US2007748A - Acoustic device - Google Patents

Description

July 9, 1935. H. F. OLSON ACOUSTIC DEVICE Filed June 1, 1953 2 Sheets-Sheet 2 ATTORN EY Patented July 9, 1935 PATENT OFFICE ACOUSTIC DEVICE Harry F. Olson, Camden, N. J assignor to Radio Corporation of America, a corporation of Delaware Application June 1, 1933, Serial No. 673,823

16 Claims.

This invention relates to improvements in translating apparatus of the electrodynamic or moving coil type. More particularly, the invention relates to improvements in the moving coil of such apparatus and to improvements in the elements associated with the moving coil. The invention is especially applicable to acoustic apparatus and in the following specification it is specifically described and illustrated as applied to loudspeakers. The invention is also applicable to microphones and to other types of translating and energy converting apparatus.

The performance of a loudspeaker is given by the ratio of the acoustical output to the electrical input. This ratio may be defined in a number of ways depending upon the loudspeaker and the method of measurement. In general, the most desirable loudspeaker is one in which this ratio remains constant over a certain frequency band; namely, the working range of the loudspeaker. It is an object of the present invention to modify the vibratile system of an electrodynamic loudspeaker so as to obtain a more uniform response over a greater frequency range than has heretofore been possible.

In the present day, flat-baffle, cone loudspeaker of the electrodynamic type, the response is generally considerably attenuated above 3,000 cycles. This reduction in response at the high frequencies is primarily due to the large mass of the heavy voice coil necessary for obtaining good low frequency response, and to the decrease in current through the voice coil resulting from its increase in electrical reactance with increase in frequency.

At low frequencies, the velocity of all parts of the vibrating system are in phase. Therefore, the low-frequency output is principally determined by the ratio of the applied force (which is the product of the field strength, length of the wire in the voice coil, and the current) to the combined mass of thecone, air load and coil. In order to obtain a good low-frequency output it is necessary to resort to a large voice coil, i. e., a coil having a comparatively large number of turns.

As the frequency increases, the effective mass of the cone at the driving point decreases. When this mass becomes comparable to the mass of the voice coil, the output can be increased by employing a lighter voice coil, and by designing this coil so that the ratio of its inductance to its resistance is small. Thus the reduction in current due to the electrical reactance will be minimized and the force will remain essentially constant as the frequency increases.

space occupied by the aluminum coil is about 65% greater than that occupied by the corresponding copper coil. Therefore, it is logical to use aluminum for a high-frequency coil, where lightness and not space is a factor, and copper for a lowfrequency coil where the number of turns in a given space is of greater importance than weight.

The primary object of the present invention is attained by vibrating a conical diaphragm of substantially uniform thickness and material, by means of a voice coil system generally similar to the voice coil system described and claimed in the copending application of Abraham Ringel S. N. 661,237 filed March 17th, 1933. The Ringel application describes and claims a voice coil system consisting of a high-frequency winding and a low-frequency winding disposed on the same coil form and separated by a compliant coupling.

The voice coil system according to the present invention, consists of a pair of coils on the same coil form with a compliant coupling therebetween. One of the coils consists preferably of a copper coil of a relatively large number of turns and the other coil consists preferably of a light aluminum coil of a lesser number of turns. The large copper coil is for driving the cone at low frequencies. The aluminum coil is for driving the cone at high frequencies.

Although an aluminum coil has certain inherent qualities which make it especially suitable for use as the highfrequency winding, in some instances it is more desirable to use a coil of copper wire of a lesser number of turns than are in the low-frequency winding, as the highfrequency winding. In the latter instance the high-frequency, copper winding preferably consists of finer wire than that used in the low frequency winding.

Another object of the invention is to provide circuits for supplying energy to the high-frequency and low-frequency windings in such a Way that the windings will function as driving coils 'in the most efficient manner.

Other and more specific objects of the invention will become apparent upon reading the following specification and appended claims in connection with the accompanying drawings which illustrate diagrammatically an approved form of a diaphragm and voice coil according to the invention, and circuits for energizing the voice coil.

