US2200490A - Loud-speaker - Google Patents

Description

May 14, 1940. 1.. B. CCJJRNWELL LOUD- SPEAKER Original Filed Dec. 5, 1955 mwzom mum I000 FREQUENCY (CYCLES PER SECOND) Patented May 14, 1940 UNETE STATES PATENT OFFICE LOUD-SPEAKER Lionel B. Gornwell, Stamford, Conn, assignor to Cinaudagraph Corporation, a corporation of Delaware Claims.

This invention relates to electro-acoustical reproducers, and more particularly to loudspeakers.

The principal. object of this invention is to pro- 5 vide a loud-speaker which can be made uniformly responsive over a wide frequency range and which is characterized by a high degree of fidelity and efficiency.

Related objects are to prevent the production of tones or relative changes. in amplitude thereof which are non-existent at the original sound. source, and to prevent change of the response characteristic when different frequencies are superimposed.

Loud-speakers heretofore available are generally subject to the disadvantage of more or-less unfaithful response to the component frequencies I of an original source of sound being reproduced.

The distorted response has sometimes taken the 26 form of over or under emphasis of portions of the total audible range, and sometimes has manifestcd itself by the creation or omission or changed emphasis of overtones at spaced intervals in the frequency range. Attempts to overcome these defects have involved compensation by a. complementary distortion in other circuits of a reproducing system, or the use of a plurality of loud-speakers to average out the defects of individual speakers, but these eiforts at compensation and averaging out add their own complications, increase the cost of the apparatus and have been unsatisfactory in the results attained.

The above noted defects of prior loud-speakers are overcome in the system of this invention by providing a loud-speaker having response characteristics which closely approach the ideal uniform response, and which can be accurately predetermined. Consequently, compensation for defective response is unnecessary.

The invention is carried out by the provision of a vibratory emitting device comprising two elements of predetermined. characteristics with a compliance member joining the elements so as to bring them into desired cooperation.

4 One of these emitting or diaphragm elementsis a light, very rigid vibratory element of relatively small area in one substantially incompressible unit with a light rigid and extremely incompressible actuating member. The combination of the actuating member and small diaphragm element constitutes a unit which is adapted to respond to a wide range of the higher audible frequencies, from the order of about 1,000 cycles per second or below to as high as 16,000

5 cycles to 20,000 cycles per second.

(Cl. ISL-31) The second of the two diaphragm elements is a heavier, less rigid element of relatively large area joined to the first mentioned smaller diaphragm element or to the actuating member through the compliance member. According to a feature of the invention the compliance member is so selected and proportioned as to transmit to the second diaphragm element the lower audible frequencies up to approximately the frequencies at which the first element starts to respond. But the compliance member is prefer ably so resilient as not to transmit vibrations in the higher frequency range.

According to a feature of the invention, the large diaphragm. element and the compliance member attaching it are so formed and positioned with relation to the vibration actuating member that while each is vibratory Within its predetermined range the two ranges are overlapped in definite desired manner, Such relationship permits proportioning of the diaphragm elements and compliance member to create a substantially uniform overall response. By suitable modification, desired. variations of frequency response may be introduced.

A feature of the invention resides in the construction of an actuating coil and its support and its attachment to the small diaphragm element.

Another feature resides in means for centering the actuating element within an air gap containing a magnetic field.

The above and other features will be better understood from the following detailed description of specific embodiments when considered with the accompanying drawing, in which:

Fig. 1 illustrates in section a loud-speaker of the moving coil type having a vibratory actuating member and a diaphragm constructed according to this invention;

Fig. 2 is a plan view of a portion of Fig. 1;

Fig. 3 shows a current carrying coil and support adapted for use in the loud-speaker according to this invention; and- Fig. 4: shows graphically typical response char" acteristics of the loud-speaker of Fig. 1.

Fig. 1 illustrates an electro-dynamic loudspeaker comprising a cylindrical magnetic core or pole piece Ill centrally mounted. in a cup yoke H, to the circular end of which is fastened a flat annular pole piece 12. An annular air gap i3 is formed between the pole pieces and I2. A coil [4 is wound around the pole piece 10 within the yoke H through which is sent a direct current for creating a uni-directional magnetic field through the pole pieces and; air gap 13.

Attached to the pole piece I2 is a supporting annulus l5 to the outer portion of which is fastened the annular bracket 16 flanged in ward'ly at if and I8 to connect with the annulus and to provide an attaching surface for the soft peripheral flange ring 23 of the cone or diaphragm C. The bracket H5 is preferably open in structure to provide for free emission of sound from the rear surfaces of the diaphragm C.

