US3079471A - Loudspeaker - Google Patents

Description

Feb. 26, 1963 M. w. WIDENER LOUDSPEAKER 2 Sheets-Shem 1 Filed March 1, 1961 MAME/CE W Mom/2 III I l3 INVENTOR.

BY /6Mm/i% ATTOF/i/E) Feb. 26, 1963 w. WIDENER 3,079,471

LOUDSPEAKER Filed March .1. 1961 2 Sheets-Sheet 2 III-16,4:

Maw/c5 kill 1405mm IN V EN TOR.

,dMPL/F/EES y W United States Patent Ofilice Fatenteti Fold. 26, 19%3 on of rest, No. sales T his invention relates to loudspeakers, and particularly to loudspeakers having diaphragms movable by electromagnetic means engaging the peripheries thereof.

in the noted art it has been recognized that 2. diaphragm moved by means engaging the periphery thereof can give improved performance if a very large part of the diaphragm is unyieldingly rigid so as to displace the air more uniformly to create the sound waves desired. The larger is the rigid area of the diaphragm, the greater is the volume of air displaced on each vibrational stroke, and the greater is the average amplitude of displacement. Also, each such diaphragm is limited in range by an upper resonant frequency at which the rigidity of the element is overcome and at which it begins to undulate or flex uncontrollably so as to produce undesirable distortions in the sound produced. Thus, the more rigid the diaphragm, the wider is the frequency range across which the speaker can be used without resonance. In effect, it is desired to n'rake the diaphragm more like the solid cad of a piston and less like the flexible head of a drum.

Previously, attempts to design a speaker embodying such a diaphragm have been limited by the circumstance that high audible frequencies require a low-mass diaphragm because very quick alternating resonse and reversal of movement of the diaphragm is needed to produce these frequencies. Assuming a given material of high strength and low unit mass (e.g., aluminum), it is nevertheless true that the lower the total mass, the thinner th diaphragm must be, and the less rigid.

Until now, no speaker is known that can reproduce the higher audible frequencies without being limited in range on the lower side at approximately 3000-4000 cycles per second.

Accordingly, it is an object of this invention to provide a loudspeaker having a non-resonant frequency range encompassing substantially all of the desirable audible fret encies above approximately 500 cycles per second.

lit is another object of the invention to provide a loudspeaker having a response flatness range of novel extent and of improved constancy.

it is still another object of the invention to provide a loud peaker of improved performance and capable of being manufactured in simple and inexpensive form for economical sale and use.

A loudspeaker in accordance with the present invention satisfies the forego and other purposes by making use of a diaphragm in the shape of an oblate dome having increased resistance to distortion under forces acting in an axial direction of the dome, and maximum rigidity in proportion to mass; and this diaphragm is acoustically coupled to a sound box the cavity of which is tuned to a lower resonant frequency than the free resonant frequency of the mass of the moving parts and the suspension thereof.

Other objects and advanta es will be explained in the following specification, considered together with the accompanying drawing, in which:

FlGURE l is a cross-sectioned perspective elevation view of a loudspeaker constructed in accordance with the invention;

FlGURE 2 is a fragmentary broken-away plan view, to an enlarsed scale, taken substantially on the plane of line 2-42 of FEGURE l;

FIGURE 3 is an enlarged View of that portion of FIGURE 1 enclosed within the arcuate line 3.--3 thereof.

FEGURE 4 is a schematic view illustrating the shape of a dome constructed in accordance with the invention.

