US2832952A - Electroacoustic transducer - Google Patents

Description

April 58 s. M. BAGNQ 2,832,952

ELECTROACOUSTIC TRANSDUCER Filed 001:. 1, 1956 IN VEN TOR.

8. 71AM CM ATTORNEY Z I Au muel mvBa rw ELECTROACOUSTIC TRANSDUCER Samuel M. Bagno, Belleville, N. J., assignor to Walter Kidde & Company, Inc., Belleville, N. 1., a corporation of New York Application October 1, 1956, Serial No. 613,103

Claims. (Cl. 340-384) The present invention relates to electroacoustic devices for transducing electrical energy to acoustic energy and vice-versa, and, more particularly, to such devices which operate at ultrasonic frequencies.

An example of the use of such devices is illustrated in my prior Patent No. 2,655,645, wherein one or more of such devices transduce electrical oscillations at ultrasonic frequencies to acoustical vibrations which are transmitted into a space and another of such devices is sensitive to and picks up the transmitted vibrations and transduces the same to electrical oscillations.

It has been 'found that a suitable device of this character, which is operable at ultrasonic frequencies, generally comprises a hemispherical vibratory shell, a pair of magnetostriction rodshaving their ends secured to the inner wall of the shell, and a'magnetostriction winding on each of the rods.

By magnetostriction is meant the change in dimension produced in a magnetic material, such as iron or nickel or alloys thereof, upon being magnetized, in this instance by the winding which causes the rods to effect vibration of the shell when the device is used as a transmitter. When the device is used as a sound pick-up, vibration of the shell changes the rods in dimension, which in turn changes the magnetization in the rods in a manner to alternate the sameand generate electrical oscillations in the winding around the rods.

In both cases, the energy transduced to another form depends upon the motion of the shell, and, in so transducing energy, maximum efficiency is desirable to transmit the vibrations of the rods as airwaves of maximum power and to pick-up the 'full power of such a wave. To achieve the foregoing, the considerations about to follow will enable the operation of such devices to be understood more clearly.

Electrical oscillations and acoustic waves are analogous in that the voltage E and the current I may be likened to the sound pressure and the vibratory particle velocity, respectively. Thus, since electrical impedance Z is equal to acoustic impedance is equal to i sound pressure T particle velocity r 2,832,952 Patented Apr. 29, 1958 In general, when wave energy flows from a source material of a given impedance to another material (a sink) of a different impedance, there is a reflection of energy at the boundary of the two materials so that only a part of the energy gets through with the rest reflected back. The reflected portion P is given by the following formula:

Z Z 1+ where Z and 2;; are the impedances of the source and sink respectively. It thus becomes apparent that when there is a large discrepancy between the source and sink impedances most of the wave will be reflected back at the boundary of the material and very little will be transmitted through. In such a case, to increase the efiiciency of the transmission an impedance matching device is utilized which is analogous to an electrical transformer. A transformer, by changing the voltage and current in opposing ratios from primary to secondary, transforms the impedance as it transforms the voltage. An analogous acoustic matching device (an acoustic transformer) transforms the acoustic impedance between its terminations as it likewise changes the sound pressure and particle velocity in opposing ratios. Such a device might be a vibrating lever having different particle velocities and pressures in opposing ratios at its ends so that the product of the two at each end is constant and no energy is lost in the transfer. Such an impedance matching device enables a source of one impedance to feed a sink of another impedance without loss.

This impedance mismatching is the condition encountered if an attempt is made to get sound energy from the magnetostriction source into air and vice versa. Air has a low impedance, whereas the material of which the shell is constructed, for example, a metal such as alumi-' num, has a high impedance. Nickel, the magnetostriction material, has a still higher impedance yet. More specifically, the acoustic resistance of air is about 40 ohms, whereas the acoustic impedances of nickel magnetostn'ction rods are about 100,000 ohms. Thus, in order to provide for maximum power, an acoustic transformer must be inserted between the magnetostrictive material and the air. This can be attained by matching the impedance of the rod to the impedance of the shell and the shell in turn to the air.

The problem of impedance matching the shell to the air has been overcome by choosing a shell material which had approximately the same transverse velocity of sound as that of air, thus by the point for point correspondence between the natural motion of the shell and the air, the air presented the best load per unit surface to the shell and the load of the air could beadded within practical limits directly at its area. The shell was given suflicient area so that the total load resistance of the air was within the same order of magnitude as the resistance of the shell.

The problem of matching the impedance of the magnetostriction rods to the shell is somewhat more difiicult.

In the past, it has been found possible to do so by making use of the standing wave patterns of the sound' in the shell. The rods were mounted in such a position that the impedance of the shell at that point in the standing wave pattern was the same as the rods. Standing waves are due to the reinforcement at different points of reflected wave energy adding or substracting to the incident energy. Thus, the amplitude and likewise the impedance at each point on the standing wave is difierent. For that reason, a match can be obtained by choosing the right point on the standing wave. However, no two shells are alike whereby such adjustment had to be made by the use of instrumentation and with great care. After such adjustment was made, very often handling of the shell slightly distorted the same and destroyed the adjustment because the standing wave pattern was altered.