The drawings also illustrate apparatus to which the invention has been applied and characteristic curves for the apparatus.

. In the drawings Fig. 1 is a cross-sectional view of apparatus embodying the invention;

Fig. 2 is a diagram of a circuit for energizing the windings shown in Fig. 1;

Fig. 3 is a graph illustrating the impedance characteristics of certain of the elements of the circuit of Fig. 2;

Fig. 4 is a graph illustrating response charac teristics of apparatus embodying the invention and of other apparatus;

Fig. 5 is the diagram of an electrical circuit which is the equivalent of the mechanical apparatus of Fig. 1;

Fig.6 is a perspective View of the diaphragm and driving coil system of Fig. 1;

Fig. '7 is a View of a detail of the apparatus illustrated in Fig. 1, and

, Fig. 8 is a diagram of another circuit that can be used for energizing the windings shown in Fig. 1.

An approved form of the invention is illustrated diagrammatically in Fig. l. A truncated conical diaphragm! issupported at its larger base by an annular rim of flexible material 2 suitably-'securedto a support 3 which may be a baffle or any other suitable member. The diaphragm is centered and supported at its smaller base: by aspider member 41 of usual construction. The spider member may be secured to the central pole piece of the pot magnet 5, as shown, or it may extend outwardly from the diaphragm to-suitable supporting means.

The diaphragm l is vibrated by the flow of current through the voice coil system located in the air gap between the pole pieces of the pot magnet 5. Only a fragment of the magnet system is shown as it is of well .known construction.

- The magnet is energized either by means of permanent magnetization or by a winding preferably surrounding the central pole.

The Voice coil system consists of a low fre-' quency winding 6 and a high frequency winding l both woundon a coil form 8. The low frequency winding 6 comprises a comparatively large number of turns of copper wire and the high frequency winding 7 according to one form of the invention, comprises a smaller number of turns of aluminum wire. In an approved construction the low frequency winding has approximately 150 turns of #32 enameled copper wire and the high frequency winding has approximately 60 turns of #34 enameled aluminum wire. Other combinations can be used, however, if found more satisfactory. Both windings are supported on a form 8 secured to the truncated. diaphragm at the smaller base thereof.

Suitable mechanical and electrical systems designed so that each winding will operate in its proper frequencyrrange, are provided for that purpose. The mechanical system consists of a compliant coupling 9 formed in the voice coil form 8 between the coils B and l. The electrical system consists of electrical reactance members arranged, for example, as in circuits 9 and :0 of Figure'2.

At-low frequencies the large coil 5 provides the driving force for vibrating the diaphragm. The flow of: current through the coil causes it to move in the magnetic field between the poles of magnet 5. The motion of the coil 6 is transmitted to the diaphragm at low frequencies through the compliant coupling 9. At high frequencies the circuit it including coil '1 and condenser l, provides a low impedance path for the actuating currents, thus causing coil 1 to be the driving coil. Both coils are energized over a band of intermediate frequencies.

The flexibility of the compliant coupling is made such that, for a coil of a given mass, there is substantially no transmission of motion between coil 6 and the remainder of the system at high audio frequencies, whereas, at low audio frequencies motion is transmitted from coil 6 to the remainder'of the systemsubstantially without loss. This is due to th'e'fact that the compliant coupling is absorbent to vibrations at high audio frequencies, while it is non-absorbent to 1 vibrations at low audio frequencies and, therefore, transmits low audio frequencies. The reason that the voice coil compliance 9 is absorbent to certain frequencies, while transmitting certain other frequencies is that the mechanical impedance of the compliance is inversely proportional to the frequency, while the mass of the Voice coil is proportional to the frequency. The compliance, therefore, shunts out'the motion-of the heavy coil and prevents transmission of the motion of this heavy coil tothe light coil and the cone. s

At an intermediate frequency rangaboth coils are energized and the coupling 9 transmits vibrationsfbetween the low frequency-winding Sand the diaphragm to a substantial extent. The compliance of the coupling 9 is adjusted so that this intermediate frequency range throughoutv which there is an overlap of the energy supplied to the two coils, occurs at a'desired part of the audio frequency spectrum.

