US1748990A - Acoustic diaphragm - Google Patents

Description

March 4, 1930. E. c. NILSON ACOUSTIC DIAPHRGM Filed Deo. 16, 1927 RGnZ vF-'IGhE FIG, 5

Patented Mar. 4, 1930l UNITED STATES EVEBETT C. NILSON, OF WORCESTER, MASSACHUSETTS, ASSIGNOR OF FORTY PER CENT TO RALPH G. NILSON, OF SAN FRANCISCO, CALIFORNIA, AND TWENTY PEE CENT T0 NILS NILSON, OF WORCESTER, MASSACHUSETTS ACOUSTIC DIAPHRAG'MA Application led .December 16, 1927. Serial No. 240,596.

-Ths invention relates to diaphragms of loud speakers of the type known in the art as cone speakers. Its primary object is to combine the advantages of the free edge cone and the fixed edge cone and to eliminate some of the faults inherent in each of these types.

A further object of this invention is to devise a' diaphragm having a plurality of aperiodic sections of differing acoustical characteristics whereby blasting at. any par: ticular sound frequency is eliminated.

A further object is to devise a diaphragm which is unconfned in its range and which is responsive to th'e highest notes as well as to the very lowest notes thereby covering the entire range of audibility.

A further object is to devise a diaphragm which has its center of inertia approximately at its point of support thereby making the driving force more effective at all volumes of received signals and especially feeble signals.

A further object is to devise a diaphragm with a plurality of sections each of which is most sensitive to a range of sounds between certain limiting frequencies.

Another object is to devise effective damping means for each of the several sections of which the diaphragm is composed.

Another object is to devise a diaphragm which is more rigid than either a free edge cone or a fixed edge cone of the same size and material.

Another object is to devise a diaphragm which has light weight but which is rigid enough over its entire surface to retain its shape when supported at a single point even when made in enormous sizes.

A further object is to devise a diaphragm which is ornamental and decorative and Well balanced from an artistic viewpoint.

Further objects and advantages will be readily apparent to those skilled in the art.

Great diiculty has been encountered in attempting to produce a loud speaker of simple construction which will give faithful reproduction over the entire range 'of audibility. Cone speakers have overcome the main fault of speakers of the horn type, eliminating constriction of the sound waves and interfering echoes. At the present development of the art, however, cone speakers have not been perfected to the extent that they will give reproduction without distortion over the entire audible range. Due to the fact that every vibrating diaphragm has its own period of mechanical resonance, sounds of a certain frequency are more readily sustained than sounds of other frequencies and this gives rise to distortion. One method of overcoming this objection is to use a diaphragm which has a natural resonance period outside the audible range. When such a diaphragm is used, other disadvantages appear which are even more objectionable than distortion due to resonance. For example, when a diaphragm is used which has a resonant point above the upper limit of audibility, volume of sound is sacrificed on account of the small size of such a diaphragm. On the other hand, a diaphragm which is so large that its natural resonance lies below the musical scale is too sluggish to respond to the higher notes of the scale unless an unduly strong operating force is applied to it. f-

`This invention overcomes these disadvan tages in a manner which will be hereinafter disclosed.

In order that the principles of this invention may be fully understood, attention is called to acompanying drawings in which:

Fig. 1 is a plan view of a dia hragm constructed in accordance with this invention.

Fig. 2 is a side view of the diaphragm shown in Fig. 1.

Fig. 3 is a cross section of a modification.

Fig. 4 is a modification having pyramidal sections. --5 I j Fig. 5 is a modification showing the invent-ion applied to a diaphragm of oval or elliptical shape.

Fig. 6 is a modification showing the invention applied to an eccentric diaphragm.

