US2373181A - Transducer - Google Patents

Description

Patented Apr. 1 0, 1945 TRANSDUCER Lawrence Fleming, Falls Church, Va.

Application February 16, 1942, Serial No. 431,160

7 Claims.

This invention relates to electro-mechanical transducers of the type used for the recording and reproduction of sound on discs, and more particularly t phonograph pickups for lateralcut records.

An object of the invention is to provide a pickup in which the mechanical impedance presented by the stylus to the record has the optimum proportions of mass, stifiness, and damp- A further object is to provide a pickup of the electromagnetic type capable of reproducing faithfully the entire range of audible frequencies while operating with a total unbalanced weight on the record so low a to permit the use of a permanent jewel-like stylus without appreciable wear of the record.

A further object is to provide a pickup of the above properties whose construction is simple and inexpensive.

Other objects will appear hereinafter.

In the drawing:

Fig. l is a greatly enlarged diagrammatic crosssectional view of a phonograph pickup stylus in position in a record groove, illustrating the forces acting thereon;

Figs. 2, 3, and 4 are graphs useful in explaining the invention;

Fig. 5 is a top view, partly cut away, of a phonograph pickup embodying the invention;

Fig. 6 is a cross-sectional view taken on line 6-6 of Fig. 5;

Fig. 7 is a cross-sectional view taken on line 1-1 of Fig. 5;

Fig. 8 is a partial cross-sectional view taken on line 88 of Fig. 5;

the groove walls tangentially as shown at 6. It is does not wear to lit the bottom of the groove but rather retain its spherical tip shape substantially throughout its life.

When the groove carries a recorded sound, it moves from side to side, and exerts a force on the stylus in forcing it to execute the same motion. This force is indicated in the form of its horizontal and vertical components F, F which are equal because the groove wall is inclined at The total unbalanced Weight of the pickup on the record is carried by the stylus tip and is shown as a force P. If at any time the force F becomes greater than the force P the pickup will be pushed momentarily upward out of contact with the groove 3 and the reproduction of the recorded sound will be distorted.

It is desirable to have the weight P on the record rather low to minimize wear of the record and the stylus. Experience has shown that the weight P should be of the order of one ounce (28 grams) or less. The pickup should therefore be designed so that while it is playing, the force F should rarely or never exceed about this value.

The magnitude of the force F is determined by the following factors: the amplitude and frequency of the lateral motion of the groove, the eflective mass of 'the vibratory element of the pickup as measured at the tip of the stylus, the stiflness of the suspension of the vibratory element, and the damping of the vibratory element.

The proportioning of these factors will be taken up after the pick-up shown in Figs. 5 to 10 is described, in order to describe the invention more clearly.

, Referring to. Figs. 5 to 8 inclusive, a U-shaped permanent magnet 9 is held in place against two pole pieces In, H by means of screws l5, l6 which pass through holes in a strap member l2 into the said pole pieces and magnet. The strap member I2 is made of non-magnetic material such as brass. Recesses 20, 22 (shown best in Fig. 7) are formed in the pole piecesill, II, and in the space thus formed is a coil of wire 30. The ends of coil 30 are connected to terminals 33 from which the electrical output of the pickup is taken.

Extending vertically downthrough the open center of coil 30 and between pole pieces II), II is the vibratory element or armature '40 of the pickup. This armaturefll'carries the stylus 4 at its lower extremity, supported through a small ferrule 45. The direction of the lateral vibration of the stylus 4 is indicated by an arrow 66 (Fig. 7). The suspension of armature consists of two spring 42 and 44. The upper spring 42 is of essentially wide, fiat shape, so that the portion 49 to which the upper end of armature 40 is secured is free to deflect torsionally and also to some extent in an up-and-down direction, but cannot deflect substantially in the lateral direction. So far as lateral motion of the stylus 4 is concerned, the spring 42 acts, for Small amplitudes, rather like a pivot, but without the disadvantages of looseness and friction. Spring 42 may conveniently take the form of a fiat cantilever member made of phosphor bronze or the like, secured to the upper end of the armature 40 as at 49 by soldering or welding, and anchored at its other end by a screw IT.

The lower spring 44 is a wire-like member which may be secured to the lower portion of armature 40 as at 50 by soldering or welding, and anchored at its other end 46 to a post IS. The combination of the two springs 42 and 44 allows degrees of freedom of motion to the stylus 4 in the lateral and vertical directions, but constrains it from motion in the direction of the groove. The degree of freedom in the vertical direction is desirable to absorb minor shocks caused by accidental dropping of the pickup head on the record, and is also necessary to take care of pinch efiect. Pinch effect is discussed in an article by me which appeared in the Journal of the Accoustical Society of America, vol. 12, pages 366-373 (January 1941),, which also describes other features of the present invention.

The armature, springs, and ferrule may alternatively be made integrally from a single piece of resilient sheet magnetic material, as illus trated in Fig. 10. The armature portion is made in a channel cross-section, and the upper and lower springs 62 and 64 are constituted from integral tabs suitably bent. The lower extremity of portion 60 is formed into a socket portion 65 into which stylus 4 is secured. It has been found that cellulose cement and stick shellac are effective agents for cementing styli in place. The cross-section of portion 60 may obviously be of circular, Z-like, or angular configuration instead of the channel-shape shown.

