US3046362A - Speaker - Google Patents

Description

July 24, 1962 s. F. w/HnE 3,046,362

SPEAKER,

Filed Nov. 6, 1956 by awww-f 16727515.

Unite This invention relates to a speaker and it is the primary object of this invention to provide a speaker which will faithfully and efficiently reproduce sound over a wide range of frequencies without requiring a cumbersome or elaborate bafiiing arrangement, and which is rugged and reliable in operation and easily and economically manufactured.

In a speaker constructed according to this invention, magnetic field-producing means, such as permanent magnets, are arranged to produce a radially oriented eld acting between inner and outer coaxial ring-shaped pole pieces which dene an annular air gap in which a voice coil moves axially. With this arrangement, the magnetic structure can be made highly efficient and it is also possible to effectively utilize special magnetic materials, as will be described. In addition, the magnet structure is open at its center so that it can have a large diameter and thus minimize acoustical interference with vibrations of a diaphragm connected to the voice coil. In a preferred arrangement, the voice coil is connected to the periphery of the diaphragm, the diaphragm being thus disposed within the voice coil, land it is not necessary to provide any support for the diaphragm additional to that provided by the voice coil.

A further feature of the invention is in the provision of an improved voice coil centering and support means which is particularly advantageous for large diameter voice coils.

Additional features of the invention reside in the construction of the diaphragm and particularly to the use of special materials and special configurations to obtain high rigidity, low weight and distortionless reproduction with a wide angle dispersion of sound.

This invention contemplates other objects, features and advantages which will become more fully apparent from the -following detailed description taken in conjunction with the accompanying drawings which illustrate preferred embodiments and in which:

FIGURE 1 is a lfront elevational view of a speaker constructed according to the principles of this invention;

FIGURE 2 is a sectional view taken substantially along line II-II of FIGURE l;

FIGURE 3 is a sectional view similar to FlG-URE 2 of a modified speaker; and

FIGURE 4 is a sectional view similar to FGURES 2 and 3, illustrating another modified form of speaker.

Reference numeral generally designates a speaker constructed according to the principles of this invention.

The speaker 10 comprises a diaphragm 11 which is ccnnected at its periphery to a voice coil 12 which is movabley in an annular gap -between an inner ring 13 and a ring 14 which is integrally secured to an outer ring 15. One or more permanent magnets 16 are disposed between the inner and outer rings 13 and 1S, the magnets being radially magnetized.

To support the diaphragm 11 and the voice coil 12, a resilient suspension ring 17 is connected to and surrounds the voice coil 12, the peripheral edge portion of the suspension ring 17 are being held between a clamp ring 13 and a spacer ring 19. The clamp ring 13 and the spacer ring 19 are held together in clamping relation and are supported by means of screws 20 extending therethrough and threaded into a support ring 21 which is tack welded or otherwise secured to the outer ring 15.

States arent ICC A gasket 22 of cork or the like is cemented to the clamp ring 18 for mounting of the speaker, and aligned holes are provided through the rings 22, 18, 19 and 21 at peripherally spaced points for passage of mounting bolts therethrough, such holes being designated by reference numeral 23.

The support ring 21 is provided with a series of openings 24 to permit free passage of air to and from the space surrounded by the voice coil 12 and the rings 14, 17, 19 and 21.

Important features of the invention reside in the construction of the diaphragm 11. In particular, the diaphragm 11 is made of a rigid material with voids or pockets distributed throughout the material to greatly decrease the density thereof. By decreasing the density, the thickness of the material is increased relative to the diameter of the diaphragm and by increasing the thickness, the rigidity of the diaphragm against bending is increased. This decreases the tendency toward production of nodal vibrations or breakup and hence decreases generation of harmonic distortions, particularly at higher frequencies. The material should also have a comparatively high lmodulus of elasticity for rigidity, and a high-tensile strength to permit maximum reduction in density.

Foam plastic materials are particularly suitable for the diaphragm 11, and a preferred material is expandable polystyrene. This material is :available in the form of beads or pellets -which may be placed between a pair of dies and expanded by heating, as by applying steam, to form a foam filling the space between the dies, after which the material -is cooled to allow it to set. The diaphragm can thus be readily formed from this material and in addition, this material has closed pores and is relatively hard. vSince the material fills the space between the dies and since the pores are closed, both the front and back surfaces are smooth, continuous and uninterrupted by pores or other openings to provide a satisfactory sound generating surface. This is to be contrasted with openpored foam materials which have inherently less stiffness and which are rough surfaced and therefore less satisfactory in generating a precise wave front.

