US2352311A - Oscillation translating device - Google Patents

Description

June 27, 1944. J D| TQRQ 2,352,311

OSCILLATION TRANSLATING DEVICE Filed Feb. 7, 1941 DECIBELS FREQUENCY m KILOC7CLE3 PER segmeng 5 m 5 Zinnentor U! I Q Mz'c/zae/ of D1 753m .3 .4 1.7. 1-5 2 3 4 7 attorney FREQUENCY --IN KlLOCYCLES PER COND lustratins Patented June 27, 1944 2,352,311 OSCILLATION TRANSLATING DEVICE Michael J. Di Tom, East Orange, N. J., assignor to Thomas A. Ediso Qrange, N. J a corporat Incorporated, West ion or New Jersey Application February I, 1941, Serial No. 377,830 19 Claims. (Cl. I'm-400.41)

This invention relates to oscillation translating devices and more particularly to translating devices which employ vibrating units having various modes of motion or vibration.

The invention has especial utility and is herein illustrated and described in connection with a translating device of the phonographic type, but it will be understood that no unnecessary limitation of the invention to'this type of apparatus is. intended. V

It is an object of my invention to provide a translating device of imprpved sensitivity,

It is another object to provide improvements in translating deviceswhich will increase their useful frequency range of response without effecting any substantial impairment in the overall efllciency of the device.

It is another object to smooth out in a novel and eifective manner the variations in the frequency-response characteristic of translating devices at their resonance frequencies.

It is another object to control in a novel and efiectlve manner-the. slope oi. the frequency response characteristic of translating devices whereby to adapt thesame toparticular applications.

It is another object to provide phonographic apparatus, for example, a phonographic recorder, having improved frequency response characteristics.

It is a further object to fulfill the above-stated objectives by the method of selectively controlling the different modes of motion of the translation vibratingu'nit as a whole.

? It is a yet further objectto fulfill these objectives by the use of simplified and improved means.

Other and allied objects will more fully appear from the following'description and the appended claims.

In the descriptionof my invention reference is had to the accompanying drawing of which:

Figure l is a vertical sectional view of a phonographic translating device of the recorder type wherein my invention is incorporated, taken on the line 1-4 of Figure 2;

Figure 2 is a horizontal sectional view of the structure of Figure 1, taken on the line 2-2 of that figure;

Figures 3a and 3b are views. illustrating th first two natural modes of motion of the vibration unit of the translating device herein shown;

Figure 4 is a group of approximate curves ildiiferent actions and efiects of my invention, particularly in relation to a conventional system; and

Figure 5 is a group of approximate curves illustrating certain damping eifects in comp fion with the related curves of Figure 4.

The illustrative embodiment of my invention herein shown constitutes a phonographic recorder of the electro-mechanical type. The frame structure of this recorder comprises a casing I of rectangular shape which is pivoted through a bracket 2 to a standard 3 partially shown. The bracket is secured to the casing by screws 5 and is pivoted to the standard through a pair of spring arms 2' on the bracket which engage the conical end portions 6' of a rod 6 carried by the standard. The pressure of the spring arms on the conical end portions are suitably controlled by an adjusting screw l which passes freely through one of the arms and threads at its end portion into the other of the arms.

The active element of the recorder comprises a bar-shaped piezo-electric bending member or unit 8 of the so-called bimorph" type. This type of bending unit consists of two or more thin crystals secured face-to-face and arranged for simultaneous voltage application thereto, through leads 9, in a manner such'that one crystal expands longitudinally as the other contracts, and

vice versa, whereby to cause a vibration of the unit in bending. This bending or vibrating unit is held in the casing, in cantilever fashion, by a clamping of the same between two similar blocks in and suitably also by cementing the clamped unit to the blocks. The blocks are clamped together at their ends and are held in the casing at the pivoted end thereof by the aforementioned screws 5, which screws pass through the bracket, the top wall la of the casing and through the upper one of the blocks and then thread into the lower one of the blocks. The clamping pressure of the blocks against the bending unit is appropriately limited by the provision of recesses I l in the adjacent faces of the blocks at the place where the blocks bear against the unit. Extending through the blocks from the inner end of the recesses II are slots l2. These slots form an opening through the blocks, when the same are clamped together, for the passage of the leads 9 therethrough.