The compliance preferably consists of 'a single corrugation pressed into the coil form. The stiffness of the compliance can bevaried by varying' the depth and shape of the corrugationTand by the application of a stiffening substance to the corrugation.

V In an approved modification using a diaphragm approximately 8- in'ches in diameter at its larger.

base, and adapted to cover'a frequency range of approximately cycles to 8,000 cycles. per second, it was found desirable to adjust the compliance so that the overlap occurred between 2,000 and 2,500 cycles'per second.

As shown in Fig; 2, the high-frequency'ancl low-frequency windings are connected in parallel. A one microfarad condenser E l is connected in series with the high-frequency winding E; The impedance of the winding 1 in series with the one microiarad condenser E! is such that appreciable current does' not flow through the winding at frequencies lower than approximately 2,000 cycles per second. The impedance of the low frequency winding 5 is such that appreciable current does not flow through the winding at higher frequencies, i. e., above 2,500 cycles per second. Theseyalues may be changed, however, if diaphragms of other sizes and physical prop erties are used, and when the operating conditions are changed.

Fig. 3 shows the impedance curves for the windings.

Due to the large inductance of the low-frequency winding 5, the simple filter comprising the condenser H, has been found to work satisfactorily; The shunting action of the low frequency coil at high frequencies (Fig. 2) is practically negligible. Due to the small capacity in the high frequency arm, the shunting effect of this branch is very small at low frequencies.

It has been found desirable in some instances to adjust the combined inductance of the two coils 6 and l and the capacity of the condenser H so that the circuit including these elements will resonate at the overlap frequency.

In Fig. 4 the output characteristic of an eight inch cone driven with a double coil of .the type described hereinbefore, is compared with theoutput characteristic of a standard single cone of the same size. The characteristic of the cone with the double coil drive is shown by a solid line whereas the characteristic of the standard cone is shown by a dotted line. The improved response of the double coil drive at the higher frequencies, is apparent.

An equivalent circuit of the mechanical system, is shown in Fig. 5. The mass of the copper coil 6 is represented by the inductor 20, the mass of the aluminum coil 1 is represented by the inductor 2!, the mass of the cone I by the inductor 22, the compliance between the coils 6 and 'l by the capacitor 23, the compliance of the cone suspending means by the capacitor 24 and the resistance of the cone by the resistor 25. The driving force f1 at low frequencies is represented at 26 and the driving force f2 at high frequencies is represented at 21. The mass and resistance of the cone at the driving point are functions of the frequencies.

Referring to the equivalent circuit, the velocity of the cone when driven by the low-frequency coil 6 is Xii and the velocity of the cone when driven by the high frequency coil 1 is Xhf. At any instant the velocity of the cone given by the expression where, referring to Fig. 5,

f=frequency, mzo=mass of the heavy coil (inductor 20 of Fig. 5),

c'zs compliance of the mechanical member separating the two coils (capacitor 23 of Fig. 5)

Z3=jwmz1+ mzi=mass of light coil (inductor 2| of Fig. 5) C24=compliance of cone suspension system,

(capacitor 24 of Fig. 5) m22=effective mass of the cone (inductor 22 of Fig. 5) rrz5=resistance due to air load on the cone and dissipation in the suspension system (resistor 25 of Fig. 5).

When the mechanical impedance Z2 equals the mechanical impedance -Z1 the velocity equals zero. The compliance C23 is adjusted so that this occurs at approximately 2200 cycles for the 8 cone hereinbefore referred to.