Accordingly, an object of the present invention is, to provide an 'elect-roacoustic transducer which-overcomes the foregoing 'difiiculties and disadvantages.

Another object is to provide such a transducer which is readily and accurately adjusted to match the impedance of the shell with the impedance of the rods.

Another object is to provide'such a transducer which can withstand rough handling without loss of adjustment. A further object is to provide such a transducer which is simple and practical in construction and is economical to manufacture and assemble.

Other and further objects of the invention will be fobvious upon. an understanding .of the illustrative embodiment about to be described, or will be indicated in the appended claims, and various advantages not referred to herein will occur to one'skilled in the art upon employment of'the invention in practice. a

In accordance with the present invention, the foregoing objects are accomplished by providing an electroacoustic transducer which comprises an acoustic sink such as a generally hemispherical vibratory shell, an acoustic source such as a pair of magnetostriction rods located within the shell spaced from andsubstantially parallel to the plane in which the edge of the shell is disposed and having its respective ends closely spaced from the inner wall of the shell. at substantially diametrically opposite points thereof, a magnetostriction winding on the rods, and a vibratory strip for each end of the pair of rods having one end secured to the rods at the ends thereof and having the other end secured to the inner wall of the shell and being disposed at a desired angle withrespect to a plane tangential to the shell at the point of securement of the'strip for impedance matching the impedance of the rods to the shell.

A preferred embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawing, forming a part of the specification, wherein:

Fig. l is a planview of an electroacoustic transducer in accordance with the present invention, illustrated as seen from the bottom thereof. 7

Fig. 2 is a sectional view taken along the line 2-2 on.'Fig.1.

Fig. ,3 is an enlarged fragmentary sectional view taken along the line 33 on Fig. 2.

Fig. 4 is a sectional view taken along the line 44 on Fig. 3 illustrating the construction of a pair of rods.

Fig. 5 is a wiring diagram illustrating a manner of connecting the transducer to an oscillator.

Referring to the drawing in detail, an electroacoustic transducer isshown which generally comprises an approximately hemispherical shell 10, two pairs of magnetostriction rods 11 within the shell, a magnetostriction Winding 12 on each of the rods, and strips 13 for securing the ends of the rods to the inner wall of the shell.

The shell is formed of a vibratory material, that is, a material capable of vibrating at or susceptible to frequencies within the ultrasonic range. For example, the shell may be formed of an aluminum alloy and may have a thickness of about .020 inch. In order to prevent distortion of the shell when handled during assembly, adjustment, installation or repair, stiffening means are provided at the edge of the shell such as a flange 14 preferably extending inwardly.

The hemispherical shell provides a large area in contact with the air, and enables sound to go to the edge and reflect back towards the dome to produce a standing wave pattern. The stiffening flange does not interfere with such sound reflection. In a practical embodiment, the shell may have a diameter of about six inches.

The rods 11 are formed of nickel because of the pronounced magnetostriction property of nickel, The pairs of rods are located Within the shell about halfway between the edge and the dome and are substantially parallel to the plane in which the edge is disposed. The pairs of rods are adjacently spaced apart, and the respective ends thereof are closely spaced from the inner wall of the shell at substantially diametrically opposite points thereof. In the practical embodiment, the rods have a length of about 4.5 inches.

When the device in accordance with the present invention is used as a transmitter, two pairs of rods preferably are employed as shown, to provide for more powerful transmission. When the device is used as a microphone, only one pair of rods is employed to provide for more sensitive reception of vibrations.

The windings 12 provide induction coils for operating the rod, and in the practical embodiment may comprise about 1000 turns of number 40 insulated copper wire.

, A pair of rods are mounted together in such a way as .to

complete their magnetic circuit, and thereby provide a magnetic path of such low reluctance that the rods once polarized by magnetization would tend to remain mag netically polarized. Such polarization is necessary for the proper functioning of the magnetostrictive material and must be providedeither externally by a direct current magnetic winding or by a permanent magnet source. By making the magnetic path of the rods of low enough reluctance, they provide their own permanent magnetism for polarization. The reluctance is further reduced by tying both rods of each pair together with a low reluctance magnetic tape M (Figs. 4 and 5) composed preferably of a 50% nickel-50% iron alloy of the type and generally known as magnetic A metal.