Damping material such as is sold under the trade name "Viscoloid may be employed to damp the natural oscillations of the armature. For the lateral motion of the stylus a small damper 10 is illustrated, and at H is a second damper designed to damp more particularly the vertical motion. While vertical movement of the armature produces no electrical output, I have found that vertical damping was effective in removing a slight ringing quality associated with the reproduced surface noise. While I do not wish to be limited to the following theory, I believe that when the vertical oscillation of the armature is undamped, the armature tends to "stutter" in and out of contact with the record at its natural frequency, under the influence of the irregularities in the record surface; and that this periodic variation in stylus pressure is reflected in the lateral motion of the armature, causing the natural frequency of vertical oscillation to become unduly prominent in the spectrum of the reproduced surface noise.

The operation of the present pickup is the same in principle as any conventional "balanced armature" reproducer; the armature 40 'is in effect pivoted about the axis of spring 42, and its motion causes it to change its magnetization, inducing a, current in the stationary coil 30 which surrounds it. The current is proportional to the velocity of motion of the armature.

Reference is now made to Figs. 2 to 4 inclusive. In U. S. Patent 1,981,793 to A. C. Keller it is shown that if the stylus of a vertical or hill-anddale t pe pickup to remain in contact with the groove, the following relation must hold:

FiVZ 1) where F is the force tending to hold the pickup down, V is the peak vibrational velocity of the motion of the stylus, and Z is the mechanical impedance which the stylus tip presents to the record.

V=Aw

where A is the amplitude of the recorded wave and w is 21r times the frequency.

where R is the damping factor of the vibrating system in dynes/cm./sec., m the effective mass in grams, and s the stiffness of the spring suspension in dynes/cm.

I have found that the same equation can be used to express the behavior of a, pickup which plays lateral-type records, provided only that the Walls of the groove are inclined at substantially 45. Referring to Fig. 2, the factor Fin Equation 2 can be considered the same-as F in Fig. 1. It is the upward component of the force which the record exerts on the stylus in overcoming the mechanical impedance of the pickup. The

weight P must be greater than duction is to be had.

A recording characteristic often taken as standard provides for an amplitude of 0.002 inch at 300 cycles and at all frequencies .below 300 cycles; and for constant vibrational velocity V at all frequencies above300 cycles. This frequency at which the recording characteristic changes from "constant amplitude" to constant velocity" is commonly called in the art the "turnover" frequency, and, as is well known, may have other values than 300 cycles, such as 800 cycles. Fig. 2 gives a plot of the force F versus frequency on the basis of this characteristic, for an assumed pickup having the following mechanical constants: Effective mass m, 50 milligrams; stiffness s F if clean repro- 6 10 dynes/cm.; damping R, critical. The condition for critical damping is The lowest point in the curve corresponds to the resonant frequency of the system, where the-mechanical impedance consists solely of the damping R.

Now, Fig. 2 represents the variation of F with frequency on an idealized frequency record, in which it was assumed that the voltage fed into the recorder was independent of frequency. In actual practice the behavoir of a pickup on such a record is of less interest than its performance on a record which carries actual music. In orchestral music for example the distribution of sound level with respect to frequency is not uniform at all, but is known to have characteristics which are summarized in approximate for in the following table:

\ Relative Fmqlllmcy voltage If the ordinates of the curve of Fig. 1 be each multiplied by the corresponding factors. in the right-hand column of the above table, a new curve is obtained which is shown in Fig. 3. This curve represents the way the reaction force F would vary with frequency on a record in which the peak vibrational velocity varied as in the above table.

While the curve of Fig. 2 is symmetrical, the curve of Fig. 3 is not, and does not represent the performance of a pickup of optimum design. According to the invention the proportions of effective mass and stiffness are now changed so that the peaks in the characteristic are of approximately'the same height, resulting in a new curve of the type shown in Fig. 4.

The desideratum that the reaction F not exceed the weight P (Fig. 1) must obviously be fulfilled at all frequencies within the range con sidered, and moreover the weight P which is required must be at least equal to the value which F attains at the highest portion of the curve. .In the curve of Fig. 2 the mass and stiffness have been proportioned so that the two highest values whichF reaches, at the limits of the constantvelocity range, are'equal. This corresponds to the condition that the resonant frequency (in this casev about 1300 cycles) be the geometric mean of the two limiting frequencies in the constant-velocity range, as is explained in the aforementioned U. S. Patent 1,981,793.

