US2175833A - Sound propagating device - Google Patents

Description

Oct. 10, 1939. c. 1.. FARRAND SOUND PROPAGATING DEVICE Filed Dec. 11, 1937 2 Sheets-Sheet 1 grwemm C/az'r- L. F or/"and WKW ATTORNEY Oct. 10, 1939. c. L. FARRAND SOUND PROPAGATING DEVICE 2 Sheets-Sheet 2 Filed Dec. 11, 1937 6 5 3 1 7 u i I M W nfl- HEW any w Q M w A TTORNEY Patented u e-meme UNITED- STATES PATENT omen I ,175,833

Larchmont,

N.Y.,aflgnorto3adio OoIporationoIAmer- -ica,aoorporatianofDelaware I Application December 11, 1937, Serial No. 179,2:4

J 3 Claims. This invention relates to loudspeakers and particularly to the sound radiating portion thereof.

'I'hisapplicationisadivisionoimy application Serial No. 94,072, nled August 3, 1936,

g inwhichthetranslatingunitdisclosedhereinis being claimed.

It is well knownin the artto allocateapart of the acoustic range oi a sound reproducing apparatustoonetypeotloudspeakerand anotherpart l or parts to another type (or other yp s) of loudspeaker.

This invention relates to the type of loudspeaker useful in a so-called two-way system, whereintheacousticrangeishandledby onLytwo ll typesoil,thetypeotioudspeakeroi' the present invention being particularly adapted to reproduce acoustic irequencies from about three himdred cycles to the upper limit, i. e.,-

eight thousand cycles or higher. a The objects 01' the present invention comprise an eilicient reproduction of sound waves over a a wide frequency range, with large power. and while attaining a substantially uniform distribution oi. the sound waves. particularly of the higher i'requencieswhichhave a tendency to remainin a The novel features of one embodiment of the invention will be described in the accompanying specification-and particularised in the appended claimsinconnectionwiththeaccompanying wherein: I'igurelisasectionalelevational viewoia i embodying the present-invention,

m throat block and is taken alongthe line 4-4 set 01' along the line H otl ig l, and

5 l'lgureilisabottompianvlewotaportion of the annular diaphragm showing the connection oi the vdce coil thereto.

Referring nowto l'lgs. 1 me use driving unitiot the'loudspeaker comprises a U shaped o magnetic iramel. havinga centraicore so:

'videdontheuppersuriaceoftheoorel',andthis assembly. including the frame 2, core 3, and I u pole piece I, issecured together-by means oi 'coii I is prcvided around the corel, and receives -asupplyofdirectcurrentsoastoproduceaconstant magnetic flux between the two separated pole pieces 4 and t. I

-As will be noted in Fig. 2, the adjacent outer l and inner edgesofthe polepieces land ,respectively, are undercut as at ll, leaving only a short length of parallel, closely adjacent pole faces at H. 'lhespace thusprovided betweenthe paralleli'aces Ii'oithepolepieceslandlaccommodates the voice or driving coil i! of an annular diaphragm it. The diaphragm II is supported belowvits inner and outer edges by means oi inner and outer concentric supporting rings II and it, respectively. Upper conso centric clamping rings II and it clamp the edges otthe diaphragm upon the supporting rings ll andll bymeansotboltsll.

A frusto-conical member II, with a coextensive conical member ll, secured togetherand to the gs uppersurraeeoi'theinnerclampingringilby means of boltsllandthe centralbolt lands casing member 2i, allconcentric with the diaphragm ",iorman-annularsumidpassagell leadingto thecentralportion oithe annulardiaso phragm il.

'lhe'a'nnularsoundpassage 2!,atthetipotthe conical member ",communicatu with a tubulare 28 formed in an adapter 24. This passageltiscoextensivewithasimilarpassagell gs formed in a throat block 2'.

