GB2096860A

GB2096860A - Piezoelectric sound transducer

Info

Publication number: GB2096860A
Application number: GB8109825A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1981-03-30
Filing date: 1981-03-30
Publication date: 1982-10-20
Also published as: GB2096860B

Abstract

A piezoelectric sound transducer comprising a disc 8 of piezoelectric material mounted on a thin metal diaphragm 9 and supported on a baseboard 1 in such a manner that the diaphragm/disc combination can vibrate in a flexural mode. A very cheap but highly reliable supporting means is obtained by engaging the diaphragm at three substantially evenly-spaced points in its peripheral region by respective pins 2, 3, and 4, which pins cause the disc 8 to be stressed against contact pins 5a and 6a. <IMAGE>

Description

SPECIFICATION Piezoelectric sound transducer This invention relates to a piezoelectric sound transducer comprising a disc of piezoelectric material fixed over an entire one of its major surfaces to a circular metal diaphragm concentric therewith which has a diameter larger than that of the disc and which forms one contact electrode of the disc, at least a major portion of the other major surface of the disc being provided with a second contact electrode, which diaphragm is supported in such a manner that the diaphragm/disc combination can vibrate in a flexural mode.

Transducers of this type are well known, one typical example being shown and described in the Mullard Application Book "Piezoelectric ceramics", second edition, January, 1974, pages 103 to 105. In the described transducer, the diaphragm is clamped around its whole periphery between the facing ends of two tubes of equal diameter and wall thickness. One of the tubes serves as the electrical connection to the diaphragm (and hence to the said one electrode) and its end remote from the diaphragm abuts and is soldered to a printed wiring track on a printed circuit board. Thus this tube serves both as a support for the diaphragm/disc combination and as an electrical contact. The other tube forms part of a Helmholtz resonator. Electrical contact is made with the other electrode on the disc via a spring blade which contacts the disc centrally.

The disc forms part of a 3 kHz oscillator of the multivibrator type comprising, in addition to the disc, two transistors, four resistors, two capacitors, and two diodes. These additional components are mounted on the printed circuit board.

This method of supporting a piezoelectric disc is referred to as edge clamping and was developed as a reasonable compromise between manufacturing cost and the maximum sound output achievable. If the diaphragm/disc combination were allowed to resonate freely- that is in an unsupported state, its nodal vibration line (a line joining all points of zero vibration, sometimes referred to as the Chladni line or pattern) would be a circle having a radius very slightly smaller than that of the piezoelectric disc.

For optimum response of any resonating plate, the plate should be supported along its nodal line so that the supporting means have minimum mechanical damping effect on the vibration. One of the problems in supporting a disc nodally, however, is that means have to be provided to ensure that the disc cannot move sideways on the (annular) support. This not only adds considerably to the expense of the transducer but, because there is some restraint on the movement of the disc, also tends to load the disc mechanically and hence reduce its efficiency as a sound emitter. In the case of an edge-clamped diaphragm/disc combination, the nodal line is moved radially outwards to the circular clamping region. The fact that the nodal line is still a circle shows that the disc is vibrating in a relatively simple, and hence reasonably efficient, mode.The relatively large increase (e.g. about 40%) in the radius of the nodal circle shows, however, that the mechanical load applied by the clamp is not inconsiderable and that the edge-clamping method is, as is well known, less efficient than nodal suspension.

In order to reduce the high mechanical load applied by the edge-clamping method, it is known to provide a ring of resilient material between the edge of the diaphragm and the two tubes. This improves the efficiency but the transducer is still markedly less efficient than is achievable by a nodally-supported piezoelectric disc. Also, of course, the provision of the resilient ring increases the cost. Even without the resilient ring, however, such a transducer still tends to be too expensive to serve as a very cheap form of audible alarm on, for example, domestic equipment (ranging from alarm clocks to washing machines), automobiles, television games, and electronic calculators.

An object of the invention is to provide a piezoelectric sound transducer in which the above disadvantage is at least mitigated and which is relatively cheap.

According to the invention there is provided a piezoelectric sound transducer of the type defined in the opening paragraph hereof, characterised in that the diaphragm is solely supported at three points substantially evenly spaced around the annular portion of the diaphragm which surrounds the disc.