The bending unit 8 extends from the blocks l0 longitudinally of the casing substantially throughout thelength thereof and carries at its free end a stylus holder l3 secured to the unit as by cementing. This holder projects down through an opening it provided in the bottom lb of the casing and carries a stylus It at its lower end for coaction with a record R. For reasons involved in the assembly of the translating device, the

bottom lb of the casing is rendered removable from the casing proper, the bottom being secured at one end by screws l6 to the lower one of the blocks Ill and being provided atthe other end with a turned-up lug II which is secured to the casing proper by a screw l8. Y

The translating device is biased by its own weight against the record R to maintain a normal I coaction between the stylus and the record. In

a recording operation, the record is rotated and the recorder is moved therealong as by a suitable progressive movement of the standard I, the means for effecting such movements being however not herein necessary to show. By such relative progressive movement between the recorder and the record, there may be formed a spiral groove on the record body by the stylus. If, dur ing the time of such groove formation, an oscillating voltage is supplied to the leads 9, the bend ing unit will move to vibrate the recorder stylus iii the type of beam constituted by the vibrating unit of the present structure-is in order;

When an elastic member is sli 'jected to forced I vibration at frequencies varying throughout a continuous spectrum, the member will at frequency intervalsassume different shapes, which shapes characterize its different .modes of vibration. The shapes which a cantilever beam assumes when vibrating in its first and second modes aretypically represented .by Figures 3a and 3b. The frequency at which the beam has a maximum amplitude of deflection at its free end for its first mode constitutes the natural frequency of that mode and may be appropriately referred to as the first mode resonant frequency of the beam. Similarly, the frequencies at which in accordance with the impressed" voltage oscillations. In the present instance, purely by way of example, these stylus vibrations are vertically directed to cause the formation of a modulated groove of the hill-and-dale type.

A well-known characteristic 'of translation vibrating unitsis that a decrease in their stiffness, other factors remaining about the same, will cause an increase in their sensitivity. A counteracting factor to such decrease in stiffness of a vibrating unit, however, is that the resonance frequencies of the unit are lowered and the useful frequency range of responsei. e., the range wherein the response is substantially uniform of the unit is reduced, this reduction in the useful frequency range following from the fact that sharp variations in the response of the unit occur at its resonance frequencies, which variations can usually not be permitted within the useful frequency band. Accordingly. to obtain a uniform response throughout a desired band, the vibrating units have been designed so that their resonances occur at frequencies well above this band-such units being thus relatively insensitive; or, in cases where some improvement in sensitivity is desirable even at the expense of a variation in response, they have been designed so that their resonances begin in theupper region of the de-- sired band and the resonance effects are then suppressed by a suitable damping of the units.

In accordance with the present invention, I

I have found a simple and effective way to smooth out the frequency response characteristics of vibrating units at their resonant frequencies without incurring any substantial impairment in the overall eiliciency of the units. I am" thus permitted to use vibrating units which have their lower order resonant frequencies occurring within the desired band; As a result, for a given fre-' quency range of response I may provide a translating device of improved sensitivity or vice versa, for a given sensitivity, I may provide a translating device of greater frequency range of response. This control over the sensitivity" and the frequency range of response, of translation vibrating units is achieved through a selective control of the different modes of motion or vibration of the units. Before however presenting a detailed description of the manner inwhich these improvements are' attained, a preliminary description of the several lower modesof motion in beam at another intermediate. point 2| which is at 'a of about .8 of the length of the beam from its fixed end. These points 20 and 2| are points of maximum and minimum amplitudes of deflection of the beam and constitute an antinode and node of the second mode of vibration. Further, in the sense that nodes and anti-.

nodes are points of zero and maximum deflection respectively, all modes of motion may be considered as having a node at the'clamped end of the beam and an antinode' at the free end thereof. The distinguishing characteristic of the second mode over the first-and thisvlis general in respect of each higher mode over the next lower mode-is that the second mode has one more antinode and one more mode'than the/first mode, this additional node and antinode being I each in the intermediate regions of the beam.

While my invention is not limited in respect of the number of modes of motion operated upon, the first several modes are for most purposes the more important and are the ones herein considered in the illustration'off-my invention.