.Above this frequency, the velocity of the cone due to the high frequency coil increases, while the velocity of the cone due to the low frequency coil decreases. The logical place to divide the current between the two coils is the point where The performance of this system may be summarized theoretically by stating that at low frequencies the heavycoil exerts the preponderant influence upon the cone. The compliant link causes this to be segregated from the cone at about 2,500 cycles, just before the cone would ordinarily have a peak of resonance. Simultaneously, the phase relations between the heavy coil and the light coil are such that their combined driving effort on the cone is properly proportioned relative to the range above and below this frequency. At high frequencies the light coil acts as the active driver. The high frequency range is limited only by the mass of this coil, the mass of the cylinder by which it is connected to the cone and the radiation characteristic of the cone. Lightening the coil and cylinder therefore increases the high frequency range.

Figure 6 illustrates a diaphragm to which the invention has been applied. It has been found desirable to fasten the voice coil leads to a part of the vibratile system, such as the apex portion of the diaphragm, as shown at 30 and 3 i. The leads for the low-frequency winding '6 are attached to the diaphragm i by suitable means 30 and the leads for the high-frequency winding '1 are attached to the diaphragm l by attaching means 3! If the leads, especially the leads for the highfrequency winding, are not supported as at 3i, their mass is added to the mass of the high-frequency winding and the mass of the portion of the voice coil form on which it is wound. This tends to increase the mass of the vibratile parts at the higher frequencies to such an extent as to materially affect the high-frequency response. In some instances it has been found desirable to increase the length of the portion of the coil form on which the high-frequency winding is supported, and to attach the leads to the coil form between the winding and the diaphragm. Care should be taken, however, not to make the high frequency portion of the coil form too long as this increases the mass of the parts vibrating at the higher frequencies. It is an object of the in vention to reduce to a minimum the mass of the parts which vibrate at the higher frequencies.

It may also be found desirable to support the voice coil leads on the spider member. The spider member is especially adapted to support the voice coil leads if it is arranged to extend from the diaphragm to the outer pole piece rather than to the inner pole piece as in Figure 1.

In winding the coils on the form it has been found desirable to first wind the copper coil, i. e., the low-frequency winding 6. The leads from the copper coil are brought out at an acute angle over the bead or compliant coupling 9 as shown at 40 in Fig. '2. They are brought to a slot 4| in the coil form 8, where they are given a sharp turn and laid in the plane of the coil form as shown at 62, longitudinally in the slot 4|. The light coil i. e., the high frequency winding, is then wound over the leads from the low frequency coil 6. The acute angles formed as at 40, prevent the deformation of the compliant coupling or bead 9 from causing sharp bends in the leads. Breakage of the leads is thereby avoided.

After the coils are wound, both'of the coils and the bead or coupling 9 maybe painted with thinned ambroid, or a similar type of varnish. The coil assembly 'is then cemented to the diaphragm in any approved manner, such as by means of suitable tabsforrned on the cone. The leads may then be secured in any approved manner, to the diaphragm, the coil form, the spider member, or to any other part of the vibratile system or to any other member, as desired.

If it is desired to increase the stifiness of the bead or coupling 9, additional ambroid may be added or a shallower corrugation may be used. If it is desired to decrease the stiffness of the coupling, less ambroid should be used or a deeper corrugation should be made. It has been found that paper works very well as material for this purpose. Where greater strength is desirable metal such as duralumin and phosphorous bronze, may be used as the compliant coupling between the sections of the coilrformr Four leads may be brought out from the windings 6 and ?,.as shown in Fig. 6. However, it has been found that the mass of the vibratile system can be reduced by connecting corresponding ends A of the two windings (assuming they are wound in the same sense) to form a common terminal and to thereby reduce the leads to three in number. V

In some instances the value of the condenser it may be reduced from one microfaradto one half or even one third of a microfarad. In general, however, it is desirable to have the condition E.5 zero Where Lz; :inductance of low frequency coil Ln inductance of high frequency coil C=capacity of condenser H V exist with the value of --fbeing substantially in the middle of the frequency range of the overlap. The overlap region is given by rent flows in both coils. When the ultimate in low-frequency efficiency is desired, the resistance per unit length in each section should be made the same regardless of whether copper or aluminum is used. Consider the circuit shown in Fig. 8. The current in the heavy coil is given by- -ZL=Impedance of light coil,

Zc mpedance of generator, zz lmpedance of heavy coil, Z3=Impedance of condenser.

