US3193627A - High fidelity loudspeakers - Google Patents

Description

July 6, 1965 w. HECHT HIGH FIDELITY LOUDSPEAKERS Filed May 51, 1962 v INVENTOR.

WILL/Al? H'C/{T United States Patent 3,193,627 HIGH FIDELITY LOUDSPEAKERS William Heclit, 11 Normandy Terrace, West Orange, NJ.

Filed May 31, 1962, Ser. No. 199,148

9 Claims. (Cl. 179-4155) This invention relates generally to high fidelity loud speakers, and particularly to an improved design for the voice coil assemblies used in such speakers, by means of which the effects of distortion are more effectively subdued over the entire operating spectrum of the speakers.

An object of my invention is to provide loud speakers apparatus in which the forces associated with transient distortion are cancelled by the magnetic damping action of the voice coil assembly.

Another object is to provide loud speaker apparatus in which the sound distorting effects of extraneous disturbances, such as, for example, the effects associated with system resonances, standing waves, sympathetic vibrations, tuned ports and the like, are more effectively nullified.

Another object is to provide loud speaker apparatus exhibiting improved constant velocity damping characteristics over the entire operating frequency range thereof, while avoiding undue complication of the design of said apparatus or increase in the cost of manufacture thereof.

As a feature of my invention I provide a voice coil assembly including the equivalent of a short circuited conductive path, the dimensions and composition of which are carefully selected in association with the magnetic field intensity between the poles of the speaker magnet, to provide critical magnetic damping at all frequencies, and thus to maintain a constant velocity voice coil response regardless of the origin of a distorting force.

These and other objects and features of my invention will be more fully understood and appreciated when considered in relation to the following description taken in connection with the accompanying drawing wherein:

FIGURE 1 is a simplified diagrammatic view of loudspeaker apparatus with reference to which my invention is explained,

FIGURE 2 is a view in perspective of a voice coil form in accordance with my invention illustrating the circuit paths followed by electrical currents induced therein, and

FIGURE 3 is a view in elevation of an alternative voice coil form configuration in accordance with the invention.

The art of high fidelity loudspeaker design has reached a highly advanced state in which further progress is confined in general to relatively small design innovations. My invention relates to one such seemingly small, but significant, innovation.

One indication of the quality of performance of a loudspeaker is its transient response, this being simply the response to sudden or rapidly varying input excitation of the type commonly associated with cymbal crashes, or the like. It is generally well known that to faithfully reproduce such transients a loudspeaker should exhibit constant velocity'frequency response. In other words the peak velocity of the speaker cone fluctuations should be constant, for a given constant level of power input, over.

manufacturer, and thus the characteristic frequencies of externally originating resonances, or other extraneous velocity distorting influences, are generally unpredictable. It would follow therefore that in high fidelity speakers provision should be made for the inclusion of means to nullify, or to compensate for, such extraneous influences at all frequencies of interest. The problem however is to provide such means without undue complication of the design, and without undue increase in the cost of manufacture, of the speaker apparatus.

Reference is had to FIGURE 1 for an explanation of the novel technique by means of which I provide the requisite compensation. As shown therein, my apparatus contains the usual stationary assembly including a permenent magnet 1, having respective inner and outer pole piece extensions 2 and 3, separated by a narrow annular recess 4 hereinafter designated the gap. Disposed symmetrically within the .gap are a voice coil form 5, of generally cylindrical shape, and a voice coil winding 6 which is wound on the form 5. One end of the form 5 is connected to 'the base of a sound producing diaphragm 7, the members 4-7 thereby constituting a movable assembly, the motion of which corresponds to the electrical variations applied to the coil 6. The free end of the form 5 extends beyond the gap, the length of this extension being greater than the maximum displacement of the movable assembly, so that the volume of form material within the gap is held constant movement of the said assembly, for reasons which are more fully discussed'below.

It is well known that for optimum efficiency the above movable assembly should be made as light and as precisely dimensioned as possible. Accordingly, the form 5 is usually obtained by precisely molding paper, or other lightweight moldable material having suitable structural characteristics, into the required shape. As it is unfeasible to cast metal forms in the required shape, and with proper thickness, and weight, metal forms are not generally used. In certain instances however metal forms are used, composed of a metal having a high strength to weight ratio, such as aluminum, brass, or the like, and which serve exclusively as a support for the voice coil.

As the coil assembly must fit precisely and symmetrically into the gap, the usual practice, with respect to metal forms, is to fashion the form out of a rectangular strip of metal into a cylinder having a longitudinal split, as at 8 in FIGURE 2, and to place the form over a precisely shaped mandrel, the diameter of the form being made to conform to the diameter of the mandrel by adjustment of the width of the split 8.

Returning to the problem of transient distortion, it is generally well known that a properly damped moving coil system will not exhibit such distortion. It is also well known, however, that a voice coil which is properly damped at one frequency will beimproperly, or rather insufiiciently, damped at other frequencies due to the usual variation in the electrical impedance of the coil over the operating spectrum. Mechanical damping structures being cumbersome, costly, and limited in frequency response,

the usual practice is to append additional electromagnetic damping structures to the moving coil structure. The latter, however, add mass to the system, thus reducing the eficiency and increasing the natural resonant frequency thereof, in addition to complicating the design of the system.