A transducer according to the invention can therefore be supported in a very simple and cheap manner by, for example three pins each of which may engage the peripheral edge of the diaphragm or may be secured to a face of the diaphragm, for example by welding, at a point between the said edge and the disc. A surprising advantage is that this method of supporting the diaphragm/disc combination appears to apply negligible mechanical damping to the combination.

When mounting disc or diaphragm/disc combinations by the known edge-clamping or nodal support methods, use has always been made of the fact that the nodal vibration lines of these discs are circles and/or diameters of the disc. The present inventive concept, however, arises from the realisation that if a diaphragm/disc combination of the type described above is rigidly supported at a single point at the edge of the diaphragm, the nodal line (Chladni pattern) is substantially an equilateral triangle the corners of which extend to corresponding points at the edge of the diaphragm. This is very surprising, since the various possible modes of vibration for discs have always been assumed to be confined to nodal circles and/or nodal diameters and the various modes are represented by the number (usually O to 2) of nodal circles followed by the number (usually O to 3) of nodal diameters.See, for example, "Analysis of flexural vibrations of a circular disc", IEEE Trans. Sonics and Ultrasonics, Vol. SU15 No. 3, July1968 pages 182-185.

Thus a vibration mode having one nodal circle and two nodal diameters is represented by 1 , 2. The various known mounting methods (e.g. edgeclamping, nodal support, and core, or central, support) are based on these modes. The triangular mode of vibration cannot, of course, be represented by this system.

If the single point of mounting the diaphragm is moved radially inwards towards the periphery of the piezoelectric disc, the other two corners of the triangle move inwardly in a corresponding fashion and the corners become progressively more rounded.

The reason for the unexpectedly triangular nodal line is not known to us. Further an unexpected result is that the intensity of the emitted sound is noticeably greater than that achieved under the same circumstances but using the edge-clamped method described above. This may possibly be due to the fact that the diaphragm/disc combination is mounted at one point only in a cantilever fashion and, since all points are therefore free to move relative to each pther, the vibration of the disc is mechanically damped to only a negligible degree.

In a first experiment, a point on the extreme edge of the diaphragm was rigidly fixed and the frequency and sound intensity of the vibrating diaphragm/disc combination were measured. The diaphragm was then rigidly fixed at each of the other two corners of the nodal triangle at the edge of the diaphragm. The frequency and sound intensity were found not to have changed, thereby showing that the two additional fixing points had not applied any mechanical damping to the vibrating combination. The experiment was repeated several times with the three fixing points being progressively moved radially inwardly of the diaphragm from experiment to experiment until all three points were close to the periphery of the piezoelectric disc.In each case the added fixing of the diaphragm at the other two corners of the equilateral triangle made substantially no difference to the intensity of the emitted sound. It was also found that, provided the three fixing points are substantially evenly spaced around the annular region of the diaphragm between the disc and the edge of the diaphragm, their respective distances from the centre of the diaphragm were not very critical. This means, for example, that a small radial slot may be made in the peripheral region of the diaphragm and that one of the supports may engage this slot to locate the diaphragm in a predetermined rotational position.

A further disadvantage of known piezoelectric sound transducers is that they use a spring or a soldered joint to make electrical contact with an electrode on the piezoelectric disc.

The use of a spring contact is not only relatively expensive, in terms of both material and labour cost, but is also far from ideal since the spring almost invariably makes contact with the electrode at the centre of the disc-i.e. it applies a mechanical load to the disc at the point of maximum vibrational excursion (antinode) of the disc.

The use of a soldered joint, generally at the centre of the disc, not only applies an inertia load to the disc due to the mass of the solder, but also gives rise to production problems. The disc electrodes typically comprise a silver layer approximately 25 ,um thick. Great care has to be taken when soldering on to the layer and a number of rules have to be strictly observed, regarding for example the soldering temperature and time, the solder, the flux, the wire diameter, and the careful removal of flux (see pages 1 70 and 171 of the above-mentioned Mullard pubiication). These rules are necessary, for example, to limit the dissoiving of the silver layer by the solder (to an extent which depends upon temperature and time) and to limit the thermal depolarisation of the piezoelectric material.

The electrical contact surface on a contact of the spring type is usually of silver, for example a layer of silver on a phosphor bronze spring blade.

