US2806905A - Sound translating devices - Google Patents

Description

Sept 17, 1957 I w. CHRISTENSEN 2,806,905

SOUND TRANSLATING DEVICES Filed Feb; 27, 1952 INVENTOR. VA L 75? d/S'RISTFA/SEN BY- 2 A771 United States Patent Ofilice 7 2,806,905 Patented Sept. 17, 1957 2,806,905 SQUND TRANSLATING DEVICES Waiter Christensen, Virum, Lyngby, Denmark Applicable-n February 27, 1952, Serial No. 273,698 1 Claim. (Ql. 179-100A1) This invention relates to sound translating devices and is, more specifically, though not exclusively, concerned with gramophone pick-ups of the type having a three-pole magnetic system comprising an intermediate pole or core, and a pair of outer poles situated substantially symmetrically with respect to the intermediate pole and separated from each other by an air gap, in which an armature, extending from the end face of the intermediate pole into the air gap and connected with a gramophone stylus is adapted to vibrate in unison with the stylus to induce varying voltages in a coil.

It is the main object of the invention to provide for certain improvements in such pick-ups, and above all to substantially reduce the inertia and other mechanical resistance opposing the oscillatory movements of the armature in the pick-ups known heretofore, while at the same time providing for a maximum linear amplitude of the armature in order to increase the output of the pick-up.

With the said construction of pick-ups, the distance between the point of the stylus and the axis of oscillation of the armature should be relatively small, proportionately to the distance between the axis of oscillation of the armature and the edge thereof situated between the outer poles, so as to insure that the linear amplitude of oscillation of the armature will be relatively great even if the dimension of the armature at right angles to the axis thereof is relatively small. Therefore, the invention may advantageously be applied to such pick-ups in which the axis of oscillation of the armature is adapted to be substantially at right angles to a gramophone record during reproduction thereof, the end of the armature adjacent the record being provided with an arm extending radially from the axis of oscillation of the armature and having a stylus, e. g. a sapphire, at its free end. Such stylus arm may be made as small as desired and is, moreover, preferably, resilient in the direction of the axis of the armature. To this effect the arm may consist of a thin metal strip.

This resilient stylus arm may be made to yield, if the load thereon, i. e. the load on the stylus, should substantially exceed the normal one. However, as in accordance with the invention the weight and the moment of inertia of the oscillating system of the gramophone pick-up is reduced so as to enable the pick-up to reproduce very high sound frequencies, at the same time as the load on the stylus is reduced, the moment of inertia of the needle arm will represent a relatively great portion of the total moment of inertia of the whole of the oscillating system, especially in view of the fact that the length of the stylus arm is substantially greater than the dimension of the armature in a direction at right angles to the axis thereof. At the same time the resonance frequency of the stylus arm may fall within the frequency range covered by the pick-up so as to produce undesired resonance peaks in the frequency characteristic of the pick-up, and this applies to torsional oscillations as well as to bending oscillations.

With a given length of the needle arm, measured from the axis of oscillation of the armature to the stylus point and being normally in the order of 0.160.6 inch, the resonance frequency of the needle arm depends of the mass and the spring characteristic thereof. These two characteristics are, with a given material, approximately determined by the normal stylus load, usually in the order of 515 grams, because the resilient strip constituting the stylus arm must yield at a stylus load not very much in excess of the normal one. This gives the required base for calculating the spring characteristic as well as the dimensions and the mass of the stylus arm.

Therefore, it is a further object of the invention to provide certain materials for the stylus arm with such properties that, with a desired spring characteristic of the arm properly suited to the normal stylus load, the resonance frequency of the arm will be outside the audible frequency range, at'the same time as the moment of inertia of the arm with respect to the axis of oscillation of the armature will be considerably smaller than heretofore.

Still another object is to provide for such screening of the stylus and the armature that practically no direct audible vibrations, neither in the form of tones nor in the form of stylus noise, will issue from the pick-up.

The invention will now be described in detail with reference to the accompanying drawing, in which:

Fig. 1 is a schematical plan view of the essential parts of a gramophone pick-up in accordance with the invention,

Fig. 2 is a longitudinal section of the pick-up, and

Fig. 3 is a facial view of the underside of the pick-up.