At the resonance frequency of the mass of the heavy section with the compliance, the velocities of the two coils differ in phase by In order that the overlap frequency shall not show a diminution in response with respect to the remainder of the range, it is desirable that the phase in the two coils differ by a fairly large angle. Due to the relatively large inductance of the heavy coil with its large number of turns and the presence of the capacitor in parallel with this coil, together with the small inductance of the light coil makes it possible to obtain this result in practice. 7

The force developed in the heavy coil is F =BZI2 Eq. (8)

where V B :fiux density, Z =length of wire in the light coil, I1=current in the light coil from Equation (2).

From a consideration of the foregoing equations, it is possible to choose the constants so that V uniform response will result in the same manner as forthe parallel connection of Fig. 2.

It isto be understood that the apparatus and circuit diagrams set forth and the constants referred to, are for explanation and illustration only and that many changes can be made in the apparatus, circuits and constants without departing from the spirit and substance of the invention, the scope of which is to be limited only by the appended claims.

What I claim is: V 1. A transducer comprising a frusto-conical diaphragm of substantially uniform thickness and rigidity, throughout its entire area, a coil form connected to said diaphragm at the smaller base thereof by a connection which is effective in transmitting vibrations to substantially the same extent at all frequencies, a compliant coupling dividing said coil form into a pair of sections, and a winding on each section. 1 1

2. A transducer comprising a conical, .diaphragm of substantially uniform rigidity throughout its area, a coil form secured to the apex portion of said diaphragm by a connection efiective in transferring vibrations at all frequencies, a winding on said form adjacent said diaphragm, a second winding of more turns and of greater mass than the first mentioned winding spaced from the first mentioned winding, and. agcompliant coupling in the coil form between said windings.

3. A coil system for an ele'ctrodynamic acoustic device comprising a coil orm, a pair of windings on said form, and a compliance in said form between said windings, said compliance having a relatively low mechanical impedance at frequencies above approximately 2,500 cycles per second and a relatively high mechanical impedance at frequencies below approximately 2,000 cycles per second.

4. A coil system for an electro-acoustical translating device comprising a pair of windings, a coil form section for each winding, and means physically connecting said coil form sections, said means beingeffective to transfer vibrations to some extent at frequencies between 2,000 cycles and 2,500- cycles per second, eifective to transfer substantially all vibrations, at lower frequencies and substantially ineffective in transferring vibrations at higher frequencies.

, 5. A transducer comprising a frusto-conical diaphragm substantially uniform in rigidity throughout its entire area, a coil form secured to the smallerbase of said diaphragm by a connection equally effective at all audio frequencies in transferring vibrations, a pair of windings on said coil form, a compliant coupling in said form between said windings, and means providing a low impedance path for high frequency electrical va riations connected in shunt with one of said windings.

6. An electro-acoustical translating device comprising a diaphragm, a voice coil form connected with said diaphragm, a pair of windings on said coil form, a compliant coupling in said coil form between said windings, one of said windings hav ing such an electrical reactance that it offers a high impedance to frequencies above approximately 2,500 cycles per second and a lower impedance to lower frequencies, and means connected in circuit with the other winding, which in connection with said other winding, offers a high impedance to frequencies below approximately 2,000 cycles per second and a lower im pedance to higher frequencies.

7 A voice coil system for electrodynamic acoustic apparatus comprising a coil form, a pair of windings on said form, a compliant coupling between said windings, and connections and electrical reactance means whereby the flow of alternating currents at frequencies higher than 2,500 cycles per second is materially less through one of said windings than the other, and the flow of alternating currents at frequencies lower than 2,000 cycles per second is materially less through the said other winding than the first-mentioned winding.