This then leaves the voice coil form as the only available, non-additional damping structure. Metal forms, however, are extremely thin and light structures, the thickness usually being on the order of 0.005 inch, and the mass being on the order of 10 percent of that of the entire moving structure which is typically on the order of 0.02 kilogram. \Because of this and also because of the apparent electrical discontinuity presented by the longitudinal split in the form cylinder, it is generally assumed that the electromagnetic damping action of a metal voice coil form' is negligible. I have experimentally verified the absence of significant damping for an aluminum form by dropping a suitably loaded aluminum form of typical dimensions into a correspondingly typical magnetic gap, and by noting the apparent absence of any viscous drag as the form drops. The same is true of almost all metals. Surprisingly enough, however, forms made out of pure copper or pure silver, and only these two metals, exhibited a marked degree of'viscous damping action approaching the optimum for wide-band fre- I quency response without overshoot, or approximately seventy percent of critical damping. Furthermore, loudspeakers which I have constructed containing copper and silver forms exhibit a marked improvement in frequency response, especially at the upper end of the frequency range, where the voice coil damping action tapers off. The compensating action of the copper and silver forms almost completely eliminates the peak in response'curve associated with the behavior of the diaphragm athigh frequencies (see, for example,.L. L..Bera11ek, Acoustics, McGraw-Hill Book C0,, p. 199, et seq.). f, 1 Thus it may be narrowly concluded, and this is so claimed below as oneattribute of 'my invention, that copper and silver voice coil forms of a given size, the dimensions of which are listed in an examplecited below, constitute almost ideal damping structures when operated in a magnetic field of a given intensity, also cited below.

However," an even more general conclusion as to the nature of my inventive discovery may be drawn from the following analysis.

The forces acting on a damped vibrating system are the system in meters, and the respective first and second derivatives thereof with respect to time;

K represents the spring constant of the moving system,-

in newtons/meter; K represents the damping constant, in newtons-meter/ sec.;

M is the mass" of the moving system in. kilograms; and

F is the total force acting on .the system, in newtons.

Substituting'A-sin wt for x, where w is thefrequency of vibration, in radians/sec, we have: i

At resonance, the reactive term K -.-MW vanishes.

where w is the resonant frequency.

Thus, the 'basic differential equation may be written- In a critically damped syst-em, the radicalof thecorresponding quadratic expression is equal to Zero. Thus For the split cylinder coil form of FIGURE 2, as

used in the system of FIGURE 1, it may be shown that inducted eddy currents follow the pattern indicated, in FIGURE 2, by dotted lineson the hidden surfaces of the cylinder, and by solid lines on the visible surfaces, one such current path being traced by the letters ABCD in FIGURE 2. The damping forces F due to eddy currents induced in the form 5,are given by:

F '=BL I.

where, V V 7 I B is' the flux density (in gap 4 of FIGURE '1), in webers/ square meter; Y p

7 Thus 4. L is the circumferential length of the form cylinder,

in meters; I represents the induced current, in amperes, given by: I=BL Tv/k where:

L is the length of the gap 4, in meters;

T is the thickness of the form, in meters;

v is the velocity, as stated above; and

k is the resistivity of the form material, in ohm-meters.

7 F eL zea L L rv/k Hence, the damping ratio R, defined as the ratio of the actual damping force F to the critical damping force F is given by: V

, R=B L L T/ 2kMw Solving for the resistivity k:

I V V k--=B?L L T/2RMw (1) Substituting 11/2 for L,,/w in Equation 1 where d is the diameter of the form cylinder, as shown in FIG- URE 1, and where his the resonant frequency in cycles/ k=B dL T/2.8Mf V (3) and Equation 1 becomes:

' k=B L L T/1.4Mw, 4)

I A typical set of'value's used in the construction of speakers, of'the type shown in FIGURE 1, is given as follows:

I B=l Weber/sq. meter 10 gauss) d=.05 meters (2 inches) L -=.0095. meters (.375 inch) T=.00013 meters (.005 inch) M=.02 kilograms, and

f cycles/ sec.

Substituting the above values in Equation 3, we have:

Referring to any table of resistivity values, for example that given on page 197 of the above-referenced book Acoustics, it is seenthatthere are two materials which approximately provide the above value of resistivity, and all the rest do not. These two materials are silver (;0163 l0- ohm-m.), and copper (.0l72 1O- ohmm.), which verifies the above-discussed experimental observations.

