Acoustic delay lines

2,913,680

Nov. 17, 1959

V. J. PORTER ET AL

ACOUSTIC DELAY LINES Filed Aug. 18, 1955

FIG.

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INVENTORS

VINCENT J. PORTER _Βγ MARK NAIMAN

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AGENT

United States Patent Office

2,913,680

Patented Nov. 17, 1959

2,913,680

ACOUSTIC DELAY LINES 5

Vincent j. Porter, Huntingdon Valley, and Mark Naiman, Philadelphia, Pa., assignors to Sperry Rand Corporation, New York, N.Y., a corporation of Delaware

Application August 18, 1955, Serial No. 529,261 io 5 Claims. (Cl. 333—30)

The present invention relates to acoustic delay lines 15 and is more particularly concerned with novel delay line structures exhibiting a larger delay per unit length than may be obtained from acoustic delay lines known heretofore.

Acoustic delay lines are presently utilized in many 20 electronic applications, and may be employed for instance in memories for use in computing apparatuses. Reference is made to U.S. Patent No. 2,629,827 to J. P. Eckert, Jr., issued February 24, 1953, for “Memory System,” which patent discusses various forms of acoustic 25 delay line structures which may be employed in computer memories. In general, such a memory operates in conjunction with recirculation means such as an amplifier, whereby information to be stored as a pulse sequence may be fed to one end of thie acoustic delay ³θ line, may be caused to propagate as an acoustic signal down the said delay line, and may then be caused to recirculate to the input of the line. Inasmuch as the acoustic propagation down such a line takes a finite time; the successive recirculation of information through an ³³acoustic delay line, of the type contemplated, effectively stores the information being recirculated.

The forbgoing Eckert patent discloses various forms of delay lines capable of functioning in the above described manner, and such delay lines may take the form ⁴θ of an elongated rod, tape, wire, or tube, for instance, of and acoustic transmission medium. One particular acoustic transmission medium described in the said Eckert patent employs, for instance, magnetostrictive materials. In general, while the particular configure- ⁴³tions of solid acoustic delay lines discussed in Eckert Patent 2,629,827, identified above, provide good results in memory systems, it has been found that the acoustic transmission mediums, when taking the configuration of an elongated substantially straight body of acoustic trans- ³⁰mission material, exhibit an extremely high velocity of sound propagation. As a result, when such lines are to be utilized at relatively low frequencies of operation, for instance at pulse rates of a hundred or so kilocycles, the storage of information in the line is relatively inefficient, ^5aand information pulses may be spread out along the line at any given instant so as to occur, perphaps, inches apart.

The present invention serves to obviate the foregoing _{.θ difficulty and provides acoustic delay lines which ate capable of storing pulses more efficiently and at higher density when lower pulse repetition rates are employed, then has been the case heretofore. In addition, due to the concepts of the present invention, the improved de- 65 lay lines to be described are adapted to provide acoustic matching of the impedance of acoustic transducers more readily than has been the case heretofore.

It is accordingly an object of the present invention to provide improved acoustic delay lines. 70

A further object of the present invention resides in the provision of acoustic delay lines which may be smaller in physical configuration than has been the case heretofore.

A still further object of the present invention resides in the provision of an elongated acoustic delay line exhibiting a greater delay per unit length than has been the case heretofore whereby shorter lines may be utilized to provide a desired delay than has been the case in the past.

A further object of the present invention resides in the provision of an acoustic delay line providing greater efficiency of operation at relatively low pulse repetition rates.

A further object of the present invention resides in the provision of an improved acoustic delay line capable of being more readily and accurately acoustically matched to the impedance of acoustic transducers than has been the case heretofore.

The foregoing objects and advantages of the present invention are effected, as will be described, by the provision of delay lines exhibiting a constantly varying acoustic impedance between spaced points on the line; In accordance with preferred embodiments of the present invention, this constant ' variation in an acoustic impedance may be caused to occur cyclically and the rate of cyclic variation in acoustic, impedance may in turn be relatively high in respect to the repetition rate of acoustic waves to be propagated down the line. Due to the aforedescribed variation in acoustic impedance, the resultant delay line exhibits a lumped characteristic whereby the velocity of propagation of acoustic waves in the medium is lower than would be the case with a delay line of constant acoustic impedance, and as a result, more pulses may be stored per unit length in the line.

The foregoing construction and operation of the present invention will become, more readily apparent from the following description and accompanying drawings, in which:

Figure 1 illustrates an improved acoustic delay line constructed in accordance with one embodiment of the present invention.

Figure 2 illustrates a modified form of acoustic delay line constructed in accordance with a further embodiment· of the present invention; and

Figure 3· illustrates a still further embodiment of the present invention wherein variations in acoustic impedance may be introduced magnetically.