In Equation 3, it will be seen that when w is small, the term m is small compared to the term and that when w is large, the term mu is much the larger of the two. Hence the magnitud of the reaction F depends mainly on the mass m at high frequencies, and on mainly the stiffness s at low frequencies; and this dependency becomes more complete as the frequency becomes more remote from that of resonance. Accordingly the stiffness s is made substantially smaller than in the pickup of Figs. 2 and 3, in order to reduce the height of the peak 61 (Fig 3) to a value approximating that of the highrequency peak 68. Fig. 4 represents th relation between reaction force F and frequency calculated in the same manner as the graph of Fig, 3, but for a pickup having a vibratory system of the following constants: m=50 milligrams, s=l.5 dynes/cm., damping R=critical. It will be seen that the ratio of stiffness to mass is one-quarter that of the prior-art design of Figs. 2 and 31 It is to be understood that the numerical values given are merely to illustrate and explain the in-.

for reproducing sound other than music, such as speech, where the frequency distribution is different, the optimum proportions will again change. The embodiments of Figs. 5 to 10 moreover are merely illustrative of the invention, and the principles of the invention in proportioning the vibratory constants could obviously be applied to other types of reproducers, such as the piezoelectric and the moving-coil.

I claim:

1. A phonograph pickup for lateral-cut records comprising, means for maintaining a steady magnetic field, a light armature of ferromagnetic material positioned in cooperative relation with said field, a stylus secured to said armature, vibratory suspension means secured to said armature comprising a vertically and torsionally deflectable spring system, and means to substantially damp vibratory motion of said stylus in both the lateral and the vertical directions.

2. A phonograph pickup for lateral-cut records comprising, a permanent magnet, pole pieces secured to opposite poles of said magnet, and a non-magnetic strap member secured to said pole pieces and magnet, said strap member carrying 1 said faces defining a space and a stationary coil in said space, an. elongated magnetic armature extending substantially vertically between said faces and through said coil, a substantially horizontal fiat cantilever spring extending between said spring-supporting means and the upper portion of said armature, a wire-like cantilever spring extending parallel to said first-mentioned spring between said spring-supporting means and the lower portion of said armature, a stylus carried by the lower portion of said armature, a block of solid viscous damping material extending between said lower portion of said armature and a stationary portion of the pickup, and an additional block of solid viscous dampingmaterial extending between the upper portion of said armature and said strap member.

3. A phonograph pickup for playing a lateralcut record which has a constant vibratory velocity characteristic between two limiting frequencies, comprising a stationary coil, a vibratory magnetic element having a stylus secured thereto, and a spring suspension for said element, the effective mass of said element and the stiffness of said spring suspension as measured at the sylus tip producing mechanical reasonance, the magnitude of said stiffness being substantially lower than that magnitude which would produce mechanical resonance at a frequency equal to the geometric mean of the said two limiting frequencies.

4.'A phonograph pickup for playing a record cut with-a substantially constant-velocity characteristic over a predetermined range of frequencies, the character of the sound on said record being such that the distribution of vibrational velocity with respect to frequency is nonuniform, comprising a vibratory system carrying a stylus, the force which the record must exert on said stylus to make it vibrate being defined by where F is the said force, V the peak vibrational velocity, and Z the mechanical impedance of the pickup as measured at the stylus tip, Z varying predominantly with the frequency and with the efiective mass of the said vibratory system at high frequencies, and varying predominantly inversely with the frequency and the effective stiffness of said vibratory system at low frequencies.

said mass and stiffness being proportioned so that the high and low frequency maxima in the relation of said force F with respect to frequency are approximately equal when the said non-uniform distribution of velocity V with respect to frequency is taken into account.

5. A reproducer for lateral records comprising a permanent magnet system, pole pieces positioned in cooperation with the poles of said magnet system, said pole pieces having opposed faces defining an elongated gap, recesses in opposed portions of said faces defining a space and a stationary coil in said space, a light elongated armature of ferromagnetic material extending through said gap, said armature carrying a stylus at one end, a substantially flat spring supporting the portion of said armature remote from said stylus, and a substantially wire-like spring supporting the portion of said armature adjacent said stylus.

6. A reproducer for lateral records comprising a permanent magnet system, pole pieces postioned in cooperation with the opposite poles of said magnet system, said pole pieces having opposed faces defining an elongated gap. recesses in opposed portions of said faces definin a space and a stationary coil in said space, a light armature of ferromagnetic material extending through said gap, said armature carrying a stylus, a substantially flat spring supporting said armature at a point remote from said stylus. a substantially wire-like spring supporting said armature at a point near said stylus, said springs constituting the sole suspension means for said armature, and means to damp vibratory motion of said stylus and armature in both the lateral and the vertical directions, the output of said reproducer being taken from the terminals of said stationary coil, the damping being to a degree between critical damping and five times critical.

7. A phonograph pickup for playing a lateralcut record which has a constant vibratory velocity characteristic between the turnover frequency and the upper cutoif frequency, said pickup comprising an elongated, substantially vertically extending magnetic vibratory element and a suspension for said element comprising a vertically and torsionally deflectable system of spaced parallel springs, the effective mass of said vibratory element and the effective stiffness of said suspension as measured at the stylus tip producing mechanical resonance, the magnitude of said stiffness being substantially lower than that which would produce mechanical resonance at a frequency equal to the geometric mean of thesaid turnover frequency and the said upper cutofi frequency and a piece of solid viscous damping material positioned between said vibratory system and a stationary point to damp said resonance.

LAWRENCE FLEMING.

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