In Fig. 1 a diaphragm 11 is shown having a central cone 12 with apex 13 and circumference 17. Adjacent cone 12 is a truncated cone 14, the smaller circumference 17Y of which coincides with the circumference of cone 12. The outer circumference 18 of truncated cone 14 coincides with the smaller circumference of truncated cone 15 and in a like manner the larger circumference 19 of the truncated cone 15 is coincident with the smaller circumference of the truncated cone 16. If desired, more sections, each comprising a truncated cone, may be added to those shown in the illustration. Under some conditions it may also be desirable to dispense with some of the sections, so that there may be as few as two sections comprising a central cone and a surrounding truncated cone. The dimensions of the several sections may be determined from several standpoints and I do not wish to be limited to any specific dimensions or proportions except as defined in the appended claims. Where the acoustical characteristics of the severa-l sections are to be the deciding factors, more sections or fewer sections may be employed, each section being designed to cover its own acousti- Cal range, regardless of the area of the other sections. The different sections may all have the same slope or different slopes as conditions may warrant.

In the particular embodiments illustrated, the dimensions and slope of the several sections are so chosen that the center of gravity and the center of inertia of the diaphragm is approximately at the apex of the central I cone 12. It is at this point that the driving force is applied. It 1s well known that a force applied at the center of inertia of any mass is far more effective than the same force applied at any other point and for this reason the diaphragm illustrated will be more responsive to forces derived from feeble currents.

Such a construction allows the diaphragm to be supported at any ang-le withoutdetracting 'from its efciency. This feature is important where the diaphragm is large and is suspended at an angle to a wall or other background.

In constructing the diaphragm so that its center of mass occurs at the driving point, sections lying proximate the driving point ma be of any desired dimensions, and prefera ly of such dimensions that they most faithfully reproduce sounds Within certain frequency ranges and the outermost section or sections may then be used to balance the diaphragm. The central section 12, for example, may be of such size and mass to respond most faithfully to the highest range of audible sounds, section 14 to an intermediate range, and sections 15 and 16 to the lowest range and also to balance the diaphragm. It should be remembered, however, that the central section 12 and the other interior sections vibrate in unison with external sections for certain frequency ranges, as hereinafter described, and this should be taken into consideration in selecting the dimensions of these sections. If desired, these sections may be selected in respect to their volume response or any other acoustic characteristic instead of to frequency response. After the dimensions of the internal section or sections have been fixed, the exterior section or sections of the diaphragm may then be used to balance the diaphragm so that its center of mass occurs approximately at the apex of the central section, calculating the dimensions of such external section or sections by means of the formula: i

Where: Q, is the statical moment about an axis passing through the apex of the central section and parallel to the plane of the base of the section.

M1, M2, M3 M are the masses respectively of the several sections which compose the diaphragm.

R1, R2, R3 Rx are the perpendicular dis- ,f

tances of the center of gravity of the sections whose masses are M3, M3, M3 MX respectively, from the axis.

' Since the statical moment about an axis passing through the center of mass of the diaphragm is equal to zero and since MIR1 and M2R2 may be predetermined and therefore of known value, the equation under these conditions becomes:

M3133 MXRX=M1R1M2R2 and where there are but three sections the equation becomes:

` Males: -MlRfMgla The right hand side of this equation is of known value and any values of M3 and R3 which satisfy the equation will determine the dimensions of the outer section which will balance the entire diaphragm so that its center of mass occurs at the apex of the central section, at which point the driving force is applied. It will be understood that the mass M3 will be different for the different materials of which the section ma be made and the distance R3 will be di erent for different slopes of the section so it is apparent that there are a number of solutions for the equation.

If desired, the outer section may also be determined by the cut and try method, that is, by making it larger or smaller until the diaphragm becomes balanced at the apex of the central section.

The invention is not limited to a diaphragm ample, will'respond to the higher notes or sounds because of its relatively small size. There will be little or no resonance in cone 12 since it is highly damped by the adjacent section 14.

Sounds next lower in the audible range will be reproduced by cone 12cand section 14 vibrating in unison. Resonant vibrations in this range of sounds are also absent by reason of the damping afforded by the adjacent section 15. f

Still lower ,sounds will vibrate cone 12, section 14 and section 154 all in unison. Damping is provided for these sections by the section 16.

