US2127110A - Acoustic device - Google Patents

Description

c. FARRAND ACOUSTIC DEVICE Aug. 16, y 1938.

Filed Aug. 3, 1956 2 SheeJts-Sheet 1 www o E L ,M ,o f

Aug. 16, 1938.4 c. l.. FARRAND.'

ACOUSTIC DEVICE Filed Aug. 3, 1956 ZISheets-Sheet 2 INVENTOR. CLA/z2 L. F.'AQQA/va ATTORNEY.

Patented Aug. 16, 1938 2,127,110:

UNITED STATES PATENT CFFICE ACOUSTIC DEVICE Clair L. Farrand, Beverly Hills, Calif., assigner to United Research Corporation, Burbank, Calif., a corporation of Delaware Application August 3, 1936, Serial No. 94,072

10 Claims. (Cl. 181-31) This invention relates to loud speakers. is provided around the core 3, and receives a It is well known in the art to allocate a part of Supply of direct current so as to produce a conthe acoustic range of a sound reproducing appastant magnetic flux between the two separated ratus toy one type of loud speaker and another pole pieces 4 and 6.

5 part or parts to another type (or other types) of As will be noted in Fig. 2, the adjacent outer 511 loud speaker. and inner edges of the pole pieces, 4 and 6, ref This invention relates to the type of loud spectively, are undercut as at I0, leaving only a speaker useful in a so-called two-way system, short length of parallel, closely adjacent,`pole wherein the acoustic range is handled by only faces at I|. The space thus provided between l two types of loud speakers, the type of loud the parallel faces II of the pole pieces 4v and 6, l l

speaker of the present invention being particuaccommodates the voice or driving coil I2 of an larly adapted to reproduce acoustic frequencies annular diaphragm I3. The diaphragm I3 is from about three hundred cycles to the upper supported below its inner and outer edges by limit, i. e., eight thousand cycles or higher. means of inner and outer concentric supportingl The objects of the present invention comprise rings I4 and |41, respectively. Upper concentric 1%? an `efficient reproduction of sound waves over a clamping rings l5 and I6 Clamp the edges 0f tho wide frequency range, with large power, and diaphragm upon the supporting rings I4 and I41 while attaining a substantially uniform distribuby means of bolts I'I.

tion of the soundwaves, particularly of the higher A frusto-conical member I8, with a co-extenfrequencies which have a tendency to remain in sive conical member I9, secured together and to 20 a narrow beam. the upper surface of the inner clamping ring I5 The novel features of one embodiment of rthe by means o-f bolts 20 and the central bolt 5, and invention will be described in the accompanying a casing member 2|, all concentric with the diaspecication, and particularized in the appended phragm I3, form an annular sound passage 22 25 claims in connection with the .accompanying leading to the central portion of the annular 25 drawings, wherein; diaphragm I3.

Fig. 1 is a sectional elevational View of a loud The annular sound passage 22, at the tip of the speaker embodying the present invention. conical member I9, communicates with a tubular Fig. 2 is an enlarged fragmentary sectional passage 23 formed in an adapter 24. This pas- View of the annular diaphragm and the accom- Sage 23 iS CO-EXGDSNB With a Similar passage 25 30 panying sound chamber forming parts. formed in a throat block 26.

Fig. 3 is a front view looking toward the mouth Adapter 24 is fitted in a Counter-bored portion portions of the nested horn assembly. on the topi of the casing member 2|, and has an' Fig. 4 isl a plan sectional y View through the annular shoulder portion 241, which is engaged by throat block and is taken along the line 4-4 of a locking nut 21 threadably secured to the cas- 35 Fig, 1 ing member 2|. 'Ihe throat block 26 is thread-r Fig. 5 is 'a sectional plan view through the ably scoured at 261 to thoupper portion of lower set of diaphragm clamping rings and is adapter 24. The upper portion of the throat taken along the line 5 5 of Fig. 1. block 26 is formed in a funnelfshape having Fig. 6 is a. bottom plan View of a portion of the diverglg flanges 28, S0 aS t0 IeCeVo the thIOa 40 annular diaphragm showing the connection of DOI'OHS Of a plurality `0f 110m SSCOHS 29`t0 44, the Voice coil thereto. inclusive. A suitable cement 54 is provided to Referring now to Figs. 1 and 5, the driving unit secure the throat portions of the various horns I of the loud speaker comprises a U shaped magtogether between the flanges 28 of the throat netic frame 2, having a central core 3 of magblock 26. These varioushornsare all identical 45 netic material secured thereto. A circular plate in Shape and SiZey having their IODgtudinal aXeS` 4, comprising an inner pole piece, is provided on diverging from each other away from their throat the upper surface of the core 3, and this asportions. This arrangement of nested horn secsembly, including the frame 2, core 3, and pole tions eliminates the well known directional or piece 4, is secured together by means of a bolt 5. beam effect of the higher'frequencies by means 50 An annular plate 6, comprising an outer pole of the approximate spherical surface formed by piece, is secured to the magnetic frame 2 by the combined mouth opening 45 thereof. That is, means of bolts with its inner edge concentric in the case of a spherical source of sound, suchl with and closely adjacent to the outer edge of the as a diaphragm in the shape of a sphere, the surinner pole piece 4. An electro-magnetic coil 9 face of which vibrates radially in accordance 55 `planar wave front on entering the combined 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. 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 opening of the center horn 43 and the chordal line passing through the throat portions of the outer horns 29 and 35 (Fig. l) is of the order of one-quarter wave-length of the highest frequency sound to be reproduced so as to prevent distortion and/ or cancellation of a nearly 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 of the longitudinal axes of the horns also forms the center of curvature for both the combined throat openings 46 and the combined mouth openings 45.