The pick-up illustrated in the drawing comprises a rectangular magnetic frame in one side of which there is an air gap and which may consist of a single horseshoe magnet, but which is preferably composed of two parallel magnet bars 1 and 2 constituting two sides of the frame, whilst the other two sides thereof are constituted by a yoke 3 and pole pieces 4 and 5, respectively. The yoke 3 is intermediate its ends connected with a preferably laminated spool core 6 arranged parallelly to the magnets 1 and 2 and extending towards, but not into, the air gap between the pole pieces 4 and 5. The core 6 is provided with a coil 7, and a movable armature 8 made of soft iron plate, for example, is arranged in immediate continuation of the core so as to extend into the air gap between the pole pieces 4 and 5. The armature is adapted to oscillate about a vertical axis defined by pivots 9, as hereinafter described in detail, and extending at right angles to the plane of the magnet frame at a location between the end face of the core 6 and the adjacent side faces of the pole pieces 4 and 5. As will be apparent from Fig. 2, the armature 8 is of relatively small dimensions in a direction at right angles to the axis 9-9 so as to present a small moment of inertia with respect to this axis, whereas its dimension in the direction of the axis 99 is substantially the same as those of the pole pieces 4 and 5 and the end face of the core 6. The air gap between the armature 8 and the core 6 is very small, e. g. of the order .004 inch, whereas the air gap between the pole pieces is large enough to insure free vibration of the armature about the axis 9-9 under the action of the undulations of the sound groove of a gramophone record without the armature making contact with the pole pieces or even without exceeding the straight portion of its magnetization curve.

The magnet frame is mounted upon a non-magnetic base plate 19 having a plastic or rubber bearing 11 for one pivot 9, while another similar bearing 12 for the other pivot 9 is provided in a plate 13 of non-magnetic material secured to the upper sidesof the magnet frame. The pivots 9 are stamped integral with the armature place 8 and are, consequently, of a rectangular cross section. Therefore, sockets 14 of a cylindrical circumference are mounted on the pivots to provide for suitable bearing faces for cooperation with the bearings 11 and 12. These bearings constitute, in addition, the required damping means for the armature in respect of undesired vibrations about other axes than that intended. Equally, a suitable elasticity of the plastic or rubber material of the bearings will cause the armature to return to its intermediate or a neutral position in the air 4 and 5 position.

v gap between the pole pieces upon each deviation from this intermediate The lower pivot 9 extends through the bearing 11 and is, at its lower end, provided with a substantially horizontal stylus arm 15 constituted by a vertically flexible,

resilient metal strip having astylus 16, e.-g. a sapphire mounted at its fr'e'eend. The arm-15 is bent slightly downwards so as to provide for 's'uflicient space between the free end of the stylus arm and the underside of the base plate 10 to permit a suitable deflection of the arm. In order to insure a maximum flexibility in conjunction with a minimum moment of inertia without the risk of producing a resonance" frequency of the arm within the audible range, special materials should be used for this arm. 7

In accordance with the invention, this object is accomplished by making the stylus arm of a material which complies with the condition E/qZ'approXimately 1.6 x 10 centimeters where E is the modulus of elasticity of the material in kilograms per'square centimeter, and q is the specific weigth of the material in grams per cubic centimeter.

When using this criterion for selecting material for the stylus arm it is surprisingly found that the materials commonly used heretofore, viz. various bronzes, are wholly unfit for the purpose, whereas a series of materials, which have heretofore been deemed to be completely out of question, are excellentlysuited to the object, as has now also been confirmed experimentally.

For example, a bronze alloy, commonly used for stylus arms and having a modulus of elasticity of 850,000 kilograms per square centimeter and a specific weight of 8.7 grams per cubic centimeter, has a quotient of 0.975 centimeters, i. e. a value considerably below the desired one. Another bronze alloy having a modulus of elasticity of 1,200,000 kilograms per square centimeter and a specific weight of 8.3 grams per cubic centimeter has a quotient of 1.45X10 centimeters, i. e. also below the value in accordance with the invention. On the other hand, steel having a modulus of elasticity of 2,100,000 kilograms per square centimeter and a specific weight of 7.75 grams per cubic centimeter, has a quotient of 278x10 centimeters, i. e. a relatively favorable value. Similar advan'tageous values are suprisingly found with such relatively soft materials as magnesium and Duralumin and with such heavy material as iridium, as will be apparent from the following table which also includes the values quoted hereinbefore.