Adapter 24 is iitted in a counter-bored portion 'onthetopofthecasingmember2Landhasanannular shoulder portion 24. whichisengaged by alockingnutl'lthreadabiysecm'edtothecaaing 40 member ii. The threat block It is threaniably securedat 26 to the upper portion of adapter ll. Theupperportionoithethroatblockltis formedin a funnel shape having diversing flanges It, so asto receivethe throatportions or a plurality of horn sections 1. to N, inclusive. A suitablecelnent ll is pr'ovided to secure the throat po'rtionsoithevariomhornstogetherbe- 'tweerfthe flanges II o! the throat block II. 'Ihesevarloushornsareallidsnticalin shapeand size, having their longitudinal axes diverging from each other away from their throat portions. Thisarrangement oi nested horn sections eliminatesthe wellknowndirectionalorbeameil'ect oi the higher frequencies by means of the aphigher frequencies, is to be distributed. in the proximate spherical surface formed by the combined mouth opening 45 thereof. That is, in the case of a spherical source of sound, such as a diaphragm in the shape of a sphere, the surface 5 of which vibrates radially in accordance with the frequency and amplitude at every point thereon, sound would be radiated through the entire frequency range, including the higher frequencies, in all directions. In case, however, the source from which the sound is radiated constitutes only a segmental portion of a sphere, this uniform sound radiation would still prevail through the solid angle subtending the segmental surfaces and the center of curvature thereof. This condition is approximated by the nested horn assembly 29 to 44, inclusive, by angularly disposing the longitudinal axes thereof so as to form at their combined mouth opening a spherical segment from which the sound is radiated into the atmosphere. It will thus be seen that the individual sound Waves propagated in the various horns will have exactly the same characteristics and will be so timed that they will unite at the combined mouth opening to form a spherical wave front. a The combined throat opening 46 of the various horns 29 to '44, inclusive, is also in the shape of a spherical segment. Preferably, the height of the segmental surface thus formed, as indicated by the dimension A, between the single throat 30 opening of the center horn 43 and the chordal line passing through the throat portions of the outer horns 29 and (Fig. 1) is of the order of one-quarter wave length of the highest frequency sound to be reproduced so as to prevent distortion 5 and/or cancellation of a nearly planar wave front on entering the combined throat opening 46 and emerging from the combined mouth opening 45. It will be seen, therefore,-that since all of the horns are of the same length, intersection B 40 of the longitudinal axes of the horns also forms the center of curvature for both the combined throat openings 48 and the combined mouth openings 45. W

As will be seen in Fig. 3, the combined thrbat portion as indicated by the dotted line 40 is rectangular in shape and opens into the sound passage 25 of the throat block 26, which is of the same rectangular shape and size in cross-section at its upper end (Fig. 4). The passage 25 and 50 the coextensive passage 23 in the adapter 24 gradually change in cross-sectional shape until'they assume a substantially circularcross-section adjacent the .tip of the plug I6.

. Preferably, the annular passage 22, tubular passages 23 and 25, and the passages forming the various individual horns 29 to 44, inclusive, have an exponential rate of taper or exponential rate of cross-sectional increase along the" lengths thereof, increasing from the throat to the mouth portions thereof, this rate being the same throughout. The size of these various passages is determined bythe upper and lower frequency limits. For examplathe size of the throat opening of the annular passage 22, adjacent the diaphragm I3, is determined by the amount of air velocity change between the diaphragm chamber and the annular passage 22. The limiting factor in making the'throat of passage22 as small as possible is the frictional resistance of the air with "the walls of passage 22 when moving at the increased velocity and the harmonic distortion which results when the instantaneous pressures depart'too far from atmospheric pressure. The

number of horns is determined by the solid angle 1 through which the sound. particularly of the tion, an annular trough 46, with relatively smaller the flexibility at the edge to allow the annular diaphragm I3, is of the order of one-quarter wave length of the highest frequency sound intended -to be reproduced, a wave originating at the furpresent case (Fig. 3), the horns are arranged in five vertical rows with three horns in each row. This particular arrangement of horns, however,

may be varied to suit the conditions of the particular auditorium or theatre in which the loudspeaker is to be placed. The length andrate of taper of the various horns is determined by the lowest frequency sound to be reproduced and are preferably such that the smallest dimension of 1 thecombined mouth opening of the various horns is on the order of one half of the wave length of the lowest frequency to be radiated.