Although my invention is applicable either to extend the frequency range of response of a trans- I lating device or to improve the sensitivity of the device, as aforementioned, the same principles are involved in each of these two objectives and the application of my invention to one will be apparent'i'rom a description of the other. In the present instance, my invention is applied to the realizationof the latter objective-that is, to improving the sensitivity of 'a translating device while maintaining its frequency range of response. Purely by way of example, the upper limit of the desired frequency range of response is set at 4500 c. p. s. A translating device of the usual design which is capable offulfilling this range of response will employ a piezo-electric unit havingits first natural frequency occurring not substantially below this upper limit of 4500 c. p. s.; such amibrating unit will have a frequency-response characteristic typically of the form represented by curve I of Figure 4, wherein the ordinate, axis represents frequency to log arithmic scale and the abscissa axis represents relative sensitivity in decibels. In the present invention, however, this prescribed frequency range bending of a cantilever (clamped-free) beam- 16 is fulfilled with the use of a vibrating unit 8 whose first natural frequency falls well within this prescribed range as at 1500 c. p. s.; this vibrating unit has a frequency-response characteristic.

typically of the form represented by curve 2 of Figure 4. By comparison of curve 2 with curve I it will be seen that in the frequency range below 1500 c. p. s. the vibrating unit 8 is at least 10 decibels more sensitive than the former unit having the higher mode resonant frequency; that at frequencies above 1500 c. p. s. the response of the vibrating unit 8 however falls ofl and in the frequency band between 2000 c. p. s. and 6000 c. p. s. it even falls considerably below that of the former unit; but that in a frequency band above 6000 ing. This loading has the eifect of reducing the c. p. s. the response of the unit 0 again exceeds the former by reason of the unit 8 having its second mode resonant frequency occurring in this region, at about 9000c. p. s. V V V Because the second and first natural frequencies of the vibrating unit 8 are widely se'pa rated-these frequencies being approximately in the ratio of 6 to 1-and because the response of the unit 8 falls greatly in the frequency range between these natural frequencies, a mere application of damping to the vibrating unit 8 is insufficient both to reduce the resonance peaks and to fill in the dip in the frequency-response 1 characteristic between the two natural frequencies. I find however that by a very simple expedient the frequency interval between the natural frequencies of two adjacent modes of motion of a vibrating unit may be reduced to permit the response of the unit to be maintained substantially uniformly at the general level of response of the unit in the region of-these natural frequencies.

The general efifect of a loading of an elastic member is to reduce the resonant frequency of that member. When the member is characterized by different modes of vibration, I find that the loading will be differentially effective as to these different modes of the member provided the loading is placed in the region of an antinode of one mode and in the region of a node of another mode, the mode whose antinode constitutes the place of loading being the one operated upon. This principle of differential loading is employed in the present instance for the purpose of lowering the natural frequency of the second mode so as to reduce the frequency interval between this natural frequency and that of the first mode. Since the antinode of the second mode of the vibrating unit 8 is at a distance of more than one-half the length of the unit from the antinode of its first modethat is, from the free end of the unita loading at this point will be highly eflective in lowering the natural frequency of the second mode but will have relatively little effect on the first mode. This differential effect on the two modes by a loading of the unit in an intermediate region thereof is even more increased when the loading is applied to the unit at a'point near the fixed end thereof, as the application of loading to the unit in the inner end region thereof is found to shift the antinode of the second mode toward the fixed end of the unit, the effect of this shift being to space farther apart the antinodes of the first and second modes or, in other words, to place the antinode of the second mode in the region of the node of the first mode. For example, a loading of the unit at a distance only one-fifth of the length thereof from its fixed'end,

witha weight about ten times the effective mass" natural frequency of the second mode about three times from 9000 c. p. s. to roughly 3000 'c. p. s., and also it has the beneficial effect of increasing the amplitude of response of this second mode, as maybe seen by a comparison of interval betweerrthe natural frequencies of the first and second modes. This reduction permits the response of the unit, in the region of the first and second mode resonant frequencies, to be now smoothed out to a generally even level by a suitable damping of the unit, as is now explained.

A simple damping means which may be effectively employed in the present invention comprises a pair of damping pads 26 which are interposed between the vibrating unit 8 and the top and bottom respectivelyof the casing l and located, for example, by suitable cups 2'! provided in the casing. These pads are to have a high internal resistance and may suitably consist of the cellulose nitrate plastic known commercially as Dupont Viscoloid. In cases where the vibrating unit is to be damped without partiality to its different modes of vibration the damping pads may be made quite wide and need not be critically located. This mode of damping-which is the mode illustrated in my Patent No. 2,177,692, issued October 31, 1939--will cause the peaks in the response of the unit at its resonance frequencies to be suppressed each in about the same degree with the result that the frequency-response characteristic will be rendered fiat and of a substantially zero mean slope. While such general damping is applicable in the present invention, I preferably selectively damp the unit in respect of its modes of vibration so as to provide the frequency-response characteristic with a predetermined slope, the slope being of positive or negative inclination depending upon whether a first or second mode of vibration is acted upon by the damping means. i

