Title:
Sound device with piezoelectric double plates
United States Patent 2270167


Abstract:
In Germany April 28, 1938 10 Claims. (CL 179-110) This invention covers a sound device with piezo-electric double plates, which is particularly suitable as a microphone or loudspeaker. The sound device can also be applied to advantage in television apparatus, in instruments for recording and...



Inventors:
Alexander, Meissner
Ernst, Burckhardt
Application Number:
US26891639A
Publication Date:
01/13/1942
Filing Date:
04/20/1939
Assignee:
GEN ELECTRIC
Primary Class:
Other Classes:
310/329, 310/332, 310/362
International Classes:
H04R17/00
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Description:

In Germany April 28, 1938 10 Claims. (CL 179-110) This invention covers a sound device with piezo-electric double plates, which is particularly suitable as a microphone or loudspeaker. The sound device can also be applied to advantage in television apparatus, in instruments for recording and reproducing sound graphs, and in so-called public address systems, In the piezo-electric loudspeakers or microphones with square or rectangular double plates, which in the process of their deformation assume a saddle-like surface, the plate systems are generally supported on three corner points of the crystal plates, while the sound cone is secured, at its apex or tip, to the fourth freely oscillating corer. The supports of the three corner 15 points are elastic, but they must suppress any motion in a direction at right angles to the plates, in order that when a potential is applied to the system the maximum amplitude deflection will manifest itself at the freely oscillating fourth plate corer. In this case, the motions of the free plate corer and of the opposite corner would be additively transmitted to the sound-cone apex. However, because of the imperfect elasticity of the supports, a part of the energy is M uselessly converted into heat by the work of bending the supports; for the oscillations at right angles to the plates are never entirely eliminated, Moreover, the supports cause, because of their natural resonance, an additional root mean- 30 square distortion or non-linear harmonic distortion, which can noticeably distort the sound effect. Besides, the necessary eccentric mounting and arrangement of the sound cone at only one crystal corner is a cumbersome device, which 3 makes the instrument sensitive to vibrations or other mechanical influence, and unsuitable for compact constructions.

According to the invention, all these disadvantages are avoided by the fact that two opposite corers of the plate systems are connected to two points of the sound membrane, which lie symmetrically on a zone of increased membrane stiffness, while the two other corners of the plate systems, which are not fastened the membrane, are weighted by means of suitable weights or are supported by suitable strips. In most cases these weighted corers need not be secured by means of strips or supports, etc., and the plate system is supported 50 directly by the membrane by means of the two corers which have been fastened..

As a result of this weighting of both free plate corers, the force necessary for causing these corners to vibrate in a direction at right angles M double plates.

to the plate is quite considerable, for the lever arm, from the center of the plate system to the weighted comers, is quite large. On the other hand, only small forces are necessary for the torsional vibrations of the corner in the plate plane or in the direction of the diagonal. In this way, the oscillations in the direction at right angles to the plate will be the more suppressed, the greater the weights and the less they extend laterally, parallel to the plate plane. With this arrangement, no energy losses can occur due to the energy transformation into heat when the supports are bent, for the energy is stored up in the mass as potential energy. Moreover, the elimination of the banding work, etc., permits an additional increase in the amplitude, that is to say, a higher efficiency. Since both opposite plate corners are connected to the sound membrane, the forces acting on the sound membrane from each individual corner are only onehalf of the force of an individual freely oscillating corer, which manifests itself on the apex of the sound membrane in the case of three-point connection. The maximum bending, however, is independent of the plate thickness, and thinner plates can be used for the same system capacity, with the two-point support, since, about only one-half of the forces are effective from each corner to the sound membrane with a given total force. The zone of increased membrane stiffness of the sound cone, which is attacked by two opposite corners of the plate system, is attained, in the case of the conical membrane, simply by depressing the apex of the cone toward the interior, in such a way that a circular zone of increased membrane stiffness is formed at the edge where the apex has been pushed in. The sound membrane has now the shape of a truncated cone, the upper diameter of which must be equal to or larger than the diagonal of the plate system, which is secured to the circular edge at two opposite corers.

The object of the invention will be now explained in reference to the accompanying drawto 45 ings, in which: Fig. 1 is a plan view of a piezo-electric doubleplate system with two weighted corers employed in connection with a sound membrane; ig. 2 is a side view of Fig. 1; Fig. 3 is a side view of the same plate system, which represents, however, a special method of fastening or suspending the weighted corners.