Referring now to FZGURE 1 there is shown a lower housing ll. defining a cylindrical cavity 12 and having a bottom wall 13 reinforced by flanges E4, the wall 13 being provided with a small vent opening 16. in the upper portion of the housing 11 there is seated and firmly affixed upper housing 17 provided with an inwardly directed flange 18; at the lower end thereof, the flange defining a central opening 19. On the flange is afiixed a toroidal permanent magnet 21 formed, for example, of Alnica, and having a frusto-conical outer surface 22. The housing 17 is part of one of the pole pieces of the magnet 21 and is preferably formed of soft iron. The central opening 23 of the magnet is in registry with the opening 19 of flange l8. Surmounting the magnet 21 is a soft iron pole piece 24, having a toroidal shape and defining an opening 26 registering with the opening 2% of the magnet. On top of the housing 17 and in the same plane with pole piece 24 is affixed a toroidal cover plate 127, which is also part of the pole piece represented by the housing 317 and is also made of soft iron. The plate 27 defines a central opening 28, so that the cover plate circuniferentially encompassing the pole piece 24 is spaced therefrom to define the opening 28 as a space between the cover plate and pole piece. A ring 29 of electrically insulating material is afiixed to the upper portion of the cover plate 27 to form a portion of the upper wall of the acoustical cavity through which electrical leads are to be passed, as described below. A diaphragm mounting ring Edi is aiiixed to the upper portion of the ring 29 and mounts a resilient collar 33;, which has a wavy cross-sectional shape to permit motion. The collar 3% in turn serves as a mounting for a thin aluminum diaphragm 3.2 in the shape of an oblate dome oriented with the concave side inward and communicating with the central openings of the pole pieces 2 and magnet 21. Also afiixed to the collar 31 is a downwardly extending cylinder 33 formed, for example, of stiff paper on which is mounted an electrical coil The cylinder 33 is extended to dispose the coil 34 (best seen in FEGURE 3) within the space 28 etween the cover plate 27 and the pole piece 24. A plug 36 made of acoustical damping material such as phenolic bonded fiberglass is mounted in the central portion of the magnet opening 23 so as to define within the interior of the apparatus the lower cavity 12 and an upper cavity 37. A damping element 3%, made of solid non-magnetic material, is centrally mounted as by means of a spider on the pole piece and proiecting upwardly in the upper cavity to a position in which the upper surface of the element is spaced closely beneath the peak portion of the dome shaped diaphragm 512, the surface ll being formed to substantially the same conformation as the confronting peak portion of the diaphragm. A pair of electrical leads 4-2 and 43 are arranged projecting through the insulating ring 29 to be coupled with the coil dd, as hereinafter described.

it is here noted that the cavities l2 and 3'7 are completely sealed to form what would be, in the absence of the vent opening 16, an air-tight chamber. Under some conditions of use or during shipping as by aircraft, there would be danger of the louds eaker blowing up. The opening to is provided to circumvent such happenings.

The structure described above is further illustrated in PlGURE 2, together with a protective cover 4-3 of rigid, acoustically transparent net material in the shape of a dome and aiiixcd to the outer portion of ring 30 to cover the diaphragm 32.

Referring now to FIGURE 3, there is shown the electrical lead 42 extending through the insulating ring 29 and electrically coupled to a flexible strip Sl of electrical conducting material having undulations, one end of which is extended to pass through a hole 52 in the paper cylinder 33 and is electrically coupled to an extending end 53 of the conductor forming the coil 34.

Referring now to FIGURE 4, one method for establishing the cross-sectional shape of the diaphragm 32 is illustrated. The shape is that of an exponential curve, and particularly a symmetrical curve or having an axis 62 of symmetry. To construct one-half of the curve, a circular are 63 is struck from the axis 62 and from a center 64 on the axis 62. The arc so has a predetermined angle define-d by a radius 66, the angle in this example being five degrees. From a second center 67, lying on the radius on and spaced from center 64 for a distance equal to ne-tenth the length of radius 66, a second are 68 is laid oil from arc 63. The arc as is also of five degrees and is defined by an outer radius 69. A third are 71 also of five degrees and of radius 72 equal to 6.9 of radius 69 is then laid off. The process is continued until the tangent '74 to the curve 61 at the outermost portion thereof lies at an angle of approximately sixty degrees to a line 76 that is perpendicular to the axis as; that is to say until the tangent '74.- lies at an angle or" approximately thirty degrees to the axis 63. It will be seen that each pair of adjacent arcs have a common tangent at the juncture of the arcs, and form a smooth curve.

In this figure, the are 81 of a circle centered on axis 2 and passing through the peak point 82 and the outermost point 83 of the curve oil, is shown for comparison. It will be noted that the curve 61 has a much sharper degree of curvature (i.e., shorter radius of curvature) in the vicinity of point 83 than has the are ill; that is, the tangent 74 of the curve 61 at point 83 is substantially closer to being parallel with the axis 62 the would be the tangent to the are 31 at point 33.