The cone C is formed as a single unit of deposited fibers continuous through the dome Z8, groove 27, high frequency element 28, the low frequency element 2! and the intermediate compliance area 22. The high frequency element 20 is molded by depositing on a suction screen or perforated surface wood cellulose fibers from a watery slurry, the fibers being felted during the precipitation and accumulated to desired thickness of .005 to .02 inch. These fibers will be quite closely and snugly contacted; at the same time the outer portion of the cone forming the low frequency annulus 2| is similarly deposited of different material involving more fluffy fibrous material, such as wool fibers, cotton fibers or rayon fibers, either alone or in desired mixture. The fluffier fibers give a more open porous structure to the portion 2| and provide an inner surface of relatively sound absorbent character. Preferably, the cone is deposited on a gauze of generally conical shape and forming the inner emitting surface of the cone but the precipitation may be in an opposite direction wherein the gauze surfaces correspond to the lower surfaces of the structure shown in Fig. 1.

During the precipitation the outer portion 2i is separated from the inner portion at, it being supplied with a different slurry, but in the region 22 corresponding to the built-in compliance these regions overlap one half an inch or less so that the different fibers are interlaced and felted into a continuous unitary formation.

After precipitation the cone may be pressed if desired and dried, and after drying is preferably stiffened by coating or impregnation of the high frequency cone portion 20 including the central dome, this impregnation giving the desired rigidity while at the same time maintaining the structural lightness. The low frequency cone portion 2! is preferably coated on its outside or lower surface throughout a portion thereof and down to near a compliance area 22, the degree of coating or impregnation being sufficient to give the desired flexibility while maintaining the surface desired sound emitting or sound absorbing characteristic.

The cone C is supported by the soft peripheral flange ring 23 at the outer edge and by the. centering ring 24 engaging an intermediate point of the high frequency plunger or annulus 20, 21, 28 by a ring 25. The centering ring 24 is supported at its outer periphery on the ridge ring 26 and is preferably corrugated as shown at 12! to give it desired flexibility to follow the motions of the cone. This ring 24 is also made with air openings through it either by forming the ring of open net work structure, such as stiffened cheese cloth or gauze with free openings between the strands, or the ring may be formed of solid material punched with openings to relieve the air pressure in the space between the ring and the pole piece I 2.

The thickness of the low frequency material will generally be greater than that of the high frequency element and may be .035 inch or even thicker. While the cone C of Fig. l is described as being made by precipitation of fibrous matcrial a similar cone may also be formed of fabric material of the closely woven cotton type, for instance, impregnated and molded to shape under heat and pressure and formed with the high frequency and low frequency elements stiffened to give the desired relative response characteristics and having between them a relatively soft area forming a compliance ring of the fabric itself. The front face of the low frequency element will preferably be napped to soft sound absorbing condition while the stilfening of this low frequency element is attained by coating with lacquer or like applied to the rear surface.

The inner circular edge of the cone C is formed with an apex groove 21 in which is cemented the edge of a thin mica cylinder 3| carrying the voice coil. The cylinder 3!, shown enlarged in Fig. 3, acts as a voice coil support. It is made from a narrow continuous strip of mica having a thickness of the order of .002 inch and having numerous cracks 4| parallel to the axis of the cylinder at short intervals around the circumference for the purpose of rendering the strip sufficiently flexible to be rolled into a substantially perfect cylinder which is, nevertheless, substantially incompressible in the direction of the axis. A current carrying coil 31 is supported at the end of the cylinder opposite that to which the annulus 27 is attached. This coil is composed of two layers 38 and 39 which may total about 50 turns of No. 36 B. & S. gauge enameled copper or aluminum wire. The cylinder may be inserted only between the last two or three turns of the two layers as shown. The entire coil assembly. as thus formed, with its enameled wire is SCLtlP rated by a suitable binder such as hard shellac so that upon drying of the binder there result. an extremely light and rigid coil and support encasing the coated wires in a solid hardened mass. The entire unit consisting of the coil, the support and the annulus 20, 21, 28 rigidly cemented to the support, constitutes an extremely light, rigid and substantially incompressible unit which is thereby enabled to vibrate at extremely high frequencies.

It has been found that in a magnetic circuit of the type shown, the field intensity which exists in the air gap at region 4-0 slightly inside the inner edge of the pole piece !2,is of practically the same value as that directly between the adjacent surfaces of the pole pieces. Consequently, it has been found possible, and even preferable, to extend coil 3'! a substantial distance below the gap of the opposed pole piece faces, as shown in Fig. l, to take advantage of the magnetic field in th s region 40. By this extension the electro-acoustical efiiciency of the loud-speaker is brought to a high value.