It is desired in the present instance that the translating device be provided with a frequencyresponse characteristic having a positive slopethat is, a characteristic wherein the response rises gradually with increasing frequency. Accordingly, the damping is arranged so as to affect substantially only the first mode of motion of the unit. This selective damping is achieved by the use of damping pads of a predetermined conbilities which they exhibit are about in direct proportion to their stiffness reactances. For instance, damping pads which are narrow in width and relatively thick, such as the pads 28 shown in Figures 1 and2, will be relatively stiff in compression but will be quite compliant in torsion and, accordingly, they will exhibit a high degree of damping to compressional deformation but will-exhibit only a small degree of damping to torsional deformation. Since, at the node of the second mode, the first mode is represented almost wholly as a.transverse movement of the unit and the second mode is represented entirely as a torsional movement of the unit, the application of thedamping pads '26 to the'vibrating unit 8 at this node, with the narrow dimension of the pads directed longitudinally of the unit as shown, will causethe unit to be damped considerably in respect of its transverse movement but only slightly in respect of its torsional movement. Consequently, such damping pads are selective in their action in that they are effective on the unit substantially only in respect of its first mode of motion in bending. V r

This selective damping of the loaded vibrating ingly, the term substantially at an anti-node of unit 8 causes its undamped frequency-response characteristic, represented by curve 3 of Figure 4, to be re-shaped into a characteristic such as is represented by curve 3 of Figure 5. By compari-.

son of these two curves it will be seen that the damping of the unit has the effect of smoothing out the frequency-response characteristic and also, by reason of the selective nature of the damping, of providing the characteristic with a represented by curve I of Figure 4, this unit must be damped to suppress the peak in its response at its first mode resonant frequency. When appropriate damping is applied to the unit, the frequency-response characteristic of the unit is reshaped into curve I of Figure 5, which curve will be seen to be about 4 decibels lower in level than curve l of Figure 4 which represents the undamped frequency-response characterist c of this same unit. Thus, on a comparable basis, determined by a comparison of curve I with curve 3' of Figure 5, the 10 decibel improvement in sensitivity of the undamped unit 8 over the aforementioned unit of the conventional design is yet p'reserved when these units are suitably damped. Accordingly it will be seen that for a given range 3 of frequency response, translating devices designed in accordance with the present invention will have a substant ally greater sensitivity than one of the more usual design.

While my invention has especial utility in connection with phonographic recorders,'lt will be understoodnthat it may also be utilized'to advantage with phonographic reproducers and other translating devices.

stood that my invention is not limited to the details of the embodiment herein shown, as the Further, it will be undersame is subject to many changes and modiflcations without departure from the scope'of my invention, which I endeavor to-express according to the following claims. In construing the claims it will be understood that the term substantially at a node of a given mode of vibration of the translator element refers to a region of the element where the element has relatively little movement in translation as compared to the movement in translation of other regions of the element, but does not necessarily refer to an a given mode of vibration of the element" refers to a region .of the element where it has a relatively large movement in translation but again not necessarily to an exact anti-nodal-point.

I claim:

1. The method of controlling the response of a beam-like translator element to an applied oscillatory force which varies in frequency within a range lying at least partially between the natural frequencies of the first and second modes of vibration of the element, which comprises locally weighting said element at a region thereof which is substantially an anti-node of one of said modes and a node of the other of said modes, said weighting being by a mass having a weight at least substantially greater than that of said element.

2. The methodof controlling the response of a beam-like translator element to an applied oscillatory force which varies in frequency within a range lying at least partially between the natural frequencies of the first and second modes ofvibrationof the element, which comprises reducing the frequency interval between said first and second mode natural frequencies by locally weighting said element at a region thereof which is substantially an anti-node of said second mode of .vibration, said weighting being by a mass having a weight at least substantially greater than that of said element.

3.*The method of controlling the response of a beam-like translator element to an applied oscillatory force which varies in frequency within a range lying at least partially between the natural frequencies of the first and second modes of vibration of .the element, which comprises loading said element with a mass having a weight substantially greater than the weight of said element and placing said mass at an intermediate region of said element which is substantially an anti-node of its said second mode of vibration.

4. The method of controlling the response of a beam-like translator element to an applied osfrequencies of the first and second modes of vibration of the element, which comprises both loading and damping said element intermediately thereof, said loading being with a mass having a weight which is a multiple times greater than the weight of said element.