In operation, an electrical signal representing the wave fluctuations of sound is supplied to the electrical leads 4?; and 43, and the varying current passing through the coil 3 causes the coil together with the cylinder 33 and dome 32 to be rapidly displaced upward and downward with espect to the magnet 21 and pole pieces 24. The wavy shape of the conductor 51 and of the collar 31 permits substantial extension of these elements in an axial direction of the apparatus so as to impose the least possible resistance to the displacement required at the greater amplitudes, and so as to prevent fatigue and wearing out of the two parts. The damping element 38 helps to reduce the amplitude of vibration of the diaphragm ll at the highest frequencies of vibration so as to control the quality of sound emitted. In part, the element 38 effects damping as by serving as an anvil against which air displaced by the diaphragm 32 in downward movement is compressed, the space between the element 38 and the diaphragm being relatively restricted so as to hinder the escape of toward the periphery of element This eil'ect is also operative during upward movement of the diaphragm 32, because the space available for radial inward rush. of air is also relatively restricted. The presence of the plug as a dividing portion between the upper and the lo: er acoustical cavities also helps to restrict the displacement of air betw en the cavities at the lower frequencies of vibration and so to improve the acoustical quality of the reproduced sound.

The cross-sectional shape of the diaphragm has an important effect in increasing the stiffness rigidity of the dome so that equivalent rigidity can be obtained with less mass of material, and converselyso that with a given mass of material a greater rigidity can be obtained, and the range of frequencies that can be reproduced is extended at the upper limit.

it will be understood that the condition of least rigidity for a diaphragm is that in which the diaphragm is hat; and that the condition of greatest rigidity would be that in which the diaphragm most closely approached being in the shape of a cylindrical wall, but a cylindrical wall would have no cross-sectional surface area suitable for displacing air in an axial direction. The provision of a spherical dome, as known in the art, is a partly successful compromise, but the oblate shape of the dome used in the present invention more closely approaches an ideal form. As illustrated in FIGURE 4, the oblate shape of the present dome provides a resolution of forces more nearly approaching parallelism with the axis of the diaphragm at the peripheral portions thereof, where the greatest strain is focused when the diaphragm is suddenly moved as by forces applied to its periphery. The mass of the dome, and the inertia of this mass, is more readily moved without distortion of the dome when the dome has a more sharply acute curvature with respect to the axis at the point where the moving forces are applied. On the other hand, at the portions of the dome near the axis where the curvature is flatter than it would be in a spherical section, the mass remaining radially inward of any selected circular section of the dome is comparatively quite small and the eilcct of the increased flatness is not deleterious. In short, the dome of the invention has a smaller radius of curvature in the portions thereof that lie at a greater radius from the axis and where greater rigidity 'is needed to withstand the inertia forces of the central or axial portion of the dome, and has greater flatness near the axial portion where the strength does not have to be so great to produce rigidity.

The range of non-resonant response of the diaphragm having been broadened by the construction above described, the loudspeaker is additionally tuned as by caretul dimensioning and proportioning of the cavities 12 and 37 so that the low r resonant frequency of the coupled cavities is lower than the frequency of tree resonance of the diaphragm and moving parts attached thereto, so that the speaker as a whole is limited in its response range only by the lower resonant frequency of the cavities and by the upper resonant frequency of the diaphragm.

In a preferred example of the device, constructed and proportioned substantially as illustrated in the drawing, a response flatness range of i1 decibel has been achieved between the frequencies of 500 cycles per second and 15,009 cycles per second.

if desired, as shown in FIGURE 5, and as an alternative to the use of damping element 38 for damping the resonance of the speaker, the lead may be coupled to a clamping circuit 4d, comprising an inductor 35, a capacitor 4e and a resistor 47 connected in parallel, the circuit 3 being in series between the coil and a constant-voltage amplifier (not shown).

Thus, there has been described in the foregoing a wide frequency range loudspeaker in which an acoustical chamber is coupled to a diaphragm in the shape of an oblate dome oriented with the concave side facing the chamber, so as to provide a very wide range of nonresonant frequency response.

What is claimed is:

1. A wide frequency range loudspeaker comprising: a housing defining an acoustical chamber closed at one end; a diaphragm mounted closing the other end of sai chamber and arranged for axial movement relative to said housing to cause air pressure variations within and without said chamber, said diaphragm having a flexible eripheral portion secured to said housing and a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber; and electromagnetic Incans engaging the periphery of said dome portion for eilectiru axial motion of said dome portion in response to variations of a control signal so as to pro duce corresponding sound waves.

2. A loudspeaker as defined in claim 1, wherein the cross-sectional shape of said dome is defined by an exponential curve.