Figs. 4, 5 and 6 show two different methods of fastening the weights to the corners of the crystal Figs. 7 and 8, one a top view, the other a side view, illustrate the arrangement of two plate systems which are fastened symmetrically to. the membrane at opposite corners.

Fig. 9 shows again a top view and Fig. 10 a side view of a plate system in which two opposite corners are connected to the sound membrane by means of suitable extensions.

In Figs. 1 and 2, the crystal double plate I is fastened with its opposite corners 2 and 3, to the loudspeaker membrane 4, which is here a cone. The two other corners of the plate system are weighted by weights 7 and 8, which suppress or store up in the above-described manner the oscillations of these corners in a direction at right angles to the plate. The apex 5 of the sound membrane 4 is pushed in, so that a circular edge 6, which has a higher stiffness, is produced on the membrane; on this edge, the points 2 and 3, where the plate system is secured, lie opposite each other symmetrically. The plate system is supported by the membrane alone, so that the sound device can have a compact, flat, and well-rounded structure, which contrasts favorably with the piezo-electric loudspeakers known so far, with eccentric fastening of a corner of the plate system to the tip of the cone membrane. When the cone membrane or any other sound-radiating surface is flat, the loudspeaker has the shape of a flat disk, which can be hung, as a wall loudspeaker, to the walls of a room.

Fig. 3 shows the same arrangement, rotated by ninety degrees in the plate plane. The corners weighted by the weights 7 and 8 are secured to the frame II by means of wires, strings, or needles 9 and 10. In the case of larger systems and weights, this arrangement has the advantage that the pressure, caused by the weight of the loudspeaker system on the membrane supporting it, can be eliminated. '40 Figs. 4, 5 and 6 represent two methods of fastening the weights 7 and 8 to the plate corner.

In order to increase the efficiency of the plate deflection, the normal bending of the plates must not be impeded by a weight at the corners. '45 Hence, the weighted mass must be secured as much as possible at points only.

Fig. 4 shows a top view of a physical embodiment, and Fig. 5 shows its side view. In this embodiment, the plate corner 12 can be bent, under the influence of a stress, without being hindered by the weight, since the weight 7 is secured between the two plates in the neutral zone 13, 14.

Fig. 6 shows another method of fastening the weight to the surface of one crystal plate only.

By increasing the contact surface 18 between the weight and the plate surface in a direction along the diagonal to the center point, it is possible to fasten the mass in a very stable manner. A narrow slit 19 has been provided between the weight and the corner edge so as to permit any movement or warping of the plates with respect to each other.

In Figs. 7 and 8 are two similar oscillating systems 20 and 21, with two opposite corners at the four points 22, 23, 24 and 25 (which are symmetrically arranged) of the stiffened circular membrane edge. The influence of the force on the membrane is doubled by the arrangement of two systems, and the effective amplitude of the irradiated vibrations is increased. With a large diameter of the circular membrane edge, it is easily possible to fasten additional double-crystal systems, at each pair of points at the membrane edge, and in this manner to further increase the effective amplitude. The securing of one or more plate systems is by no means limited to the physical embodiment we have just described, which deals with a conical membrane.

Any kind of membrane, such as a corrugated membrane, to give an instance, which is suitably arranged, makes it possible to obtain the above advantages resulting from the connection in groups of two, of the points of the plate system with the sound-radiating surface.

The arrangements according to Figs. 9 and 10 refer especially to microphones. In order to increase the oscillation amplitude, which is transmitted by the membrane edge 6 to the corners 2 and 3, we have connected the two opposite corners 2 and 3 of the crystal system I (by means of extension 26 and 27, which lengthen the leverage), with the membrane edge 6, so that the microphone properties of the sound device are improved. All these physical embodiments, involving the two-point fastening of the plate system, give besides the possibility of further increasing the sensitivity of the sound instrument by arranging several plate systems, one beside the other or one above the other.

For the oscillating double-plate system, it is advisable to use artificial plates made of Rochelle salts which have been grown in a proper solution.

These plates must contain no impurities or further nucleus for crystal growth, the plates are cut to the proper size, and their electrodes must consist preferably of a layer of silver or a similar material sprayed directly on the cut crystal surfaces by thermal spraying or cathode spraying.

Compared with a plate system on which the electrodes have been glued onto the crystal surface, the capacitance of the plate system on which the electrodes have been sprayed is considerably increased, the electrode surface remaining, the same, and the efficiency is therefore much higher. The two Rochelle salt plates are joined, or "glued" together, by a cohesive layer of Rochelle salts, so that the bending of the plates can in no wise be changed by any foreign adhesive material having other physical properties.