Thus the cost of the materials used is relatively high. In the case of a soldered joint, the material costs are lower but the labour costs are relatively high due to the above-mentioned precautions that have to be taken. Further, the use of a spring or a soldered joint at the vibration antinode inevitably imposes a mechanical load on the disc.

These disadvantages are at least mitigated in an embodiment of the invention wherein the three supporting pins are mounted on a baseboard and engage the diaphragm so as to hold the combination substantially parallel to a baseboard and stressed against at least one contact stud thereon in contact with the second electrode in a region thereof adjacent one of the pins. The advantages arise from the recognition that the triangular nodal line would allow an electrical contact to be made with an electrode of the disc in a nodal region adjacent one of the pins, since there would be very little vibration in the region of the apex of the triangle. Preferably the contact point or points should lie on the nodal triangle itself. This results in that the contact stud (or perhaps studs) applies no mechanical damping to the vibrating combination and therefore does not reduce the efficiency.

The stressing of the disc electrode against a contact stud provides further major advantages.

Firstly, the arrangement is very robust and experiments have shown that the disc cannot readily be induced to vibrate at any of its overtone frequencies. Secondly, the electrical contact between the contact stud and the plated electrode does not suffer from the abovementioned disadvantages arising from the use of soldered joints or contact springs and is therefore not only more reliable in use but is also very considerably cheaper to manufacture.

Due to the fact that the points of suspension are nodal points, any change in the fixing force applied to the diaphragm at any of the fixing points will have very little effect. It was in fact necessary to apply force to other parts of the diaphragm in order to make it resonate at an overtone frequency and when this force was removed the frequency reverted to the fundamental. Thus the three-point mounting arrangement is not only very robust mechanically but is also substantially immune to the abovementioned handling problem.

The diaphragm/disc combination may also be stressed against a second contact stud provided on the baseboard adjacent the first contact stud, both studs being in contact with the second electrode. This had the advantage that the combination is more positively located on the baseboard (by the use of two studs instead of one) whilst the pins are being bent during assembly. The two studs may conveniently be connected together electrically.

Alternatively, the said other major surface of the disc may be provided with a third electrode thereon, which electrode extends substantially to the edge of the disc, in which case the combination may also be stressed against a second contact stud provided on the baseboard adjacent the first contact stud, the second contact stud being in contact with the third electrode. In this case, each of the two contact studs is connected to a respective electrode. The third electrode is generally referred to as the feedback electrode F and enables the oscillator circuit to compromise, in addition to the disc, only a single transistor and two resistors.

The baseboard may be a printed circuit board having respective wiring tracks connected to at least one of the three pins and to the contact stud(s). Thus the baseboard not only supports the diaphragm/disc combination, but also serves as a mounting board for associated electrical components and provides the circuit wiring.

In the above-described prior art transducer in which the diaphragm/disc combination is clamped between two tubes, one of the tubes serves as at least part of a Helmholtz resonator. In an embodiment of the invention the transducer includes a further board which extends parallel to the baseboard and which carries a tube thereon extending coaxially towards, but not touching, the disc so as to form a Helmholtz resonator for the disc. The gap between the tube and the disc forms the effective small opening of a Helmholtz resonator tuned to the fundamental frequency of the disc. By providing the resonator tube on a separate board such that it is out of direct mechanical contact with the diaphragm/disc combination, there is no direct load on the combination and, hence, any movement of the tube cannot affect the frequency of vibration of the disc.

Embodiments of the invention will now be described in detail with reference to the accompanying drawing, of which: Figures 1 and 2 show a plan and a side view of a baseboard and mounting arrangement prior to mounting a diaphragm/disc combination thereon, Figures 3 and 4 show corresponding views with a said combination mounted thereon and, in the case of Figure 4 only, with a resonator, Figure 5 is a circuit diagram of an oscillator suitable for use with a piezoelectric element provided with a feedback electrode, and Figure 6 shows a combination mounted in the same manner as for the combination shown in Figures 3 and 4 but provided with a feedback element.