However, it is important that the material be properly used, or otherwise it will provide no appreciable advantage over conventional felted paper diaphragms. Contrary to what might be expected, it is very important that the material be expanded to the maximum extent commensurate with its strength against cracking and crushing. By so expanding the material, it will have a low density, a large thickness relative to the diameter ofthe diaphragm, and the highest possi-ble rigidity. It 'is additionally important that the diaphragm have the proper mass. The efficiency of the speaker drops -oif above a certain frequency which is dependent to Ia certain extent upon the power level and which is increased and decreased as the mass is decreased and increased. However, with a given density, decreasing the mass decreases the thickness of of the diaphragm and decreases rigidity to reduce the frequency 4at which break-up of the diaphragm occurs.

In order to provide the highest possible frequency of efficient distortionless operation, the mass should be such that break-up and the mass-effect efficiency drop-off (with fthe normal power `level of operation) occur at approximately the same frequency.

It is possible that materials other than expandable or foam plastic materials may ibe used, such as papiermache, for example. In lany case, it fis important that the diaphragm have voids or air pockets distributed throughout the material to obtain a density as low as possible commensurate with the strength against cracking and crushing, and that the mass be such that break-up and the mass-effect efficiency drop-off (with normal power aca-esse level of operation) occur at approximately the same frequency.

The diaphragm may be fiat or yof the more rigid conical shape as is conventional, but preferably has a special shape forming a further feature of the invention. In particular, the diaphragm 111 comprises a generally domeshaped center section having a convex forward surface 26 and a concave rearward surface 27. The outer edge of the center section 25 is integrally secured to the inner edge of a frusto-conical section 2S or brim which has an vou-ter edge portion -secured to the voice coil, the outer edge of the frusto-conical section 23 being displaced forwardly from the inner edge thereof. Thus the central dome-shaped portion 25 is re-entrant, i.e., projects in the opposite direction from the brim portion 28 which holds it, to produce a diaphragm which has an axial dimension which is yonly about `one-half or less of that of conventional speaker cones, While possessing greatly increased inherent rigidity.

This construction has certain very important advantages. In particular, the forward surface of the domeshaped section, being convex and unobstructed, provides a wide angle dispersion `of sound waves especially at the higher frequencies which is very important. In addition, this construction provides a truss-like design to obtain a rigidity which is much greater than that which would be obtained with a conical diaphragm of the same mass, material and axial depth.

It can be shown that for maximum rigidity relative to the mass and axial depth of the diaphragm, the axial depth of the dome-shaped center section 25 and the axial depth of the outer frusto-conical section 28 should be approximately the same with the dome-shaped center section 25 having an outside diameter approximately equal to or slightly greater than one-half `of the diameter of the outer section 23.

It should 'be noted that in `order to obtain maximum rigidity, the center Isection 25 should have the general form of a catenary while to obtain optimum dispersion of sound waves, the front surface 26 should have a parabolic shape. Actually the catenary shape is very close to the parabolic shape, `and the exact shape is not cnitical, so that any generally parabolic shape is satisfactory.

According to a further feature of the invention, the thickness of each portion of the diaphragm is proportioned to the bending stresses applied thereto, so as to obtai-n maximum rigidity relative to the mass of the diaphragm. With the illustrated diaphragm configuration, vibration cf the diaphragm will produce a greater bending stress `at the center of the dome-shaped center section 2S than that produced adjacent the periphery thereof and hence the dome-shaped section is thicker at the center and is made progressively thinner as the outer edge thereof is approached. For the same reason, `the outer section 28 is thicker `at an intermediate point indicated by reference numeral 29 than it is at points 30 and 3d respectively adjacent the outer and inner edges thereof. At the extreme `outer edge, the `diaphragm is made thicker to facilitate `securing it to the voice coil, and 'the diaphragm is also made thicker at the junction between the inner and outer sections to facilitate forming of the same.

The optimum design can be determined by applying design procedures such as used to obtain a maximum strength of beams.

The suspension ring 17 forms a specific feature of the invention. This ring is preferably of a sponge-like elastomeric material. In assembly of the speaker, the voice coil l2 is cemented to the outer edge surface of the diaphragm, and the ring I7 is then cemented to the voice coil. The assembly is then installed in the speaker frame, and is so positioned that the voice coil moves freely in the air gap. The clamp ring 18 is then moved toward the spacer ring t9 by means of the screws Z0, to securely hold the outer edge portion of the suspension ring ll'7 in position. The speaker is thus readily assembled.