In order to join the plate corners with the weights and the loudspeaker diaphragm, one can likewise use molten Rochelle salts to the best advantage.

As a result of the elimination of the resonance regions created by the supports, the frequency curve of the sound device depends only on the properties of the plate system and of the sound diaphragm.

The stiffness and the shape of the sound membrane can be adapted to the amplitude of the motion of the crystal corners, in the known manner.

The weighted mass will in itself favor the low frequencies. Because of the size and shape of the weights, the frequency curve can be altered in any desired manner. The mounting of several plate systems for different natural frequencies on one and the same sound diaphragm offers the additional advantage of adjusting the frequency curve in an easy and cheap manner. The voltages which can be obtained by means of the sound device according to the invention reach, with normal speech, and at a distance of ten to twenty cm., with only one plate system, the very high value of about six volts. Thus, it is possible to obtain without any intermediate amplifier a direct two-way conversation by means of loudspeakers. The sound device is especially suitable as a microphone or loudspeaker for speech apparatus or conference systems, which are similar to the magneto-phone, and in which the Rochelle-salt plate system can be used for speaking or hearing with or without changing over the connections. As a result of the flat compact shape, and of the piezo-electric system, which can be easily shielded, and which has no leakage, the sound instrument according to the invention offers advantages, besides in the case of television receivers, in which any magnetic field from the loudspeaker would cause disturbances in the service.

What is claimed is: 1. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness, two diagonal corners of each one of said plates supported on the bent rim of said cone, and a pair of weights suspended on diagonally opposite corners of each one of said plates and at right angles to the support corners to increase the efficiency of said device.

2. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in 3( to face its base portion thus forming a rim membrane portion of increased thickness, two diagonal corers of each one of said plates supported on the bent rim of said cone, a frame surrounding said sound membrane, a pair of weights sus- 3 pended on diagonally opposite comers of each one of said plates, said weights secured to said frame to increase the efficiency of said device.

3. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle 41 salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness, two diagonal corners o cne of each one of said plates supported 4 on the bent rim of said cone, a frame surrounding said sound membrane, a pair of weights suspended on diagonally opposite comers of each one of said plates, said weights secured to said frame by a pair of wires to increase the efficiency 5 of said device.

4. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness having a diameter equal to the diagonal of said plate system, two diagonal corners of each one of said plates supported on the bent rim of said cone, E a pair of weights suspended on diagonally opposite corners of each one of said plates and at right angles to the support corners to increase the efficiency of said device.

5. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness having a diameter greater than the diagonal of said plates, two diagonal corners of each one of said plates supported on the bent rim of said cone, a pair of weights suspended on diagonally opposite corers of each one of said plates and at right angles to the support corners to increase the efficiency of said device.

6. A sound device comprising a plurality of pairs of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated cone-shaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness, two diagonal corers of each one of said plates supported on the bent rim of said cone, a pair of weights suspended on diagonally opposite corners of each one of said plates and at right angles to the support corners to increase the efficiency of said device.

7. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness, two diagonal corners of each one of said plates supported on the bent rim of said cone, a pair of weights suspended on diagonally opposite corners of each one of said plates and at right angles to Sthe support corers by means of molten Rochelle salts to increase the efficiency of said device.

8. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneSshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane portion of increased thickness, two diagonal comers of each one of said plates supported on the bent rim of said cone, a pair of weights 0 suspended on diagonally opposite corers of each one of said plates and at right angles to the support comers by means of a cohesive layer of Rochelle salts to increase the efficiency of said device.

9. A sound device comprising a pair of recti5linear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped corrugated sound membrane having its Sapex bent in to face its base portion thus forming a rim membrane portion of increased thickSness, two diagonal comers of each one of said plates supported on-the bent rim of said cone, a pair of weights suspended on diagonally opposite comers of each one of said plates and at right angles to the support comers to increase the efflciency of said device.

10. A sound device comprising a pair of rectilinear piezo-electric crystal plates of Rochelle salts forming a plate system, a truncated coneshaped sound membrane having its apex bent in to face its base portion thus forming a rim membrane having a diameter greater than the diagonal of said plates, a pair of extension members secured to two diagonalf corners of each one of 15 said plates and supported on the bent rim of said cone to increase the leverage on said sound membrane.

ALEXANDER MEISSNER.

ERNST BURCKHARDT.