In operation of the loud-speaker, a constant direct current is sent through coil 31 thereby creating a steady uni-directional magnetic hold at air gap I3. The audible frequency currents corresponding to the original sound are im pressed on the voice coil 37 thereby causing this coil and its rigid mica support 3! to vibrate parallel to the cylindrical axis at every frequency present in coil 31 and with proportionate amplitude. The coil 37 is maintained concentrically within the annular air gap 13 by the centering device which readily yields to vibration in the axial direction of the coil support, but permits no movement in the radial direction.

Owing to the extreme lightness and rigidity of the coil, the coil support and the annulus 20, 21,

2.3., these members vibrate as a unit at high frcquencies of 1,000 cycles and above. The area of the annulus is adequate for radiating such high frequencies, but is insufficient to support appreciable sound radiation at lower frequencies, below the order of about 1,000 cycles per second for example. At still higher frequencies, for example about 4,000 cycles per second and up to about 16,000 to 20.000 cycles per second, I find that the area in full vibration will decrease and only the inner portion of the annulus at and near the inverted V 01 is in vibration.

The ability of the speaker to radiate at such high frequencies is dependent on the coil vibrations reaching the annulus, or at least its inverted v. This requires that the coil support be extremely incompressible in its axial direction so that practically none of the amplitude of vibra tion is absorbed in the support. The sheet mica used for the support is best suited for the purpose, for it is a crystalline substance having an extremely hiya ratio of stiffness to its weight. Youngs modulus of elasticity for this sheet mica is approximately 30 10+ lbs. per square inch and its specific gravity is only about 2.9. Hence the ratio of modulus of elasticity to specific gravity is as much as about l 10+ This fact coupled with the fact that the mica can be prepared in a sheet as thin as .002 inch makes it adaptable for transmitting the vibrations from the coil to the annulus 20, 21, 20.

The outer part of cone 0 is not affected by the vibrations in the higher frequency range, for example above 1,000 cycles per second, because the compliance ring 252 is made too resilient to transmit such high frequencies from the coil. support to the cone. In the lower frequency range, for example from about 1,000 cycles per second down to about 30 cycles per second, however, the compliance ring is sufficiently stiff to transmit the vibrations to the cone. Although the annulus 20, 2'1, 28 also vibrates at these lower frequencies it does not support much sound radiation because of its small area. The lower the frequency the greater is the vibrating area required; so the relatively large area of the outer part of cone supports practically all of the low frequency radiation.

The cone or diaphragm C may be formed of a single fibrous material paper or textile and according to my invention will generally have a front emitting surface rough or napped to be absorbent and non-reflective of relatively high frequency vibrations. The rear surface of the paper or cloth sheet forming the cone may be stiffened as by lacquering to give rigidity according to the low frequency characteristics desired.

A feature of fundamental importance in the diaphragm construction is the fact that the inner annular element 2.0, 21, is separately vibrating. t is also generally of relatively smallarea as shown. A larger area for this element results in an overall response curve which is over-accentuated in an intermediate region of the frequency range, such as around 1,000 or 2,000 cycles per second. I have discovered that this over-empha sis can be avoided and the characteristics of the response controlled as desired by attaching the outer cone 2! to the small rigid high frequency element through the medium of a compliance de vice. This permits the response of element to be accurately predetermined and combines their performance as desired.

There is a range of frequency in the neighborhood of 1,000 cycles per second, for instance,

wherein the inner and the outer diaphragm elements, while falling off in audible emission both contribute toward sound radiation. The diaphragms can be designed so that in this range of transition the radiations from the two diaphragm elements complement each other and build up a total response substantially equal to that of each separate diaphragm element in its own range of full efficiency. This is shown graphically in Fig. i, which is a typical graph of response plotted against a logarithmic scale of frequency. Curve A illustrates in somewhat idealized form the response of the outer cone 2!, and curve B is a similar curve showing the response of the inner annulus 20. Curve C is the summation of curves A and B and indicates that the response may be maintained substantially uniform in the transition region where both response curves fall off.

If the inner annulus be made too large it will contribute too much response in this transition range which results in the above noted over-emphasis in an intermediate range.

An important effect of segregating the inner and outer vibratory elements by the compliance ring is that the development or over-emphasis of some harmonics and the cancellation of other harmonics, with reference to the original sound, is minimized. Such distortion effects would tend to be developed in the larger outer cone if it were allowed to be vibrated at high frequencies above about 1,000 cycles per second. Another very beneficial result is that the response at one frequency is not appreciably altered by the simul taneous presence of vibrations of another frequency. Such ohiectionable alteration of re spouse is commonly experienced in prior known speakers. The effect of joining the larger outer cone through a compliance ring at a position close to the voice coil support or inner annulus may be succinctly stated to be a resultant definite control of the response characteristics to give any desired response curve from the lowest tones to 16,000 or even 20,000 cycles. There may be obtained a nearly ideal uniform response charac teristic over a wide range of frequencies (about 30 to 16,000 cycles per second) in which peaks and dips in the response are minimized, and which is not subject to variation in the presence of a plurality of simultaneous frequencies.