The suspension ring i7 has a further advantage in that it tends to prevent canting of the diaphragm 1I which might cause the voice coil to rub against one or the other of the pole pieces. In particular, because of the use of a sponge-like material, the suspension ring 17 can be made relatively thick, so as to be connected to the diaphragm at points spaced axially a substantial distance apart compared to the radial distance between the periphery of the diaphragm and the annular support defined by the rings I3 and I9. With this arrangement, the tendency toward canting of the diaphragm is greatly reduced. The ratio of the thickness of the centering ring 17 to the distance between the rings i8 and I9 and the outer periphery of the diaphragm should be at least 0.1 and preferably 0.25 or greater.

It is to be noted that by this voice coil at the periphery of the diaphragm, the diaphragm` is supported solely by the voice coils, and no separate support means is required. This arrangement has the further advantage in that it permits use of the magnetic field structure in which the air gap is between two rings, and there is no structure directly behind the diaphragm to block or interfere with sound waves emanating from the back surface of the diaphragm. It has been found that by this feature, the fidelity of reproduction is greatly increased.

The magnetic field structure in which the air gap is between two annular pole pieces is, by itself, an important feature of the invention. In particular, the field structure can be of comparatively large diameter and, for a given Weight of magnet, can have a very short axial length, to provide a compact construction. Conversely, for a given axial depth of speaker, the field structure can be very heavy. A further advantage is that the voice coil can have a large area compared to the volume thereof, to increase heat dissipation and the power handling capabilities of the speaker.

Still another advantage of this arrangement is that it permits optimum use of certain special magnetic materials. In particular, the magnets 16 may be of a ferrite material which is low in cost and has a high energy relative to its mass, but which has a low saturation of flux density. Because of the use of annular pole pieces, the magnets can have a short length in the direction of magnetization and a relatively large area transverse to the direction of magnetization, to make optimum use of this material.

A single annular magnet could be used, but preferably a number of segments of ferrite material are used, to facilitate manufacture of the same. By way of illustrative example and not by way of limitation, 66 ferrite blocks are used, each having a thickness of about 0.375 inch, a length of about 2.25 inches and a width or arcuate dimension of about 0.75 inch, the outside diameter of the ring f3 being 15.345 inches and. the inside diameter of the ring l5 being 16.145 inches. The inner and outer faces ofthe magnets may be curved to approximately the curvature of the cylindrical surfaces of the rings 1S and l5, but preferably are curved to correspond to the smaller diameter rings, to allow use of the same magnets with smaller sized speakers. After the magnets I6 are installed in position, a ring 32 may be cemented to the rear face of the magnet assembly to prevent removal of the magnets and to improve the appearance of the speaker. The ring 32 may be of any non-magnetic material, preferably plastic or rubber.

With the magnet assembly constructed as above described, the diaphragm Il has a diameter of 15.356 inches with the other dimensions being in the proportion illustrated in the drawings. The diaphragm 11 is of expandable polystyrene, having a density of approximately 2-3 pounds per cubic foot, the total weight of the diaphragm being approximately 1.75 ounces.

The voice coil I2 comprises a single layer of No. 29 wire wound on a form having a thickness of 0.005 inch and the gap between 'the rings 13 and 14 is approximate- 1y 0.042 inch.

The suspension ring 17 is preferably of an elastomeric foam plastic material, most preferably polyurethane, which is comparatively resistant to cold flow. A large number of the holes 24 should be provided, preferably 48 holes equally spaced.

It will be understood that the above dimensions and characteristics are given by way of illustrative example, to fully apprise those skilled in the art as how to practice the invention, and that they may be varied as practical considerations require, and to make optimum use of new or ditferent materials.

Referring now to FIGURE 3, reference numeral 33 generally designates a modied form of speaker which is very similar to that of FIGURES l and 2 and comprises a diaphragm 34 substantially the same as the `diaphragm 111 which -is secured at its periphery to a voice coil 35. The Voice coil 35 moves in an air gap between an inner ring 36 and a ring 37 integrally secured to an outer ring 38. One or more magnets 39 are disposed between the inner and outer rings 36 and 38. The magnets 39 are preferably of alnico, or a similar material, which is not li-mited with respect -to saturation flux density, so that the radial dimension of Ithe magnets may -be much greater than the radial dimension in the arrangement of FIG- URES l and 2 designed for ferrite materials or the like having a lower saturation flux density. With this arrangement, a large amount of magnetic material can be used to obtain high efficiency and high power-handling capabilities, and yet the speaker is comparatively compact.