Referring now to Figure 1, it. will be seen that, in accordance with the present invention, an acoustic delay line may take the form of an elongated bar, tube, tape, or wire 10, constructed of a suitable acoustic transmission medium. Acoustic transducer 11 may be disposed adjacent one end of the said transmission medium 10 for the selective propagation of acoustic waves down the medium 10; and a further acoustic transducer 12 may be disposed adjacent the other end of the said medium 10 for the detection or interception of acoustic waves propagated in medium 10 by transducer 11.

As has been mentioned previously, the medium 10 may comprise magnetostrictive material for instance, in which event. the acoustic transducers 11 and 12 may comprise, as shown, magnetic transducers. When this particular embodiment of the present invention is employed, the transducers 11 and 12 may each include a pair of windings, as shown; and these windings may in turn take the form of signal windings 13 and 14 and bias windings 15 and 16. In addition, if the structure of Figure 1 is to be employed in a recirculating memory, for instance, a recirculating amplifier may be disposed between signal output winding 14 on transducer 12 and signal input winding 13 on transducer 11 thereby to provide a continual recirculation of information through the acoustic transmission medium 10. The foregoing form of acoustic

2,913,680 delay line is essentially shown in the aforementioned patent to Eckert No. 2,629,827, and in particular, in Figures 23 through 25 thereof.

If the acoustic transmission medium 10 should take the form of a substantially straight rod, tube, wire or the like, as described in the said Eckert patent, whereby the medium exhibits a substantially constant acoustic impedance between the acoustic transducers 11 and 12, the operation of the device at relatively low acoustic pulse rates becomes inefficient. In accordance with one embodiment of the present invention, the efficiency of acoustic delay lines such as have been described, may be enhanced considerably by causing the said line to exhibit a plurality of variations in acoustic impedance between acoustic transducers adjacent the said line.

In the particular embodiment of Figure 1 these variations in acoustic impedance may be effected by causing the transmission medium 10 to exhibit a plurality of changes in cross-dimension. Thus, referring to Figure 1, it will be seen that the surface of transmission medium 10 may assume a configuration exhibiting knobs or lumps 17 whereby the cross-dimension of the medium 10 is caused to change regularly betv/een values arbitrarily termed “A” and “B,” in Figure 1. The medium 10 may take the form of a flat rod or wire whereby the knobs 17 may be formed as a series of corrugations on the upper and lower surfaces thereof. In the alternative, the several knobs 17 may exhibit circular symmetry with respect to the elongated axis of medium 10 whereby the changes in cross-dimension take the form of a plurality of bulbs adjacent one another along the length of medium 10.

In accordance with preferred forms of the present invention, the variations' in acoustic impedance defined by the configuration shown in Figure 1, may occur cyclically and this cyclic variation should occur at a rate which is relatively high in comparison to the repetition rate of acoustic waves being propagated down the line, i.e. there should be a plurality of variations in acoustic impedance in each space between successive pulses being propagated down the delay line. The particular curvature defined by the peripheral surfaces of the several knobs 17 may conform to any desired law, but in accordance with a preferred embodiment of the present invention, the curvature may follow, in both expansion and contraction, an exponential law. When this latter form of the invention is practiced, an analogy may be drawn between the resultant acoustic delay line and exponential horns acting as acoustical transformers, for it is known in the art that by providing an exponentially expanding acoustical wave guide, transformations in impedance may be effected substantially without reflection.

Whatever the particular law utilized in defining the increasing and decreasing cross-dimension of the transmission medium 10 however, it will be appreciated that by the plurality of variations in acoustic impedance the delay line tends to assume a lumped characteristic whereby pulses are propagated down the line at a lower velocity than would be the case with an unlumped line; and accordingly, a higher density of pulse storage may be effected, with a resultant increase in efficiency of operation.

The particular transmission medium 10 shown in Figure 1 may assume a tubular configuration, in which event the portions 17 may take the form of an increasing and decreasing thickness of tube wall. When tubular delay lines are to be employed, however, the invention may be practiced more readily in accordance with the embodiment illustrated in Figure 2. Thus, referring to Figure 2, it will be seen that an improved acoustic delay line exhibiting the lumped characteristic described above, may be provided by a tubular transmission medium 2® associated with a pair of acoustic transducers 21 and 22. The variation in. cross-dimension and resultant variation in acoustic impedance, may be effected in this latter embodiment of the present invention by forming an elongated slot 23 in a wall of tube 20 and this slot may in turn take the configuration shown in Figure 2, whereby the slot exhibits cyclic variations in width. By so providing a slot 23, the effective cross-dimension of acoustic transmission medium varies as acoustic waves are propagated down the line; and this variation in acoustic impedance should, preferably, once more be at a rate relatively high in comparison to the repetition rate of acoustic waves being propagated.