Referring now to Figures 1 and 2, a baseboard 1, comprising a printed circuit board, is provided with five identical headed pins 2 to 6 having respective heads 2a to 6a. All the pins are inserted in the baseboard 1 and are soldered to respective copper printed circuit tracks 2c, 5c, 6c and iands 3c, 4c (see Figure 1). The pins 2, 3 and 4 are used as mounting means for a diaphragm/disc combination and the heads 5a, 6a of pins 5 and 6 serve as contact studs, pins 5 and 6 being inserted in the baseboard 1 in the opposite sense to pins 2, 3 and 4.

Figures 3 and 4 show views of the baseboard 1 which correspond to those of Figures 1 and 2 and further include a diaphragm/disc combination 7 comprising a thin disc 8 of piezoelectric ceramic material fixed over an entire one of its major surfaces to a circular metal diaphragm 9 concentric therewith. Diaphragm 9 has a larger diameter than the disc and serves as a first contact electrode for the disc. Substantially the entire surface of the disc remote from diaphragm 9 is provided with a thin (e.g. 25 ,um) layer of silver which serves as a second contact electrode for the disc, the outline of the disc and of the second electrode being shown as broken line 11 in Figure 3.

Diaphragm 9 is provided with a radiallyextending peripheral locating slot 12 which engages the pin 2 and pins 2, 3, and 4 are disposed at substantially equal intervals around the periphery of diaphragm 9. The free ends of the pins are bent inwardly toward the axis of the diaphragm to engage the diaphragm in such a way that the diaphragm/disc combination 7 is held substantially parallel to baseboard 1 with the second electrode of disc 8 stressed against the two contact studs formed by the heads 5a and 6a of pins 5 and 6 respectively.

Prior to the assembly of the combination 7 on to the baseboard 1, it is convenient for pin 2 to be pre-bent to the shape shown in Figure 4 and for pins 3 and 4 to be straight. To assemble the combination 7 on to the baseboard 1 , the pin 2 is first slid into the slot 12 with the second contact electrode of the disc 8 in contact with the two contact studs formed by heads 5a and 6a. At this stage, the combination 7 encloses an acute angle with the baseboard with the lower end of the diaphragm (as viewed in Figure 4) more remote from the baseboard than the upper end. The lower end is then pressed towards the baseboard until the combination 7 is substantially parallel with the baseboard 1 and pins 3 and 4 are bent inwardly as shown in Figures 3 and 4.This causes the combination 7-and hence the second contact electrode-to be firmly stressed against each of the contact studs formed by heads 5a and 5b.

It is to be noted that each of the heads 5a and 5b is in contact with discs 8 in a region of the latter closely adjacent pin 2; that is to say that the heads 5a and 6a are as close as is practical to the pin 2 and, hence, to the nodal point defined by pin 2. Further, head 5a lies substantially on the nodal line extending from pin 2 to pin 3 and, similarly, head 6a lies substantially on the nodal line extending from pin 2 to pin 4. Thus despite the fact that the combination 7 is stressed against these two contact studs, the latter have substantially no damping effect on the vibrations and, hence, the sound output from the vibrating combination is not reduced. Also, of course, the nodal pattern is substantially not altered by the presence of the two contact studs.Since there is no tendency for the disc to vibrate at its points of contact with the two contact studs, the electrical contact has maximum reliability from this aspect.

Figure 4 also shows a Helmholtz resonator (not shown in Figure 3) formed by a short tube 13 mounted on, or formed integrally with, a further board 14 which extends substantially parallel with baseboard 1. the tube 13 is mounted coaxially with the combination 7 and the free end of tube 13 does not extend as far as the combination. Thus the resonator applies no direct mechanical load to the combination and therefore any slight distortion or movement of the resonator due to handling has substantially no adverse effect on the transducer.

The fixing means (not shown) which fix board 14 relative to baseboard 1 may be made adjustable so that the gap between the combination 7 and the open end of the tube 13 may be adjusted. The board 14 may, for example, be part of an outer wall of apparatus which houses such a transducer. Alternatively, the board may be, or may form part of, a fascia panel of somewhat larger apparatus, such as a spin drier or washing machine.