The speaker 33 also has a modified voice coil suspension arrangement. In particular, a pair of rings 40 and 41 of flexible Isheet material are arranged in spaced relation and are cemented or otherwise secured to spaced points of the voice coil 35. The outer peripheral portions of the rings 40 and 41 are held in fixed relation to the magnet structure by means of three rings 42, 43 and `44 secured to the ring 37 by means of a plurality of screws 45. The peripheral portion of the suspension ring 40 is held between the rings 42 and 43 while the peripheral portion of the suspension ring 41 is held between the rings 43 and `44. A gasket ring 46 may be cemented to the forward ring 44. The rearward ring 42 is preferably provided with a large number of openings 47 extending radially therethrough between the screws 45, to provide for free flow of air into and out of the space defined by the rearward suspension ring 40, the voice coil 35 and the ring 37.

It will be apparent that with this suspension arrangement, canting of the diaphragm 34 is minimized by virtue of the fact that the rings 40 and 41 are secured to the voice coil 35 at points spaced a substantial distance apart.

Referring now to FIGURE 4, reference numeral 48 generally designates another modied form of speaker which is in most respects the same as the speaker of FIG- URES l and 2, like reference numerals being applied to like elements. Instead of the magnets 16 used in the speaker 10, the -speaker 48 has a winding 49 between the inner and outer rings 13 and 15, to thus provide an electromagnet type of speaker.. A ring 50 of magnetic material is substituted for the non-magnetic ring 32 of the speaker 101.

It should be noted that in the embodiments of FIG- URES 1 3, the magnets are radially magnetized which is desirable in many respects. It is however possible to use an arrangement in which the magnets are axially magnetized. In the arrangement of FIGURE 3, for example, the outer ring 38 may be eliminated and an auxiliary ring may be provided in engagement with the rearward edge of the ring 36 to extend radially outwardly therefrom in parallel relation to the ring 37. An axially magnetized magnet or magnets may then be disposed between such an auxiliary ring and the ring 37, the magnets being spaced radially outwardly from the outer surface of the ring 36.

d With such an arrangement, the magnet pole faces can be at rather than curved.

It will be understood that modifications and variations may be effected without departing from the novel concepts of this invention.

I claim as my invention:

l. In a speaker the combination comprising an annular magnetic structure having a hollow rectangular cross section with `an annular axially extending air gap formed in one of the ends thereof, -means in said structure for establishing ux across said air gap, a diaphragm having a dome-shaped central portion surrounded by a relatively narrow reinforcing brim portion, with the periphery of said brim portion terminating adjacent said air gap, a voice coil secured to the periphery of said brim portion and extending into said air gap, means mounted on said annular magnetic structure for resiliently supporting said voice coil so that the latter is constrained to move axially in said air gap accompanied by movement of said diaphragm, said diaphragm being wholly composed of stiff plastic ymaterial having appreciable thickness and with voids distributed throughout the mass so that said diaphragm moves as a unit free of localized breakup in response to magnetic forces acting upon said voice coil.

2. In a speaker the combination comprising an annular magnetic structure providing an annular axially extending air gap, means in said structure for establishing flux across said air gap, a diaphragm having a -dome-shaped central portion surrounded by a brim portion, flexible means for supporting the diaphragm at the periphery of the brim portion, a voice coil secured to the idiaphragm inside of said flexible supporting means and extending into said air gap, said diaphragm being wholly composed of stiff plastic -material having smooth surfaces on each side with voids distributed throughout the mass and with a thickness which varies from point to point radially being maximum at the regions of greatest bending stress so that said diaphragm moves as a unit free of localized breakup in response to vmagnetic forces acting upon said voice coil.

3. In a speaker the combination comprising an annular magnetic structure defining a shallow central space and having an axially extending air gap formed therein, means in said structure for establishing flux across said air gap, a diaphragm having a dome-shaped central portion surrounded by a reinforcing brim portion with the domeshaped portion occupying said shallow central space and with the periphery of said edge portion terminating adjacent said air gap, a voice coil secured to said periphery and extending into said air gap, means mounted on said annular magnetic structure for resiliently supporting said voice coil so that the latter is constrained to move axially in said air gap as a unit with said diaphragm, said diaphragm being wholly composed of stiff material with voids distributed throughout the mass and said diaphragm having a thickness proportioned to the bending forces acting thereon with maximum thickness at the center.