The strips 13 are'formed of a vibratory material such as an aluminum alloy which is relatively stiflE but is bendable to facilitate adjustment as described hereinafter. In the practical embodiment, the strips have a thickness of about .020 inch, a width of about .125 inch and a length of about .375 inch. One end of the strips is provided with a tab 15 adapted to be folded about the tape M at the ends of each pair of rods and then permanently secured thereto. The other end of the strips is permanently secured to the shell by riveting or otherwise, for example by deforming an eyelet or grommet 16, whereby discrepancies due to variations in the mass of a brazed, soldered or welded connection are averted.

The eifective length of the strips is the distance between the points at which the ends of the strips are respectively secured to the shell and the ends of the rods, and, in the practical embodiment, thatrlength is about /6 of an inch. This distance is equal to approximately one quarter the length of the waves at which the shell is to be vibrated. For example,'if the shell is vibrated at a frequency of about 17 kilocycles per second, the length of the wave'is about of an inch or about four times as long as the effective length of the strips.

.As "shown more particularly in Fig. 3, the strips are disposed at some angle with respect to a plane P tangential to the shell at the point of securement of the strip. In the practical embodiment, the strips are slightly bowed and the angle as well as the curvature of the strips are adjusted by bending with respect to the plane P. The adjustment is for best impedance matching so as to get the highest efficiency of energy transfer. This is individually set for each transducer during a functional testing and adjusting operation. A better than 10 to 1 impedance variation can'be obtained by such an adjustment as the vibratory-motion of the strip is changed from transverse to longitudinal or combinations thereof.

The shell is provided with cushioning means 17 near its edge for mounting the shell on suitable suporting is connected to the plate of an electron tube 21 and its other side is connected to the grid of the tube through a capacitor 22. A resistor 23 is shunted across the grid and cathode of the tube and is connected to negative. A capacitor 24 is connected across the winding 20, and a tap 25 on the winding is connected to positive.

From the foregoing description, it Will be seen that the present invention provides a new and useful electroacoustic transducer which is readily adjusted and remains in adjustment during the life thereof, even when subjected to such rough usage which is normally encountered.

As various changes may be made in the form, construction and arrangement of the parts herein, without departing from the spirit and scope of the invention and without sacrificing any of its advantages, it is to be understood that all matter herein is to be interpreted as illustrative and not in any limiting sense.

I claim:

1. An electroacoustic transducer comprising a generally hemispherical vibratory shell, magnetostriction rods located within said shell spaced from and substantially parallel to the plane in which the edge of said shell is disposed and having their respective ends closely spaced from the inner Wall of said shell at substantially diametrically opposite points thereof, a magnetostriction winding on said rods, and a vibratory strip for each end of said rods having one end secured to said rods at the ends thereof and having the other end secured to the inner Wall of said shell and being disposed at a desired angle with respect to a plane tangential to said shell at the point of securement of said strip.

2. A transducer according to claim 1, wherein said shell has stiifening means at the edge thereof for preventing distortion of said shell.

3. A transducer according to claim 2, wherein said stiffening means is a flange.

4. A transducer according to claim 3, wherein said flange extends inwardly beneath said shell.

5. A transducer according to claim 1, wherein said strips are formed of relatively stiff bendable material and thereby are adapted for adjustment with respect to said shell.

6. A transducer according to claim 5, wherein said shell has stifiening means at the edge thereof for preventing distortion of said shell after adjustment of said strips.

7. An electroacoustic transducer comprising a generally hemispherical vibratory shell, two pairs of parallel adjacently spaced apart magnetostriction rods located within said shell spaced from and substantially parallel to the plane in which the edge of said shell is disposed and leaving their respective ends closely spaced from the inner Wall of said shell at substantially diametrically opposite points thereof, a magnetostriction winding on each of said rods, and a vibratory strip for the ends of each '7 pair of said rods having one end secured to its said rods at the ends thereof and having the other end secured to the inner wall of said shell and being disposed at a desired angle with respect to a plane tangential to said shell at the point of securement of said strip.

8. A transducer according to claim 7, wherein said strips are formed of relatively stilf bendable material and thereby are adapted for adjustment with respect to said shell.

9. An electroacoustic transducer comprising a generally hemispherical vibratory shell, magnetostriction rods located within said shell spaced from and substantially parallel to the plane in which the edge of said shell is disposed and having its respective ends closely spaced from the inner wall of said shell at substantially diametrically opposite points thereof, a magnetostriction Winding on said rods, and a vibratory strip for each end of said rods having one end secured to said rods at the ends thereof and having the other end secured to the inner wall of said shell and being disposed at angle with respect to a plane tangential to said shell at the point of securement of said strip, said strips having an effective length approximately one quarter wave length.

10. In combination, an acoustic source having a given impedance, an acoustic sink having an impedance within a given range, and a vibratory strip having its ends connecting said source and said sink adapted to be adjusted to match the impedance of said source and said sink at its respective ends by changing the configuration thereof adjacent its ends, whereby a better than 10 to 1 impedance variation can be obtained as vibratory motion of the strip is changed from transverse to longitudinal and combinations of such motions.

No references cited.

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