface defined by points substantially equidistant from a point of origin within said small volume-and oneIor more planar surfaces which are co planar with at least some of the bounding surfaces of said large volume, said planar surfaces being along radii which cit tend from said point of origin to said curved surface, and means responsive to an electrical signal for setting up pressure variations of substantially equal magnitude in said medium at and substantially orthogonal to said surface. I i 22. For generating pressure waves in an acoustic medi: um, apparatus comprising a surface defined bypoints substantially equidistantgfrom a point of origin, means including saidsurface for enclosing a substantially fluidtight volume, and means responsive to electrical signals, including 16w frequency electrical signals, for imparting in substantialtime coincidence substantially equal forces upon said medium .at and substantially orthogonal to said surface along nearly the entire area of said surface to propagate a substantially spherical wave through said me-' dium whereby a substantially smooth matching of the boundary conditions along said surface is obtained, the distance between said point of origin and said surface being less than the wavelength of a wave generated in said medium in response to said low frequency signals.

said input terminals.

24. Apparatus for generating sonic waves in air comprising, a substantially fluid-tight volume bounded by three intersecting orthogonal planes and a continuous surface havingportions thereof which are oriented relative to said planes at an angle different from ninety degrees,

' and means responsive to an electrical signal for setting up, pressure variations of substantially equal magnitude in the air at and substantially orthogonal to said surface along nearly the entire area of said surface.

25. Apparatus for generating sonic waves in air com-,

prising, a substantially fluid-tight volume bounded by three intersecting orthogonal planes and a continuous surface having portions thereofwhich are oriented relative to said planes at an angle different from ninety degrees, a plurality of closely-spaced loudspeakers disposed upon substantially the entire continuousv surface and oriented to emit sonic Waves outward relative to said volume in response to an electrical signal, input terminals, a voice tive to said continuous surface in response to the application of an electrical signal to said input terminals.

11 l y u 26. Apparatus for generating sonic waves in air .com pri i g, a substantially fluid-tight volume bounded by three intersecting orthogonal planes and a continuous'surface having portions thereof which" are oriented relative to said planes at an angle different from ninety degrees, and a plurality of closely-spaced loudspeakers disposed upon substantially the entire continuous surface.

27. A loudspeaker baffie comprising, a surface defined by points substantially equidistant from a point of origin, means including said surface forsubstantially fully enclosing a volume, and a plurality of closelyspaced openings therein arranged whereby a. loudspeaker maybe secured to said surface about an opening the spacing between adjacent openings being less than the maximum distance across any one of said openings. d

28. A loudspeaker baffle comprising, a surface defined by points substantially equidistant froma point of origin, a plurality of closely-spaced openings therein which are small enough to enable anassociated loudspeaker to surround each opening when secured to said surface and large enough to enable substantially the entire loudspeaker diaphragm to be exposed, and means including said surface for substantially fully enclosing a volume, the spacing between adjacent openings being less than the maximum distance across any one of said openings.

29. The loudspeaker baffle of claim 28 wherein said 33; Apparatus for radiating sonic waves into a first relativelyfflarge volume comprising, means for enclosing a substantially fluid-tight second volume much smaller thansaid first volume; thatflportionj ofsaid" enclosing means exposed to said first volume being a substantially spherical surface, any. other portions of said; enclosing means being surfacesgener'ally paralleljto bounding surfaces of'said large volume, said spherical-surface being formed with a plurality of closely-spaced opejningsppcovered by vibratable diaphragrns; andr'neans responsive to an electrical signal forvibrating said diaphragms in phase in a direction generally perpendicular to said spherical 34QApparatus1in accordance with claim 33 wherein said portion of s aid enclosing means exposed to said first volume is a fullspherical surface.

' 35. Apparatus in accordance w thiclairn 33 wherein said portion dffsaidjenclosing means exposed to said first volume. is ahemisphericalsurface; r p p p d 36. Apparatus in accordance with claim 33 wherein said portion'ofjsaidenclosing means exposed to said first volumesubtends a solidangle'of 1r/2 radians. l

h 37. Apparatus in accordance with clairn33 wherein said portion of said enclosing means exposed to saidfirst volume "subtends a solid angle of 1r/ 4 radians.

' References Cited in the file of this patentf UNITED STATES. PATENTS

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