4. In a speaker the combination comprising an annular magnetic structure having a hollow rectangular cross section with an annular axially extending air gap formed in one of the ends thereof, means in said structure for establishing ux across said air gap, a diaphragm having a dome-shaped central portion surrounded by a relatively narrow reinforcing brim portion with the periphery of said brim portion terminating adjacent said air gap, a voice coil secured to the periphery of said brim portion and extending into said air gap, means mounted on said annular magnetic structure for resiliently supporting said voice coil so that the latter is constrained to move axially in said air gap accompanied by movement of said diaphragm, said diaphragm being Wholly composed of stiif expanded plastic material presenting smooth faces but with voidsI distributed throughout the mass so that said diaphragm moves as a unit free of localized breakup in response to magnetic forces acting upon said voice coil.

5. In a speaker the combination comprising an annular magnetic structure having a hollow rectangular cross section with an annular axially extending air gap formed in one of the ends thereof, means in said structure for establishing ilux across said air gap, a diaphragm having a dome-shaped central portion and a relatively narrow reinforcing brim portion with the periphery of said brim p0rtion terminating adjacent said air gap, a voice coil secured to the periphery of said brim portion and extending into said air gap, an annulus of elastomeric foam mounted on said annular magnetic structure along its outer edge and secured to said voice coil along its inner edge so that the voice coil is constrained to move axially in said air gap accompanied by movement of said diaphragm, said diaphragm being wholly composed of sti" plastic material having appreciable thickness and with voids distributed throughout the mass so that said diaphragm moves as a unit free of vibrational breakup in response of magnetic forces acting upon said voice coil.

6. 1n a speaker the combination comprising an annular magnetic structure having a hollow rectangular cross section deiining a shallow central space, said structure having an annular axially extending air gap formed therein, means in said structure for establishing ux across said air gap, a diaphragm having a dome-shaped portion occupying said central space and having a relatively narrow reinforcing brim portion having its periphery alined with said air gap, a voice coil secured to the brim portion and extending into said air gap, means mounted on said annular magnetic structure for resiliently supporting said voice coil so that the latter is constrained to move axially in said air gap accompanied by movement of said diaphragm, said diaphragm being formed of stiff light-Weight material but having substantial thickness at regions subject to bending stress so that the diaphragm resists canting in said air gap and moves as a unit free of vibrational breakup in response to magnetic forces acting upon said voice coil.

7. 1n a speaker the combination comprising a circular frame, a diaphragm in said frame, said diaphragm having a frustoconical brim portion and a central domeshaped portion reentrantly arranged with respect thereto and occupying a major portion of the diameter, the brim portion and central portion of said diaphragm being formed of stiff material with voids distributed throughout and of sufficient thickness so that the diaphragm is rigid with substantial freedom from the localized vibrational breakup which characterizes conventional speaker diaphragms, an annular support of resilient material interposed between said frame and the outer edge of said diaphragm and secured to both of them so that the diaphragm is free to move axially with respect to said frame within prescribed limits of movement, a voice coil secured to the periphery of said diaphragm and extending axially therefrom, an annular magnetic structure secured to said frame at the periphery of said diaphragm and having a magnetized axially extending air gap for receiving said voice coil, said annular support having substantial axial thickness so as to provide support for said diaphragm at points spaced axially apart thereby to minimize canting of said voice coil relative to said air gap.

References Cited in the file of this patent UNITED STATES PATENTS 1,690,840 Round Nov. 6, 1928 1,713,006 Round May 14, 1929 1,917,012 Blattner July 4, 1933 1,962,374 Woolf et al June 12, 1934 2,237,298 Baumzweiger Apr. 8, 1941 2,249,160 Mott July 15, 1941 2,408,038 Brennan Sept. 24, 1946 2,442,791 Nente June 8, 1948 2,478,642 Root Aug. 9, 1949 2,520,646 Mott Aug. 29, 1950 2,534,157 Zwerkes Dec. 12, 1950 2,535,757 Root Dec. 26, 1950 2,567,365 Dalton Sept. 11, 1951 FOREIGN PATENTS 306,839 Great Britain Feb. 25, 1929 596,893 Great Britain Jan. 13, 1948 1,036,975 France Sept. 14, 1953

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