As will be seen in Fig. 3, the combined throat portion as indicated by the dotted line 461 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 the co-extensive passage 23 in the adapter 24 gradually change in cross-sectional shape until they assume a substantially circular cross-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 by the upper and lower frequency limits. For example, the size of the throat opening of the annular passage 22, adjacent the dia: phragm 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 passage 22 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 through which the sound, particularly of the higher frequencies, is to be distributed. In the present case (Fig. 3), the horns are arranged in ve 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 loud speaker is to be placed. The length and rate 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 the combined mouth opening of the various horns is on the order of one-half of the wavelength of the lowest frequency to be radiated.

Referring to Fig. 2 it will be noted that the annular diaphragm I3, which in the present case is constructed of Duralumin on the order of .002 inch in thickness, comprises, in radial cross-section, an annular trough 46, with relatively smaller annular corrugations 41 and 48 on either side thereof. These corrugations 41 and 48 increase the flexibility at the edge to allow the annular 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 surfaces Aof the ring I6 and member I8, closely follows the contour of the cross-sectional shape of diaphragm I3. That is,-the under surfaces ofthe ring I6 and member I8, directly above the active portion of diaphragm I3, are similar in shape to the cross-sectional form of the annular trough 46 and corrugations 41 and 48. However, the curvature of the bottom surfaces of the ring I6 and member I8, directly opposite the trough portion 46, and which extend thereinto, is slightly less 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 49, or in other words, to the supported edges of the diaphragm I3, is of the order of one quarter wavelength of the highest frequency sound intended 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 cycles generated in the chamber 49 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 sound opening 55 and the time required for a corresponding wave generated at the central portion of the trough 46 of diaphragm I3 to enter the same sound opening 55. If, for example, this distance between the supported edges of diaphragm I3 and the opening 55 were as great as one-half the wavelength of the highest frequency sound intended to be reproduced, a wave originating at the furthermost point in the chamber 49 would be 180 out of phase with a wave originating at a point directly opposite the opening 55 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 tro-ugh 46, directly below the annular sound passage 22. This construction allows the driving force, i. e., the coil I2, to be directly in the center of the plunger portion, i. e.. the trough 46 of diaphragm I3, and also directly in line with the exit of sound through the passage 22. .As will be noted in Figs. 2 and 6, the drivingl or Voice coil I2 is secured to the central portion of diaphragm I3 by means of a cylindrical support 56 of cardboard or the like material around which coil I2 is wound. One edge of theV support 56 is serrated with alternate serrations 51 and 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 45.

Vents 59 (Figs. 2 and 5) in the form of a plurality of holes arranged in a concentric path about the diaphragm I3 are provided in the inner l pole piece 4 to communicate the inner back chamber 60, formed between the back surface of diaphragm I3, pole piece 4 and ring I4, with the outer atmosphere. A second series of vents in the form of radial slots 6I, provided in the diaphragm supporting ring M1, communicate the outer back chamber 62, formed by the back surface of diaphragm I3, outer pole piece 6 and ring M1, with the outer atmosphere. These two series of vents thus assist the very narrow spaces between the voice coil I2 and the adjacent inner and outer edges II of the pole pieces 6 and 4', respectively, in relieving the damping ei'ect on the diaphragm due to the practically enclosed back chambers 60 and 62.

I claim:

1. An acoustic device comprising an .annular diaphragm having a single annular trough there- Y rectly opposite said sound ein't opening in said i housing.

3. An acoustic device in accordance with claim 1 in which said housing forming said sound box has a convex portion thereof extending into said sound box and into the concave trough portion of said diaphragm and conforming to the outline of said trough. l

4. An acoustic device comprising a sound producing unit having an annular diaphragm, iiexible corrugations at the edges thereof, a casing forming a sound box for supporting the edges of said diaphragm, said sound box having a sound exit opening therein, a common throat portion for said sound box, a plurality of horns connected to said throat portion, the inlets of said horns being arranged .along an arc concentric with the arc along which the outlets of said horns are arranged, the distance from the chord connecting the inlets of the outside horns, to the inlets of the center horns, and the distance from the edges of the annular diaphragm to the sound exit opening of said sound box portion being of the order of one-quarter wavelength of the highest frequency to be produced, and means for driving said diaphragm.

5. An acoustic device in accordance with claim 4 in which the back of said diaphragm is connected with the open air through a plurality of vents.

6. An acoustic device in accordance with claim 4 in which said last mentioned means comprises a voice coil attached to the center of the convex side of said diaphragm, said attachment comprising a serrated medium uniformly distributed on said diaphragm about the line of attachment to said coil.

7. A sound reproducer comprising an annular diaphragm having a central annular curved trough body portion, a casing forming a sound box for said diaphragm, and means for mounting said diaphragm, said body portion being supported by said mounting means by flexible integral inner and outer circumferential corrugated portions concentric with said diaphragm.

8. A sound reproducer in accordance with claim "7 wherein the corrugations of said corrugated portions are small in comparison with said curved trough body portion.

9. A sound reproducer in accordance with claim 7 in which said sound box casing has a convex portion thereof extending into and conforming to the curvature of the annular curved trough body portion of said diaphragm and also conforming to the curvature of said corrugated portions.

10. An acoustic device comprising an annular diaphragm having a central annular curved trough portion with corrugated flexible inner and outer portions, a casing forming a sound box surrounding said diaphragm, said casing having a convex portion thereof extending into and conforming to the trough body portion of said diaphragm, and a driving coil mounted along the central portion and on the convex side of the trough body portion of said diaphragm.

CLAIR L. FARRAND.

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