While the particular embodiment of Figure 2 has illustrated the provision of a single variable width slot disposed substantially parallel to the elongated central' axis of tubular transmission medium 20, it will be appreciated that the effective variation in cross-dimension may be accomplished by the provision of a plurality of such slots or by a plurality of differently dimensioned slots giving the desired changes in effective cross-dimension at adjacent points along the line. In addition, the slot or slots defined in the wall of tube 20 may be substantially straight, as described, or they may he helically disposed; about the periphery of the tube 20 as may be desired.

Each of the embodiments, described in reference to Figures 1 and 2 has effected the desired variation in acoustic impedance by a change in the physical dimensions of the acoustic transmission medium between spaced acoustic transducers. This desired variation in acoustic impedance may be effected in other ways, one such method is illustrated in Figure 3 wherein the variation is accomplished magnetically..

Reference is made to the copending application of Theodore H. Bonn, Serial No, 499,285, filed April 5, 1955, for: “Variable Delay Line” now U.S. Patent No. 2,814,793, issued November 26, 1957. This patent is assigned to the assignee of the instant application and discusses the provision of a delay line wherein variations in time delay may be effected by disposing an elongated coil adjacent a magnetostrictive transmission medium between magnetic transducers and by applying suitable bias potentials to the said elongated coil thereby to cause the magnetostrictive material to operate over a selected portion of its hysteresis curve.

As has been discussed in the above identified patent, the velocity of acoustic transmission in a magnetostrictive material is represented by theequation:

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where E is Young’s modulus for the particular magnetostrictive material; and p is the density of the magnetostrictive material. The aforedescribed change in bias upon the magnetostrictive material effects a change in both Young’s modulus and in p, but the change in Young’s modulus, E, greatly predominates, whereby a variation in bias, in fact, effects a variation in the velocity of acoustic propagation through the magnetostrictive material. In the particular form of delay line disclosed in the patent, the bias coil has a uniform winding pitch between spaced acoustic transducers, and the bias potential is caused to be constant or to vary relatively slowly in comparison with the repetition rate of acoustic waves, thereby to effect the desired control in time delay. For any given acoustic wave being propagated, however, the acoustic impedance of the line is substantially constant between acoustic transducers. This particular form of delay line, however, may be readily modified in accordance with the concepts of the present invention to provide a more efficient delay line.

Thus, referring to Figure 3, it will be seen that, in; accordance with this further form of the present invention, an acoustic delay line may comprise an elongated transmission medium 3® associated with acoustic trans-ducers 31 and 32. This medium 30 may, as described, take the form of a magnetostrictive rod, tube, wire or tape, in which event the transducers 31 and 32 may comprise magnetic transducers. An elongated bias coil 33 is disposed about the transmission medium 30 between

2,913,680 the transducers 31 and 32, and the said coil 33 is coupled to terminals 34, to which terminals may be applied a suitable source of bias potential. If the winding 33 exhibits a substantially uniform pitch between transducers 31 and 32, the concepts of the present invention may be practiced by causing the bias source 34 to vary at a rate which is relatively high in comparison with the repetition rate of acoustic waves being propagated through the medium 30, whereby the said medium 30 exhibits an acoustic impedance which cyclically changes at a time rate which is substantially higher than the frequency of acoustic waves.

The particular form of the invention shown in Figure 3 may also effect the desired variation in acoustic impedance by causing the winding pitch of coil 33 to vary between the transducers 31 and 32. Figure 3 has been particularly directed to this form of the invention wherein a constant variation in winding pitch is employed. When this varying winding pitch is provided, the bias source applied to terminals 34 may assume a relatively constant value or may vary at a rate relatively low in comparison to the repetition rate of acoustic waves. It will be appreciated, however, that the flux induced at any portion of the magnetostrictive material 30 by the elongated winding 33 is related to the number of turns per inch of the coil 33, in the area of the portions. A soft iron outer sleeve 35 may be placed over the medium 30 and coil 33 to provide local magnetic return paths.

Thus, due to the constant or cyclic variation in winding pitch of coil 33, the effective magnetic bias provided by source 34 cyclically varies from portion to portion along the transmission medium 30, and this cyclic variation in bias effects a cyclic variation in the velocity of acoustic propagation (in accordance with the formula given above), whereby there is a resultant cyclic variation in acoustic impedance along the line. Again, therefore, the resultant acoustic delay line exhibits a lumped characteristic whereby the effective velocity of wave propagation in the line is slower, and the efficiency of the line at low pulse rates is higher, than has been the case heretofore.

While preferred embodiments of the present invention have been described, many variations will be suggested to those skilled in the art. The foregoing description is therefore meant to be illustrative only and should not be considered limitative of our invention, and all such variations as are in accord with the principles described are meant to fall within the scope of the appended claims.