E, kilograms q, grams per square per cubic E/q, centicentimeter centlmeters meter 420, 000 1. 74 2. 41x10 5 735, 000 2. 8 2. 63x10 5 5, 200, 000 22. 4 2. 32x10 5 2 100, 000 7. 75 2. 78x10 5 850,000 8 7 0. 915x10 5 Bronze II 1, 200, 000 8 3 1. 45x10 5 As a consequence of using the materials in accordance with the invention, the stylus arm may be made to yield upon a very slight increase of the stylus load in excess of the normal one, and still it will have no audible resonance frequency.

The yielding of the arm is intended to allow the pickup to engage the gramophone record with relatively soft projections on its underside which are better suited to the purpose of transmitting an incidentally increased load upon the pick-up to the record than is the stylus, without causing any damage to the record. Such projections on the pick-up are already known, but in accordance with the invention they may in addition be so formed as to prevent any direct sound transmission from the vibrating stylus arm during reproduction of a record.

This object isaccomplished as follows:

' On'the underside of the base plate 10 there is secured 4 a circular disc 17 of some suitable elastic fibre material or other suitable, e. g. plastic material and provided with a recess 18 for receiving the arm 15. The edge of the recess is formed with a ridge 19 which, around the stylus 16, isof such height that, when the stylus resiliently engages a gramophone record 20 under the action of the normal pick-up load, the point of the stylus will project about one hundredth of an inch below the ridge.

From the area about the stylus 16 the height of the ridge becomes gradually smaller towards the pivot 9, and likewise the side of the ridge remote from the recess 18 is sloping towards the disc 17. The width of the recess at the free end of the arm 15 is such that the angular clearance of the arm within the recess is smaller than the angular clearance of the armature 8 in the air gapbetween the pole pieces 4 and 5 so that, consequently, the disc 17 will also prevent the armature from making contact with the pole pieces.

The effect of the disc 17 with the ridge 19 is, in the first hand, that the stylus point 16 is unable to do any harm to the record 20 because, in case of an incidental increase of the load on the pick-up, the' arm 15 will yield in such a manner that the portion of the load exceeding the normal one is transmitted to the record 20 through the. elastic ridge 19 so as to be distributed over a relatively large area of the record. Furthermore,

of the record, acoustically enclosed in the recess 18 so that practically no audible sound can escape therefrom. In order to prevent a direct, acoustic sound transmission from the vibrating armature 8, non-magnetic filling members 21, 22 are arranged between the coil 7, the magnets 1, 2 and the pole .pieces 4, 5 which latter have an equally non-magnetic butt-strap 23 secured to their outsides to cover the airgap between them so that the armature 8 is completely enclosed between the members 10, 13, 21, 22 and 23. These members, made of nonmagnetic and preferably fibrous material, will, therefore, absorb the sound vibrations of the armature which latter will thus be acoustically insulated from its surroundings.

I claim:

An electro-magnetic sound translating device comprising a substantially U-shaped magnetic yoke, a metallic core magnetically connected with said yoke intermediate the ends thereof, a coil around said core, a pair of pole pieces at the ends of said yoke, said pole pieces being substantially symmetrically arranged with respect to said core and being separated from each other by an air gap substantially in alignment with said core but spaced from the endxface thereof, the end faces of said pole pieces and of said core facing said air gap being substantially equal In length in a direction at right angles to the plane of said U-shaped yoke, an armature consisting of a flat form-magnetic strip approximately the length of said end faces and formed with pivots at itsends, non-magnetic beanng members mounted on said pole pieces and said core and ncluding bearings for said pivots, said pivots and bearlngs being so arranged as to define a pivot axis for sa1d armature situated in the plane thereof and intermediate the edges thereof in the space separating the end face of said core from said air gap and to support said armature with one edge thereof adjacent said end face of said core and with the opposed edge in said air gap between said pole pieces; and a stylus arm associated with one of said pivots.

References Cited in the file of this patent UNITED STATES PATENTS A nsar Jan. 30, 1951

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