6. The method of conditioning a beam-like translator element so that the same will have a substantially even response to an applied oscillating force varying in frequency below and above the first mode resonant frequency of the element,

which comprises loading said element at an intermediate region thereof which is substantially an anti-node of the second mode of vibration of the element, and applying damping tosaid element, said loading being by a mass having a weight substantially greater than-the weight of said element.

exact nodal point of the element. Correspond- 7. In a translating device: the combination of a beam-like translator element mounted in cantilever fashion for oscillatory movement in both first and second modes of vibration of the element; means for driving said element at varying frequencies above its first mode resonant fre-- of the element; and means for controlling the response of said element to said applied oscillatory force comprising a weight associated locally with said element at an intermediate region thereof which is substantially an anti-node of a higher mode of vibration of the element, said weight being substantially greater than the weight of said element.

9. In a translating device: the combination of a beam-like translator element mounted for oscillatory movement in both first and higher modes of vibration: means associated with said element for applying an oscillatory force thereto at varying frequencies extending beyond the first mode resonant frequency of the elementnmd damping means associated with a relatively short length of said element at an intermediate region thereof which is substantially a node for a higher mode of vibration of the element.

10. In a translating device: the combination, of a beam-like translator element mounted for oscillatory movement in both first and higher modes of vibration; means associated with said element for applying an oscillatory force thereto at varying frequencies extending beyond the first mode resonant frequency of the element; means damping said element intermediately thereof; and means loading said element intermediately there:- of. said loading means having a mass that is substantially greater than the mass of said element.

11. In a translating device: the combination of a casing; a beam-like translator element mounted in said casing for oscillatory movement relative thereto in both first and second modes of vibration of the element; means for applying an oscillatory force to said-element at varying frequencies within a band at least partially lying between the natural frequencies of the first and second modes of vibration of the element;

substantially a node for the second mode of vibration of the element, said damping means resonant frequency of the unit and a substantial frequency region therebeyond, which comprises loading said unit at an intermediate region thereof which is atv leastsubstantlally an antinode for the second mode .of vibration of the unit and applying damping to said unit at an'intermediate region thereof which is at least substantially a node for said second mode of vibration.

13. The method of controlling the frequencyresponse characteristic of a resilient translation vibrating unit which is vibratable in first and in a higher mode of vibration, which comprises subjecting said unit to loading at substantially an antinode of said higher mode of the unit and to damping at substantially a node for that higher mode, said loading being substantially ineffective as to the first mode of said unit and said damping being effective substantially only as to that first mode.

14. A phonographic translating device comprising an active vibratable element supported for vibration at one end thereof, record-coacting means associated with said element at the free end thereof, a localized mass mounted on an intermediate portion thereof, and damping means associated with said element at a place thereof between said record-coacting means and said mass.

15. An active element for a phonographic' translating device comprising a piezo-electric unit supported at one end and arranged for coaction with a record at the other end, said unit having diiferent modes of motion; a localized mass mounted on an intermediate portion of said unit at substantially the antinode for the second mode of vibration of the unit; and damping means associated with the intermediate region of said unit constituting substantially a node for said second mode.

16. Means for damping the resilient active element of a translating device in its first mode of motion without substantially damping the same in its second mode, comprising damping pads applied to'said element at substantially the node thereof for its second mode of vibration, said pads having a high damping factor in compression and being narrow in width so as to have a low damping factor in torsion.

17. A phonographic recorder comprising a frame, a piezo-electric vibrating unit supported by said frame in cantilever fashion, a recorder stylus secured to the free end of said unit, a localized mass mounted on said unit at an intermediate region thereof, and damping means operatively interposed between said frame and saidunit, said damping means being locally applied to another intermediate region of said unit.

' 18. A phonographic recorder including a piezoelectric bending unit supported in cantilever fashion and vibratable in first and second modes of vibration, a localized mass mounted on said unit at substantially an antinode of said second mode of vibration and eflective substantially only on said unit in its said second mode of vibration, and damping means applied to said unit at substantially a node of said second mode and elective substantially only on said unit in its first mode of vibration.

19. In a translating device: the combination of a beam-like translator element mounted in cantilever fashion for oscillatory movement in both first and second modes of vibration; means for driving said element at v ryin frequencies lying within a range extending above the first mode resonant frequency of the element; and a localized mass associated with an intermediate portion of said translator element for controlling the response of the' element in its second mode of vibration to said driving means. said mass having a weight substantially greater than the weight of the free end portion of the translator element extending beyond said mass.

mcnam. J. or Tom;

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