. Referring to Fig. 2, it will be noted that the annular diaphragm I3, which inthe present case 15 is constructedof duralumin of the order of .002 inch in thickness, comprises, in radial cross-secannular corrugations 41 and 48 on either side thereof. -These corrugations 41 and 40 increase on trough portion 46 to be reciprocated as a rigid section by the voice coil I2. The sound chamber or sound box .49, as defined by the upper surface of the diaphragm I3 and the lower surface of g the ring I8 and member I8, closely follows the contour of the cross-sectional shape of diaphragm l3. That is, the under surfaces of the ring I6 and member I8, directly above the active portion of diaphragm I3, are similar in shape to a the cross-sectional form of the annular trough 48 and corrugations 41 and 48. However, the curvature of the bottom surfaces of the ring II and member I8, directly opposite the trough portion 46, and which extend thereintmis slightly less 3 than that of this trough portion. The distance from the annular throat opening 55 of the annular passage 22 to the inner and outer edges of the diaphragm sound chamber 4!, or in other words, to the supported edges of the-.40

to be reproduced. In the present case, the dimension is about 0.4 inch. Thus, there will be no undue cancellation of a sound wave of 8000 5 cycles generated in the chamber 40 due to the difference between the time required for a wave originating at the extreme inner or outer edge of the diaphragm I3 to enter the soundopenins II and the time required for a corresponding wave generated at the central portion of the trough 46 of diaphragm II to enter the same sound v H opening '55. If, for example, this distance between the supported edges of diaphragm I! and the opening 55 were as great as one-half the me 3 length of the highest frequency sound intended thermost point in the chamber 4! would be out of phase with a wave originating at a point directly opposite the opening ll and therefore the two waves would cancel each other.

Referring to Figs. 2 and 6, the voice coil I2 is secured to the central portion of the annular trough 46, directly below the annular sound pnssage 22. This construction allows the driving force, 1. e., the coil I2, to be directly in the center of the plimger section, 1. e., the 48 of diaphragm I3, and also directly inlinc with the exit of sound through the passage 22.

As will be noted in Figs. 2 and 6. the driving 1. orvoice coil l2 issecuredtothc ccntralportion of diaphragm II by means of a. cylindrical sup-e port 56 of cardboard or thelike material around which coil I2 is wound. One edge of the support 56 is serrated with alternate serrations l1 and:

back chambers 00 and 02.

58, etc., extending on the same side, horizontally. to form a base which may be cemented or otherwise secured to the under surface of the annular trough portion 4!.

Vents I! (Figs. 2 and 5) in the form of a plurality of holes arranged in a concentric path about the diaphragm it are provided in the inner pole piece 4' to communicate the inner back chamber 60, formed between the back surface of diaphragm ll, pole piece 4, and ring I, with the outer atmosphere. A second series of vents in the form of radial slots ll, provided in the diaphragm supporting ring I4 communicate the outer back chamber ll, formed by the back surface of diaphragm I I, outer pole piece 0 and ring il with the outer atmosphere. These two series of vents thus assist the very narrow spaces between the voicecoil l2 and the adjacent inner andcuter edges ll of the pole pieces I and 4, respectively, in relieving the damping effect on the diaphragmduetothepracticallyenclosed Iclaimasmyinvention: v Lanacousticdevicecomprisingapluralitycf nested horns of equal length and identical shape, the inlets to said horns being arranged on a curve concentric withthe curve upon which the outlets of said horns are arranged, the distance from the inlets of the center horns to a chord connecting the inlets of the outer horns being of the order of one-quarter of the wave length of a frequency at least as high as 8000 cycles.

2. An acoustic device in accordance with claim 1 in which the area of each of said horns increases logarithmically i'rom their inlets to the respective outlets thereof.

3. An acoustic device comprising a plurality of nested horns of equal length and identical shape the inlets to said horns beingarranged on a curve concentric with the curve upon which the outlets of said horns are arranged, the distanoefrom the inlets of the center horns to the center of a chord connecting the outermost, horn inlets beingof the order of one-quarter of the wave highest frequency to be transmitted through said horns.

' cumn FARR-AND,

length of the go

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