To determine the proper characteristics of a proposed compliance, which characteristics may, of course. be varied, to vary somewhat the relation between the high frequency element and the low frequency cone, it is only necessary to fasten the outside of the compliance ring and determine the frequency response of the attached inner high frequency element and then fasten the inside of the compliance ring stationary and determine the low frequency response of the outer cone element. These characteristic responses are substantially reproduced if precisely the same compliance ring is used between these parts connected to the same voice coil.

A 12" diameter should be sufficient for most purposes for the low frequency cone, the plunger part being correspondingly less than i" in diameter. The diameter of the smaller sizes is 6" or less for the cone with a closed-over plunger of a diameter of about 2" including the flange, the dome itself being not much greater than 1" and the diameter of the voice coil not much in excess of 1".

Owing to the extra area supplied by the dome 28, the response of the inner diaphragm element extends down to somewhat lower frequencies than if the dome were omitted; and because of the increased yielding of the compliance ring the upper frequency at which the outer cone is responsive is lower than in the arrangement of Fig. 1. Thus the proportions of the inner and outer elements may be regulated to provide an overall uniform response or any desired modification thereof.

The response curve has a growing amplitude starting with hardly audible reproduction at 30 and increas ng gradually at 40, 50, 60 and '70, at which point it has begun to round off to a substantially flat value at around 80. This low frequency curve will remain reasonably flat with small variation to about 800, and then will. taper on" in controlled manner to become inaudible at some point above 1,990, depending upon the pickup characteristics of the high frequency ring. The high frequency ring or plunger will not be audible to any substantial extent at the lower frequencies below 500, and will very gradually build up to around 700 or 800, where it will come in at somewhat the same rate as the low frequency cone is tapering off so that the sums of the responses of these two elements will remain very flat and very much the same, and above the point of disappearance of the low frequency reproduction the high frequency will remain substantially flat or have such other characteristics as are best adapted for cooperation with the particular system in which it is installed, the controls being predetermined in accordance with the preceding description.

Where as is rarely desired, the low frequency reproduction is to be somewhat suppressed, this can best be done by properly relating the area between the high frequency element and the low frequency cone reducing the area of the latter the desired amount.

Where the higher frequencies are to be ac centuated the dome 28 may take on a more sharp- 1y conical form with the flange 2U enlarged to considerably greater area than the dome. The low frequency cone 2! of a single layer for instance, could be attached by a softened portion 22 or compliance ring, and beyond the softened portion may be stiffened as by lacquering to give the desired low frequency characteristics.

The invention is not confined to the specific embodiments shown and may be advantageously embodied in other constructions. For instance the outer portion of the conical diaphragm may be directed re-entrantly backward to give a fiatter more compact assembly and various other modifications may be resorted to within the principle of the invention and within the scope of the appended claims.

This application is a divison of my application Serial No. 53,018, Patent No. 2,084,945 of June 22, 1937.

I claim:

1. In a loud-smaller, a generally conical diaphragm consisting of a single continuous unit of deposited fibers having an inner high frequency sound producing portion, an outer low frequency sound producing portion and an annular compliance portion interposed between and connected to said inner and outer portions, and a voice coil attached to the inner portion substantially within its outer edge, the inner or high frequency portion being light, rigid, of rela tively small area and formed of relatively nonfluffy fibers closely compacted, the outer or low frequency portion being heavier, less rigid, of relatively large area and formed of relatively soft fibers less closely compacted and the compliance portion being formed of a blended mixture of the fibers of the inner and outer portions, said compliance portion being so positioned and formed as to transmit from the inner or high frequency portion to the outer or low frequency portion such vibrations as to render the outer portion vibratory and sound-producing Within a predetermined range of low frequencies with the upper portion of said range so overlapping the lower portion of the useful range of response of the inner or high frequency portion as to maintain a generally uniform response in said overlapping range.

2. A loud-speaker as set forth in claim 1 in which the inner or high frequency portion of the diaphragm is formed substantially of wood cellulose fibers and the outer or low frequency portion of the diaphragm is formed substantially of cloth-like fibers.

3. A loud-spealrer as set forth in claim 1 in which the inner high frequency portion of the diaphragm includes a forwarding protruding central dome positioned radially within the periphery of the voice coil.

4. A loud-speaker as set forth in claim 1 in which an exterior centering device for the diaphragm and voice coil is provided with the device engaging the inner portion of the diaphragm around a perimeter positioned between the outer edge of the inner portion and the point of attachment of the voice coil to the inner portion.

5. In a loud-speaker, a generally conical diaphragm consisting of a single unit of deposited fibers having an inner high frequency sound- LIONEL B. CORNWELL.

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