US1778386A - Sound reproducer - Google Patents

Description

Oct. 14, 1930. H Q HARR|50N 1,778,386

SOUND REPRODUC ER Filed June 30, 1928 #EQ/Ry C. HAM/50N A TTH/VEY Patented Oct. 14, 1930 UNITED STATES PATENT OFFICE HENRY C. HARRISON, OF PORT WASHINGTON, NEW YORK, ASSIGNOR T BELL TELE- PHONE LABORATORIES, INCORPORATED NEW YORK OF NEW YORK, N. Y., A CORPORATION OF SOUND REPRODUCER Application led J'une 30,

This invention relates to sound reproducing mechanisms and its object is to provide a moving system for reproducers which will respond faithfully to applied forces at the needle point which are of such small magnitude that they will not disrupt or tear the soft wax material in which the original of a production is recorded. l

A moving system suiiiciently low` in mechanical impedance to fulfill the objects of this invention results from the use of a very light, stiff needle which may be connected directlyA to the translating element of the system and may be made sufficiently long that its l5 mass is the controlling factor in reducing the needle point mass to its lowest value.

A hollow conical needle, the active point of which is an integral part of the cone, has been found quite suitable for the practice of 2u this invention although other forms will readily suggest themselves. In the drawing Fig. l is a sectional assembly view of the reproducer embodying this invention. Fig. 2 is a perspective view of one form of t-he moving system for a play-back reproducer. Fig. 3 is a representation of the` analogs of the mechanical elements of this reproducer in the elements of a corresponding electrical network. Fig. 4 shows a compute a0 frequency response of a mechanical'system such as that of Fig. 3. Fig. 5 gives the impedance-frequency curves for various 'systems with the constants indicated; and Fig. 6 gives the effective values of mass for needles of $5 various lengths.

The stationary structure of this reproducer may be of the general type `described in my copending matter 3. The diaphragm 4 is arranged to be flexed about a diameter by the reaction of the application, Serial No. 191,012, filed May 13, 1927, and' consists of electro-` 40 magnets 1, l in a casing 2 containing viscousv 192s. serial No. 289,544.

grooves of the 'record 6 on the needle 5 in the same manner as described in that application except that in the present. invention the needle is directly attached to the diaphragm making t-he usual stylus holder unnecessary.

In order that a play-back reproducer may be capable of faithful response over the whole range of interest, it isl necessary not only that the moving system have a very low mechanical impedance but also that the overall response of the reproducer be substantially uniform and reasonably efficient for the various frequencies. In the present invention the above requirements are met by a combination of the following distinct features: (l) a new order of magnitude of the ratio of needlepoint stiffness to total mass moving with the needle; (2) the proper value of mechanical 60 resistance or damping; (3) a moving system having no natural periods of vibration within the frequency range to which the reproducex` is required to respond; and (4) low balance stiffness of the moving system.

A clearer understanding of the considera-, tions involved in the design of this device may be obtained by considering the mechanical ele" ments as analogs of the corresponding electrical system as set forth in Fig. 3 of the draw- IQ ing. In this ligure the shunt stiffness S1 represents the needle elasticity of the reproducer the series stiffness S2 represents the balance p elasticity of the system or the difference between the stiffness of the diaphragm and the 5 negative stiffness due to the magnetic effect of the polorizing field m1 represents the effective mass of t-he moving system at the needle point; land r1 represents some means of 80 termined forany frequency when constants of the mechanical system from the equation:

ratios and plotted against the ratio wom ' have been given in Fig. 5 for different values of mechanical resistance T1. From curve 11" ofthe ligure, it is evident that the conditions for smoothness of frequency response also lead to a smoothness in the impedance characteristic if the series stiffness S2 is small as it is in 'this case. If the series stiffness is made larger, keeping the resistance at @om as before, the impedance response is very materially affected, particularly at low frequencies as indicated by the curve la.

' These two curvesshowthe importance of low effective balance elasticity if the mechanical impedance at low frequencies is not to become excessive. .It is evident then that by the value of the terminating impedance r1 .so that the impedance of the system at terminals 1 and 2, corresponding to the needle point, will be substantially uniform for all the frequen- @H2O-K-talfHHK-i cies involved. The natural periods of the component parts of the moving system, such as the'diaphragm and the needle, -must be made very high to prevent the introduction of high shunt impedances which would make it impossible to obtain a uniform characteristic over the whole range.

The balance stiffness of the moving system, as represented by S2 in the drawing, must be made very low to prevent high impedance at low frequencies. While the actual value of the balance stiffness cannot be reduced below a certain value without causing instability, the effective stiffness is made very small due to the unusual length of needle employed.

In the design of this reproducer the comutedfrequency response for the structure for constant input velocity is given by the following equation:

proper choice of mechanical resistance and series stiffness a reproducer of approximately constant impedance at `all frequencies up t0 resonant frequency can be made.

The importance of the first' of the above considerations is due to the fact thatthe natural frequency of the system is directly'proportional to the square root of this ratio 'as l 1. 27|' Will the mechanical impedance of a system in parallel resonance is very high, the response of the system will fall off very appreciably as this natural frequency is approached. It is evident then that for a high quality reproducer the natural frequency should be as hiUh as possible.y In this reproducer this ratlo,

shown by the equation fo Sn-l has been made very high by reducing the 1 mass of the moving system to the lowest practical value and by-employing a needle of unusual stiffness so that the natural frequency of the system is in the neighborhood of the upper limit of the frequency range to be reproduced.