3. A Wide frequency range loudspeaker comprising: a housing defining an acoustical chamber closed at one end; a diaphragm mounted closing the other end of said cavity and arranged for axial movement relative to said housing to cause air pressure variations Within and without said chamber, said diaphragm having a flexible peripheral portion secured to said housing and a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber, said dome having a cross-sectional shape characterized by a symmetrical curve having an axis of symmetry, each half of said curve having a plurality of adjacent arcuate portions extending from said axis outwardly, each pair of adjacent portions having a common tangent at the juncture thereof, the radius of each portion being a predetermined tunction of the radius of the adjacent axially inward portion, the angle of each portion being a predetermined function of the angle of the adjacent axially inward portion, and the tangent to the outermost part of re outermost portion being at an inclination of approximately thirty degrees to said axis of symmetry; and electromagnetic means engaging the periphery of said dome portion for eitecting axial motion of said dome portion in response to variations of a control signal so as to produce corresponding sound waves.

4. A wide frequency range ioudpeaker comprising: a housing defining an acoustical chamber closed at one end; a diaphragm mounted closing the other end of said cavity and arranged for axial movement relative to said housing to cause air pressure variations within and without said chamber, said diaphragm having a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber, and a flexible peripheral portion of substantial radial width constituting a cantilever mounting for said dome, whereby said dome is axially movable for maximum excursion with minimum unit distortion of said peripheral portion; and electromagnetic means engaging the periphery of said dome portion for effecting axial motion of said dome portion in response to variations of a control signal so as to produce corresponding sound waves.

5. A wide frequency range loudspeaker com-prising: a housing defining an acoustical chamber closed at one end; a diaphragm mounted closing the other end of said cavity and arranged for axial movement relative to said housing to cause air pressure variations within and Without said chamber, said diaphragm having a flexible peripheral portion secured to said housing and a rigid cen tral portion in the shape of an oblate dome oriented with the concave side facing said chamber; electromagnetic means engaging the periphery of said dome portion for effecting axial motion of said dome portion in response to variations of a control signal so as to produce corresponding sound waves; and said chamber being tuned to a resonant frequency lower than the free resonant frequency of said dome together with the moving parts attached thereto.

6. A wide freouency range loudspeaker comprising: a housing defining an acoustical chamber closed at one end; a diaphragm mounted closing the other end of said cavity and arranged for axial movement relative to said housing to cause air pressure variations within and without said chamber, said diaphragm having a flexible peripheral portion secured to said housing and a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber; said chamber being tuned to a resonant frequency lower than that of said dome; electromagnetic means engaging the periphery of said dome portion for effecting axial motion of said dome portion in response to variations of a control signal so as to produce corresponding sound waves; and a quantity of acoustical damping material disposed in said chamber intermediate the ends thereof.

7. A Wide frequency range loudspeaker comprising: a housing defining an acoustical chamber closed at one one; a diaphragm mounted closing the other end of said cavity and arranged for axial movement relative to said housing to cause air pressure variations within and without said chamber, said diaphragm having a flexible peripheral portion secured to said housing and a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber; electromagnetic means engaging the periphery of said dome portion for effecting axial motion of said dome portion in response to variations of a control signal so as to produce corresp acting sound waves; and a damping element mounted Within said chamber, said element having a portion facing said dome and spaced therefrom so as to restrict the air fiow immediately behind said dome when said dome is axially moved.

8. A wide frequency range loudspeaker comprising: a housing defining a chamber closed at one end; a toroidal magnet mounted co-axially within said housing and spaced from said closed end so as to define a first end compartment of said chamber communicating with the central opening of said toroidal magnet; a diaphragm covering the other end of said housing and spaced from said magnet so as to define a second end compartment of said chamber communicating with said central opening of said toroidal magnet, said diaphragm having a flexible peripheral portion secured to said housing and a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber; an electrical coil co-axially atiixed to the peripheral portion of said dome and extending toward said magnet; and means for supplying electrical current to said coil, whereby said coil is attracted to and repulsed from said magnet in response liO variations in the strength and direction of said current, and whereby said dome is conjointly displaced to generate sound waves in the air within and without said chamber.