, Sounds at the extreme lower end of audible range will vibrate the diaphragm as a whole and all sections will be in vibration for such sounds. Resonance is not important at this stage since the mass of the diaphragm prevents sustaining such resonance.

Since the several sections of the diaphragm are adapted to vibrate independently to the extent indicated, each section will be damped by an adjacent section which may not be.

vibrating at the same frequency. Damping is extremely effective under such circums'tances and the entire absence of resonance in diaphragms of this type seems to indicate that the damping thus afforded is substantially instantaneous in its result. The damping effect provided by the exercise of this invention is in this manner an active force as distinguished from the passive force applied in certain other damping arrangements.

It will be obvious that such a construction will produce a diaphragm in which more frequencies are reproduced by plunger action of the diaphragm, as distinguished from transverse vibratory waves in the surface of the diaphragm.- The small central section will act as a plunger type diaphragm on the higher frequencies, while the larger surfaces act as a plunger in reproducing the lower frequencies. By the automatic allocation of sounds in this manner to surfaces of different areas, there is a tendency forthe amplitude of vibration `to remain more nearly con-l stant for all of the frequencies provided for in the design of'the diaphragm.'

n speakers of previous design 1t was desirable to use a large diaphragm so that low frequencies might be reproduced without too great a movement of the diaphragm. With such la diaphragm, however, the higher frequencies were distorted because the diaphragm prevented plunger, action at these frequencies. On the. other hand, when a smaller diaphragm is' used so that the higher frequencies are more faithfully reproduced, the lower frequencies cause such a large .amplitude of vibration that the pole pieces of the electro-magnetic unit are frequently struck by the diaphragm, thereby setting up Iharsh metallic noises. ln the dynamic, or

moving coil, type of speaker the metallic noises are eliminated, but there is a detrimental effect on the iiexibility and elasticity of the delicate coil supporting spider, thereby destroying its effectiveness and occasionally causing the arms of the support to break because of mechanical fatigue. Such disadvantages are obviously eliminated by the use of a diaphragm in which high frequencies are reproduced by a small area and the low frequencies by a large area, both having substantially the same relatively small amplitude of vibration.

Another important feature of this invention is also due to the individuality of the several sections and that is the automatic allocation of sounds of different intensities. Feeble forces which are too weak to vibra-te the whole mass of the diaphragm will vibrate cone 12 alone and the sound will be heard as if a small speaker having a cone of this size were being used. As the intensity of the signals increases, more and more of the sections will come into play and signals which are loud will make use of the whole diaphragm. f

Since feeble forces act upon a relatively small surface and strong forces vibrate a relatively large surface, the amplitude of vibration at the driving point will tend to remain more nearly constant over the entire volume range thereby giving rise to extremely important advantages.

Thus, inI a diaphragm constructed in accordance with the present invention, there is a double allocation of the sound emitted from the diaphragm, one determined by the intensity of the sound and the other determined by its frequency, both allocations having a beneficial effect on the performance of the reproducer and increase the amplitude of vibration under conditions which would otherwise cause the amplitude of vibration to be too small and decrease it when it would otherwise be too great.

Ridges 17 ,.18 and 19 formed at the junctions of the several sections make the structure very rigid and diaphragms may be made in enormous sizes without requiring further support or reenforcement. This construction does away with the bulky and awkward frame-work required on previous types of cone speakers of large size. A diaphragm three feet in diameter is very rigid and retains -its shape in any position when constructed according to this invention and no further reenforcement of any kind is'needed. Diaphragms of greater diameter Kthan three feet are also self-supporting. Smaller diaphragms made in accordance with this invention are extremely rugged in` comparison with those of previous design.

Another feature of the diaphragm of this invention is that the outer sections serve as a baille for sounds emanating from the inner llO relatively smaller area is provided for the louder sounds, where the need of a baiile is .less important.