With respect to the second of these features, care must be taken in adjusting the Y Since where I0=input velocity to the network shown in Fio'. 3 in cms. per second; 12T-"the output velocity from the network cms. per second; m1 is the effective mass; S1 the shunt elasticity; S2 the series balance elasticity and r1 the mechanical resistance all effective at the needle point; w=27rf, f being the frequency considered; w0=21rf0; #rw-17; ,Si 1 being'the needle free resonant frequency;

and I l: since las equal to j! a f01- o Y o lows that the above equation glves the relativev response of the system for a'frequency range which is independent of the absolute efficiency of the structure under consideration.

The needle free resonant frequency havin g yfective values of r1 .expressed in terms ofthe resistance at resonant frequency. It is evident from these curves that best results are obtained when 11=w0m1; in this case the variation up to resonant frequency is less than 1 TU; but in cases wherejsmaller values of r1 are used a peak appears in theresponse curve .near resonant frequency, while for other -wall material taken about the diameter of its base is'given byv #gaat @Rame where R2 is inside radius of the base; h is the 'length of needle; t is the thickness of the wall and p is the density of material. If to this we add the necessary moment of inertia of the armature denoted by A, we have then for the effective mass at the needle point:

gmt@ .1 .fg

F or constant needle stiffness with variation in length L, it is necessary to maintain the ratio h constant. If we differentiate the r above expression with respect to for m1 minimum, we find that yhggtpegy] but gives' no indication as to the y wide range of values of h and various values .of .altitud i.

With the bi-polar reproducer structure which has been found satisfactory for this device, the moment of inertia of the armature together with that of the effective diaphragm y mass is approximately .04 gm. cms?. If -E-? is equal to .08, i=.004 cms. p=8.0. We

find for a minimum mass m1 (1.010 gms.)

that L=2A6 cms. It is then necessary to determine if a needle with these constants is stiff lenough to give a suiiiciently high needle free resonant frequency for goodquality reproduction. The stiffness of a hollow conical needle which has a wall thickness ,very

much less than the radius of the base is given very approximately by the formula:

where E is Youngs modulus and the other symbolsare of the same significance as before. For the values assumed above, we find that the needle stiffness is equal to 12.8 X 106dynes/Cm. Corresponding to this, we find the resonant frequency to be:

l 12.8 X l06 fo 5700 cycles.

' This frequency being near the upper limit of the frequency range recorded, such a reproducer will cover the whole range at substantially constant efficiency. f

As already stated, it is found that the impedance is more nearly constant over the whole frequency range when r1 is made equal to wom/1. If then we have for this reproducer w0=27r 5700, and m1=.010, then 711 is equal 'to 360 mechanical (c. g. s.) ohms effective at the needle point distance of 2.46 cms. This has been found to require a resistance in the gap between the diaphragm and pole pieces of approximately 10,000 mechanical ohms, which can be obtained by using a gap of these dimensions Z= 3/8. width b=11 and the distance between the damping surface d=.009 approximately.

These dimensions of Z and b correspond to the dimensions used in the bi-polar armature of my application previously referred to, so that when using a structure of this type, it is only necessary to set the air gap at approximately .009. If this condition, which corresponds to 1'1= 0m is obtained and S2,

is made less than -S-1 or 50 lin this case to a value less than .25 X 106 dynes, then for any frequency above about 100 the series stiffness,

cycles or .02 `-in Fig. 5, 'we shall have for 0 the maximum value of impedance Z1 2 maxi- Taking the maximum of R1,2 and X1 2 from curves 17' and 1m we get Z1 2 maximum:

In the case considered. at 50 cyclesthe im-,`

efficiency must be .sacrificed to obtain the necessary smoothness of response but 1t 1s of course desirable that the eflciency shall be as high as is consistentwith the quality desired. By actual test reproducers designed in accordance with the description given have been found to have an eliiciency m excess of -llb of one per cent.

What is claimed is:

1. In a sound reproducing mechanism, means for producing mechanical vibrations, a vibration translating device, and a stylus needle pivoted directly on the translating device and cooperating with the mechanical vibrational means.

2. In a sound reproducing mechanism, a

grooved record surface, a vibration translating device, and a stiff light needle pivoted damping eective at the stylus point is sub- `directly on the translating device and cooperating with the record surface.

3. In a sound reproducing mechanism, a grooved record surface, a vibration translating device, and a hollow conical needle connected directly with the vibration translating device and engaging the grooved record surface.'

4. A moving system for sound reproducing devices in which the eii'ective values of the mass and stiffness are reduced to a minimum by means of a long light needle connected directly to the vibration translating device.

5. A moving system for sound reproducin devices in which the ratio of needlepoint-sti ness in dynes per centimeter to the total effective mass in grams of said system is of the order of 109.

6. A moving system for sound reproducing 7devices including a needle with an unsupported length in excess of 1/2, the system having its mass and stillness so proportioned that its lowest resonant frequency is higher thanv 5000 cycles.

7 In a sound reproducer, a magnetic circuit carrying a polarizing flux and having `a stationary portion and a moving portion, an

elastic mounting and damping means for the moving portion and a stylus connected to the moving portion, the polarizing flux, the` ystillness of the mounting and the length of the needle being so proportioned that the efvtective series stiffness of the moving system is reduced to the minimum value consistent with stable operation.

8. In a sound reproducer, a moving sysy tem comprising a stylus, an armature and mounting means therefor, having its mass and its elasticities expressed in c. g. s. units so proportioned that its lowest resonant frequency f., is higher than 5000 cycles and means for damping the system so adjusted that the stantially equal to QI'IfOm mechanical ohms. In witness whereof I hereunto subscribe my name this 29th day of June, 1928.

' HENRY C. HARRISON.

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