Thus it has been'shown that for the specific set of speaker construction values listed above, only a coil form made out of silver or copper-will provide optimum damping (R =0. 7), and further that for any set of speaker construction values, a value of resistivity k, may be deterf mined, by means of Equation 3,0r 4, above, ,such that acoil form constructed of, a material having a resistivity value nearest to' the determined mum system damping.

value will provide opti- 1 An alternative coil for-m construction in accordance with my invention is shown in FIGURE 3, wherein the ment the parallelogram, indicated by the dotted outline 11, 12, is formed into a cylinder over the above-mentioned m-andrehwith the cylinder lengthwise edges 9'and 19 preferably abutting. The coil is then wound in the manner stated above. By thus reducing the gap 8, the induced form current, and therefore the damping eltect, may be slightly increased, for a given mass of form material, but at the expense of an additional design and manufacturing complication with respect to alignment of the parallelogram edges 9 and 10.

While the invention has thus been shown and discussed in connection with specific embodiments, it should be clearly understood that these have been given solely by way of example and not as limitations to the scope of the invention as set forth in the objects thereof, and in the accompanying claims.

Accordingly, I claim:

1. In a moving coil loudspeaker, a moving coil structure comprising:

(a) a form defining a constant velocity damping element composed of material selected from the group consisting of relatively pure copper and relatively pure silver, to be positioned in a magnetic gap,

(b) a voice coil winding on the form,

(c) a magnet having inner and outer pole piece extensions disposed to define a magnetic gap to receive between the inner and outer pole piece extensions the voice coil and form,

(d) a sound producing diaphragm connected to one end of the form,

(e) the form longitudinally dimensioned sufficiently greater than the longitudinal dimension of the gap so that the free other end of the form will be beyond the magnetic gap at maximum displacement of the voice coil.

2. In a moving coil loudspeaker, a structure according to claim 1 wherein the form provides approximately seven-tenths of the force required to critically damp said moving coil structure.

3. A moving coil loudspeaker according to claim 1 in which the copper has the approximate resistivity value of .O172 10- ohm-m. and the silver has the approximate resistivity value of .0163 1()- ohm-m.

4. In a moving coil loudspeaker including a magnetic gap having a transverse flux density B, a moving coil structure comprising:

a voice coil winding,

a sound producing diaphragm,

and a constant velocity damping element interconnecting said winding and said diaphragm,

said element being composed of a material having a resistivity k determined by the relationship:

6 where; L is the mean cross-sectional length of said element taken transverse to the axis of said magnetic gap; L is the length of said gap parallel to the axis thereof; L :L sufiicient that at maximum displacement of the winding, the free end of the element will be beyond the magnetic gap, and the element itself in the magnetic gap; T is the mean cross-sectional thickness of said element measured transverse to the said axis of said gap; M is the mass of said structure; w is the angular frequency of said structure at the point of natural resonance thereof; L is greater than L and where all of the above factors are measured in the MKS system of units. 5. In a moving coil loudspeaker, a structure according to claim 4, wherein:

the resistivity k determined by the said relationship is equal to .016 10 ohm-meters. 6. In a moving coil loudspeaker, a structure according to claim 3, wherein:

the said element is made'or" material selected from the group consisting of copper and silver. 7. In a moving coil loudspeaker having an annular magnetic gap, a structure according to claim 4 wherein: the said element is a split-cylinder form on which said winding is wound. 8. In a moving coil loudspeaker having an annular magnetic gap, a structure according to claim 4 wherein: the said element is a closed cylinder form on which said winding is wound. 9. In a moving coil loudspeaker having an annular magnetic gap, a structure according to claim 4 wherein: the said element is a cylindrical form, fashioned out of a slightly oblique parallelogram of the said element material, on which the said winding is wound.

References Cited by the Examiner UNITED STATES PATENTS 1,767,837 6/30 Davis et al. 179-1155 1,971,452 8/34 Herrmann 179-1155 2,590,554 3/52 Lukacs 179-1155 2,769,942 11/56 Hasson 179-1155 2,897,291 7/59 Burke 179-1155 ROBERT H. ROSE, Primary Examiner.

Claims (1)

1. IN A MOVING COIL LOUDSPEAKER, A MOVING COIL STRUCTURE COMPRISING: (A) A FORM DEFINING A CONSTANT VELOCITY DAMPING ELEMENT COMPOSED OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF RELATIVELY PURE COPPER AND RELATIVELY PURE SILVER, TO BE POSITIONED IN A MAGNET GAP, (B) A VOICE COIL WINDING ON THE FORM, (C) A MAGNET HAVING INNER AND OUTER POLE PIECE EXTENSIONS DISPOSED TO DEFINE A MAGNETIC GAP TO RECEIVE BETWEEN THE INNER AND OUTER POLE PIECE EXTENSIONS THE VOICE COIL AND FORM, (D) A SOUND PRODUCING DIAPHRAGM CONNECTED TO ONE END OF THE FORM, (E) THE FORM LONGITUDINALLY DIMENSIONED SUFFICIENTLY GREATER THAN THE LONGITUDINAL DIMENSION OF THE GAP SO THAT THE FREE OTHER END OF THE FORM WILL BE BEYOND THE MAGNETIC GAP AT MAXIMUM DISPLACEMENT OF THE VOICE COIL.

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