8. A transducer comprising a diaphragm, a voice coil form secured to said diaphragm, a pair of windings on said form, connections and electrical reactance means whereby there is a low impedance path for bypassing high frequency alternating currents around one of said windings and providing a low impedance path for low frequency alternating currents through the said winding, said means permitting an overlap of said high and low frequencies over a band of approximately 500 cycles, and a compliant area in said coil form between said windings, said compliant area having a relatively high mechanical impedance at cer tain frequencies, a relatively low mechanical impedance at certain other frequencies, and a moderate value of mechanical impedance over a band of frequencies substantially the same as the band of frequencies of said overlap.

9. A transducer comprising a diaphragm, a voice coil form secured to said diaphragm, a pair of windings on said form, circuit arrangements whereby there is a larger flow of alternating currents above 2,500 cycles per second through one of said windings than through the other and a larger flow of alternating currents below 2,000 cycles per second through the said other winding than throughthe first-mentionedwinding, said circuit arrangements permitting an overlap through the band from approximately 2,000 cycles to 2,500 cycles per second, and a compliant area in said coil form between said windings, said compliant area being effective in transferring vibrations at frequencies below 2,000 cycles per second, substantially ineffective in transferring vibrations at frequencies above 2,500 cycles per second, and effective to a varying degree in transferring vibrations throughout the range between 2,000 and 2,500 cycles per second.

10. A loudspeaker comprising a conical diaphragm of substantially uniform rigidity throughout its entire area, a voice coil form, a connection between said coil form and the apex portion of said diaphragm which is substantially equally effective in transferring vibrations at all audio frequencies, a high frequency winding and a low frequency winding on said coil form, a compliant coupling between said windings, a condenser connected in series with said high frequency winding, leads connecting said high frequency winding and condenser in parallel with said low frequency winding, said low frequency winding having a high impedance to high frequencies and said condenser and high frequency winding having a high impedance to low frequencies, said low and high frequencies having an overlap over a small range of frequencies.

11. A loudspeaker comprising a conical diaphragm, a voice coil form, a connection between said coil form and the apex portion of said diaphragm which is substantially equally effective in transferring vibrations at all audio frequencies, a high frequency winding and a low frequency winding on said coil form, a compliant coupling between said windings, a condenser connected in series with said high frequency winding, leads connecting said high frequency winding and condenser in parallel with said low frequency winding, said low frequency winding having a high impedance to high frequencies and said condenser and high frequency winding having a high impedance to low frequencies, said low and high frequencies having an overlap over a small range of frequencies, said windings and condenser being adjusted to resonate at the overlap frequency.

12. In an electrodynamic acoustic device, the combination of a diaphragm consisting solely of a conical member of substantially uniform rigidity throughout its entire area, and a translating coil secured thereto, said translating coil comprising a plurality of turns of a suitable conductor and a plurality of turns of a second conductor, a unit length of which has materially less mass than a unit length of the first-mentioned conductor, the plurality of turns of the winding of less mass being positioned between the diaphragm and the plurality of turns of the first-mentioned conductor.

13. A driving coil for vibrating the diaphragm of an electrodynamic loudspeaker, said diaphragm consisting solely of a conical member of substantially uniform rigidity throughout its entire area, comprising a plurality of turns of conductive material arranged so that the greatest mass is located at the end of the coil farthest from the diaphragm.

14. A driving coil for vibrating the diaphragm of an electrodynamic loudspeaker, said diaphragm consisting solely of a conical member of substantially uniform rigidity throughout its entire area, comprising a plurality of sections, the first section consisting of a large number of turns distributed over a comparatively small area, the second section consisting of a small number of turns distributed over an area which is large compared to the area occupied by a similar number of turns in the first mentioned section.

15. A driving coil of the type described in claim 14 in which the second mentioned section is arranged between the first mentioned section and the diaphragm.

16. A loudspeaker comprising a sound wave 'effectivein transierring vibrations at-all audio producing device constructed and arranged for frequencies, a compliance div'iding sa'id (1011 form reproducing all frequencies within the audio f-reinto sections, a winding "on each-section, and

quency range, said device having the shape of a "electrical filtering means associated with one'of 5 frustum of a cone, a voice coil forma connection saidwindings.

between the smaller base of-the cone and the I v HARRYF. OLSON. voice coil form which is substantially equally

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