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United States Patent	3,665,352
Dietrich ,   et al.	May 23, 1972

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Current U.S. Class:	335/231 ; 381/420; 381/422
Current International Class:	H01F 7/02 (20060101); H04R 9/00 (20060101); H04R 9/02 (20060101); H01f 007/00 ()
Field of Search:	335/231 179/115.5,117,119

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References Cited

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U.S. Patent Documents

1976874	October 1934	Brzeski

Foreign Patent Documents

	1,280,324		Oct., 1968		DT
	367,998		Mar., 1932		GB

Primary Examiner: Harris; George

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Claims

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WHAT IS CLAIMED IS:

1. In a permanent magnet system for a loudspeaker which consists of axial magnetic magnets and soft iron parts forming an air gap therebetween the improvement comprising making a portion of the ferromagnetic circuit in the system from a mechanically machinable, dense, alloyed ferritic steel with higher electrical and lower magnetic resistance than soft iron and a saturation magnetization at least equal to the airgap induction required by the system.

2. A permanent magnet system according to claim 1 wherein the portion of the ferromagnetic circuit of the system in the region of the air gap consists of alloyed ferritic steel.

3. A permanent magnet according to claim 2 wherein the soft iron part is located concentric with respect to the magnets including first and second plates for the system which are attached to the longitudinal ends of the magnets and soft iron part, said plates being made of ferritic steel.

4. A permanent magnet according to claim 1 wherein a portion of the ferromagnetic circuit in the system consists of up to 0.15% C., 0.3 to 2.5% of Si, 0.2 to 2.0% Al, 5 to 25% Cr and balance iron.

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Description

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The invention is concerned with a permanent magnet system for loudspeakers that consists of a bar or ring shaped axial magnetized permanent magnet and soft iron portions such as bottom plates or top, upper plates and ore.

Loudspeaker magnet systems are known in various styles. Loudspeaker magnet systems must, as is known, show a high air gap induction. On the other hand, it must be considered in the building of a loud speaker magnet system that no overtones are produced in the speech coil which is disposed in the annular clearance of the loud speaker magnet system which lead to sound distortion. The characteristic distortion factor for the distortion is connected in such a way with the magnetic induction of the system that a decrease of the distortion factor for the suppression of the distortion normally causes a lessening of the magnetic induction.

The problem of the present invention is the creation of a loudspeaker magnet system by which the distortion factor becomes reduced without injury to or lessening of the air gap induction.

A solution of this problem is known through German patent 1,280,324. The proposal made there tends to produce poles on both sides of the air gap of the permanent magnet system in the region of motion of the speech coil from concentric dynamo laminations which are formed one on top of the other in the direction of the length of the gap. This known solution to be sure fulfills the above-named requirement after lowering of the distortion factor with retention of the air gap induction required by the system, however, it exhibits in the manufacture of such loudspeaker magnet systems technical production difficulties. The known system structure requires a high manufacturing cost. Primarily the rings must be produced from the laminated dynamo and be made in a crude manner. Then the loudspeaker system must be broken in the soft iron parts, especially in the upper plates and the soft iron core grooves, in which the plated rings should be inserted. Subsequently, the rings must be once more overwound or overground in order to form the ring gap in the required tolerance.

According to the invention a loudspeaker system is proposed which is simpler and cheaper to produce than known systems and which fulfills the above-named magnetic and acoustical requirements.

For the solution of this problem according to the invention it is proposed to produce part of the ferromagnetic circuit in the system from a mechanically workable, dense alloyed ferritic steel with higher electrical and lower magnetic resistance than soft iron and a saturation magnetization equal to or greater than the required air gap induction of the system. Especially the part of the ferromagnetic circuit found in the region of the air gap should consist of alloyed ferritic steel. This can be accomplished for example by production of the upper plate of the system and a cover plate from alloyed ferritic steel on the soft iron core.

Unless otherwise indicated all parts are by weight.

As a ferritic steel which fulfills the above-named provisions there are preferably used one such as

C up to 0.15% Si 0.3 to 2.5% Al 0.2 to 2.0% Cr 5 to 25% Mn 0.6 to 0.8% Ni up to 0.5% Mo up to 0.35% Fe balance

In a specific example there was used

C 0.1% Si 1.0% Mn 0.7% Al 0.9% Ni 0.25% Cr 18.0% Mo 0.15% Fe balance

For another example there was used

C 0.1% Si 1.2% Mn 0.7% Al 0.9% Ni 0.3% Cr 13.5% Mo 0.2% Fe balance

In contrast to the above-named known system structure with laminated ring segments the structure of the invention shows the advantage that part of the ferromagnetic circuit of the system can be produced from a dense raw material and not be required to be formed by relatively expensive layers of laminations. An especially simple solution results if according to the preferred development of the system the entire upper plate of the system and a cover plate on the soft iron core are produced from alloyed ferritic steel.

In the drawings there are shown non-limiting examples of various loudspeaker magnet systems useful in the invention.

FIG. 1 is a sectional view illustrating one form of the invention;

FIG. 2 shows an alternative embodiment; and

FIG. 3 illustrates a sectional view of still another embodiment of the invention.

In the drawings like numbers indicate like parts.

Referring more specifically to FIG. 1 of the drawings in the structure of a conventional loudspeaker system there is provided an axial magnetized ring-shaped permanent magnet 1 of hard ferritic material in which is inserted concentrically the soft iron core 3 while leaving free the annular clearance 2. The core 3 is, for example, combined with the bottom plate 4 as a single piece. According to the invention the upper plate 5 and a cover plate 6 on the soft iron core 3 can be made of alloyed ferritic steel. This material has first the required high electrical resistance at low magnetic resistance and besides possesses a saturation magnetization which corresponds to the air gap induction required by the system.

Instead of the embodiment shown in FIG. 1, nearly the same effect can be obtained as illustrated by FIG. 2 employing a stray field poor loudspeaker system by inserting rings 7 and 17 of alloyed ferritic steel in the upper plate 5 and in the soft iron core 3 on both sides of the air gap in the region of the speech coil disposed in the annular clearance. In contrast to the magnet system produced in FIG. 1, the system of FIG. 2 is distinguished in the additional arrangement of a permanent magnet 8 of hard ferritic material below the intermediate plate 9 of soft iron and a casing of soft iron surrounding the system.

In FIG. 3 there is shown another known construction for loudspeaker magnet systems. In a soft iron casing 10 there is arranged a rod shaped permanent magnet 1 of preferably Alnico material with a soft iron core 3 erected thereon. The upper plate 5 according to the invention is likewise entirely or partially made of alloyed ferritic steel.

Further non-illustrated example possibilities include a ring of alloyed ferritic steel disposed in the upper plate 5 and the core 3 entirely of alloyed ferritic steel. Additionally the combination of a cover plate on the soft iron core and in the ring 7 disposed on the upper plate at times can be made of alloyed ferritic steel.

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