9. A wide frequency range loudspeaker comprising: a housing defining a chamber closed at one end; a toroidal magnet mounted co-axially within said housing and spaced from said closed end so as to define a first end compartment of said chamber communicating with the central opening of said toroidal magnet; a diaphragm. covering the other end of said housing and spaced from said magnet so as to define a second end compartment of said chamber communicating with said central opening of said toroidal magnet, said diaphragm having a flexible peripheral portion secured to said housing, said peripheral portion being of substantial radial Width and having a plurality of concentric folds for expansion of said peripheral portion in either axial direction, said diaphragm also having a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber; an electrical coil co-axiaiiy afiixed to the periphery of said dome and extending toward said magnet; and electrical leads for supplying electrical current to said coil, said leads being mounted to extend radially inward through said housing and being affixed at the ends thereof to the ends of said coil so as to form an electrical connection therewith, the portions of said leads tending between said housing and said coil being rovided with a plurality of transverse folds for resilient extension of said leads upon axial movement of said coil; whereby said coil is attracted to and repulsed from said magnet in response to variations in the strength and direction of said current, and whereby said dome is conjointly displaced to generate sound waves in the air within and Without said chamber.

10. A wide frequency range loudspeaker comprising: a housing defining a chambe closed at one end; a toroidal magnet mounted co-axially within said housing and spaced from said closed end so as to define a first end compartm at of said chamber communicating with the central opening of said toroidal magnet; a diaphragm covering sc ee the other 3rd of said housing and spaced from said net so as to define a second end compartment of said chamber communicating with said central opening of said toroidal magnet, said diaphragm having a flexible peripherai portion secured to said housing, said peripheral portion being of substantial radial Width and having a plurality of concentric folds for expansion of said peripheral portion in either axial direction, said diaphragm also having a rigid central portion in the shape of an oblate dome oriented with the concave side facing said chamber; an electrical coil co-axially afiixed to the peripheral portion of said dome and extending toward said magnet; a pair of electrical leads for supplying electrical current to said coil, said leads being mounted to extend radially inward through said housing and being atlixed at the ends to the ends or" said coil so as to form an electrical connection therewith, the portions of said leads extending between said housing and said coil being provided with a plurality of transverse folds for resilient extension of said leads upon axial movement of said coil; a quantity of acoustical damping material mounted in the central opening of said toroidal magnet; and a damping element mounted within said second end compartment, said element being shaped to conform to the concave side of said dome and being spaced therefrom so as to restrict the flow of air immediately behind said dome when said dome is axially moved, said element being mounted by means of a spider attached thereto and to the adjacent portions of said toroidal mag net, whereby said coil is attracted to and repulsed from said magnet in resnonse to variations in the strength and direction of said current, and whereby said dome is conjointly displaced to generate sound Waves in the air within and without said chamber.

References tjited in the file of this patent UNlTED STATES PATENTS 1,812,389 Wcnte June 30, 1931 1,926,187 Young Sept. 12, 1933 2,047,777 Hartmann July 14, 1936 2,141,595 CornWell Dec. 27, 1938 2,238,741 Laufier Apr. 15, 1941 2,549,963 De Boer et a1. Apr. 24, 1951 2,848,561 Gorilte Aug. 19, 1958 2,971,597 Gorike Feb. 14, 1961 FOREIGN PATENTS NR. 190,987 Austria July 25, 1957 reat Britain Dec. 19, 1956

Claims (1)

1. A WIDE FREQUENCY RANGE LOUDSPEAKER COMPRISING: A HOUSING DEFINING AN ACOUSTICAL CHAMBER CLOSED AT ONE END; A DIAPHRAGM MOUNTED CLOSING THE OTHER END OF SAID CHAMBER AND ARRANGED FOR AXIAL MOVEMENT RELATIVE TO SAID HOUSING TO CAUSE AIR PRESSURE VARIATIONS WITHIN AND WITHOUT SAID CHAMBER, SAID DIAPHRAGM HAVING A FLEXIBLE PERIPHERAL PORTION SECURED TO SAID HOUSING AND A RIGID CENTRAL PORTION IN THE SHAPE OF AN OBLATE DOME ORIENTED WITH THE CONCAVE SIDE FACING SAID CHAMBER; AND ELECTROMAGNETIC MEANS ENGAGING THE PERIPHERY OF SAID DOME PORTION FOR EFFECTING AXIAL MOTION OF SAID DOME PORTION IN RESPONSE TO VARIATIONS OF A CONTROL SIGNAL SO AS TO PRODUCE CORRESPONDING SOUND WAVES.

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