Fig. 3 illustrates a diaphragm of somewhat different appearance. It is readily seen that an infinite number of shapes and designs may be made without departing from the scope of this invention. The dimensions and slopes, either one or both, may be varied giving rise to a wide range of atterns which are pleasingrto the eye as we l as to the ear. j

ig. 4 illustrates the invention as applied to a diaphragm having pyramidal sections. What has been explained in regard to a diaphragm composed of conical sections applies equally well to the pyramidal diaphragm.

Various kinds of conical sections may be employed and the invention is not limited to right circular cones. Elliptical cones, as shown in Fig. 5, or oblique cones, as shown in Fig. 6 are examples of some other cones which may be used.

Fig. 5 illustrates a diaphragm constructed in accordance with this invention having elliptical or oval shape. Such a diaphragm has the same advantages over the present oval cones as the circular cone described hereinblefore has over the simple cone of circular s a e.

ig. 6 illustrates the invention applied to a diaphragm having an eccentric apex.

Many other embodiments of this invention will be apparent to those skilled in the art without departing from the spirit of this invention as defined in the appended claims.

I claim:

1. In a sound amplifying device, a diaphragm, said diaphragm comprising a cone and a. truncated cone surrounding said cone,

ythe lesser circumference of said truncated cone and the circumference of said cone being coincident, the statical moment of said diaphragm about an axis passing through the apex of said cone and perpendicular to the axis of said cone bein approximately zero.

2. In a sound amp 'fying device, a diaphragm, said diaphragm bemg composed of a plurality of sections said sections comprising a cone and a pluralit of truncated cones, each of said sections a joining an adjacent section along a common circumference, the statical moment of said diaphragm about an axis passing through the apex of said cone and perpendicular to the axis of said cone being approximatel zero.

3. In a sound amplifyin g device, a dia-v phragm, said diaphragm comprising a cone and a truncated cone surroundin said cone, the lesser circumference of sai truncated cone and the circumference of said cone being coincident, the center of inertia of said diaphragm being located approximately at the apexcof said cone.

4. In a sound amplifyin device, a diaphragm, said diaphragm bemg composed of a plurality of sections, said sections comprising a cone and a plurality of truncated cones, each of said sections adjoining an adjacent section along a common circumference, the center of inertia of said diaphragm being located approximately at the apex of said cone.

5. In a sound amplifying device, a diaphragm, said diaphragm comprising a cone and a truncated cone surroundin said cone, the lesser circumference of sai truncated cone and the circumference of said cone being coincident, the center of mass of said diaphragm being located approximately at the apex of said cone.

6. In a sound amplifying device, a diaphragm, said diaphragm being composed of a plurality of sections, said sections comprising a cone and a plurality of truncated cones, each of said sections adjoining an adjacent section along a common circumference, the center of mass of said diaphragm being located approximately at the apex of said cone.

7. In a sound amplifying device, a diaphragm, said diaphragm bemg composed of a plurality of sections, said sections comprising a cone and a plurality of truncated cones, each of said sections adjoining an adjacent section along a common circumference, said sections being individually designed to have predetermined acoustic characteristics.

8. In a sound amplifying device, a diaphragm, said diaphragm com rising a cone and a truncated cone surroun ing said cone, the lesser circumference of said truncated cone and the circumference of said cone being coincident, the statical moment of said diaphragm about an axis passing through the apex of said cone and parallel to thebase of said cone being approximately zero, said cone and truncated cone being individually vdesigned to have predetermined acoustic char-- acteristics.

9. AIn a sound amplifying device, adiaphragm, said diaphragm belng composed of a plurality of sections,'said sections comprising a cone and a pluralit of truncated cones, each of said sections adjoining an adjacent section along a common circumference, the center of inertia of said diaphragm being located approximately at the apex of saidcone and said sections being individually designed to have predetermined acoustic characteristics.

10. In a sound amplifying device, a dia-

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