Electrical connection to the piezoelectric elements is established via the printed wiring tracks Sc (and/or 6c) and 2c but, of course, other track configurations can equaliy well be used. For example, all the pins 2, 3, and 4 could be electrically connected together by one printed circuit wiring track and pins 5 and 6 could be connected together via a further wiring track. A suitable oscillator circuit for driving the combination 7 at its resonant frequency is given on page 104 of the above-mentioned Mullard publication and the ten additional components can be mounted on and electrically interconnected by means of the printed circuit baseboard 1.

The circuit can be very considerably simplified, however, by using a so-called feedback-type of piezoelectric element which, in addition to the two major electrodes referred to above, has a third (relatively small) electrode. The contact electrode formed by the metal diaphragm is generally referred to as the common electrode C, the second electrode occupies a major portion of the other face of the disc and is referred to as the main electrode M, and the small third electrode is referred to as the feedback electrode F. A suitable oscillator circuit using these references is shown in Figure 5, which circuit only uses three components (a transistorT1 and two resistors R1 and R2) in addition to a disc 9a. This is a very simple and well-known basic oscillator circuit which needs no further description herein.

Figure 6 shows such a disc 9a fixed by pins 2, 3 and 4 to a baseboard 1 in an identical manner to disc 9 as shown in Figures 3 and 4. In this case, the contact stud formed by head 5a makes electrical contact with the main electrode M, the contact stud formed by head 6a is in contact with feedback electrode F, and at least pin 2 is in electrical contact with the common electrode C formed by the diaphragm. The baseboard 1 is not shown in this Figure but is the same as that shown in Figure 1 , the three connections to the electrodes C, M and F being effected via the respective printed wiring tracks 2c, 5c, and 6c and respective pins 2, 5, and 6.

In a practical embodiment, the pins 2 to 6 were of brass with a copper flash and a plating of tin.

Each pin was straight-knurled just below the head to provide a positive grip with the baseboard and the head had a diameter of 1.6 mm and a depth of 0.5 mm. The diameter of the stem was 0.9 mm and the length was 7.25 mm.

The diaphragm/disc combination 7 comprised a diaphragm 9 of nickel-plated half-hard brass having a diameter of 35 mm and a thickness of 0.2 mm, and a disc 8 of a piezoelectric material having a diameter of 25 mm and a thickness of 0.2 mm. The diameter of the second contact electrode was 24 mm. Such combinations 7 as shown in Figures 3 and 6 are available, for example, from Mullard Limited and Philips Electrical Limited as Type Numbers 4322 020 08850 and 4322 020 08890 respectively. The output frequency in each case was slightly less than 3 kHz and, in the case of the Figure 3 arrangement, the measured sound output level was 94 dB at a distance of 30 cms, using the circuit shown in Figure 6 with R1=180 kOhm, R2=1.5 kOhm and T1 =Type BC 548 (Philips).

In the above-described embodiments, the diaphragm is supported at three points on its peripheral edge by respective pins which engage the edge. As stated above, the diaphragm may be alternatively supported at three points on its major face evenly-spaced around the annular region of the diaphragm between the periphery of the disc and the edge of the diaphragm. In such a case, the diaphragm may for example be welded or soldered at these points to the heads of three respective studs arranged in a similar manner to the studs 5 shown in Figures 1,2, and 3.

Claims

1. A piezoelectric sound transducer comprising a disc of piezoelectric material fixed over an entire one of its major surfaces to a circular metal diaphragm concentric therewith which has a diameter larger than that of the disc and which forms one contact electrode of the disc, at least a major portion of the other major surface of the disc being provided with a second contact electrode, which diaphragm is supported in such a manner that the diaphragm/disc combination can vibrate in a flexural mode, characterised in that the diaphragm is solely supported at three points substantially evenly-spaced around the annular portion of the diaphragm which surrounds the disc.

2. A sound transducer as claimed in Claim 1, wherein the diaphragm is supported at each of the three said points by a respective pin, which three pins are secured to a baseboard and engage the diaphragm so as to hold the combination substantially parallel to the baseboard and stressed against a contact stud thereon in contact with the second electrode in a region thereof adjacent one of the pins.

3. A sound transducer as claimed in Claim 2 wherein the combination is also stressed against a second contact stud provided on the baseboard adjacent the first contact stud, both studs being in contact with the second electrode.

4. A sound transducer as claimed in Claim 2 wherein the said other major surface of the disc is provided with a third electrode thereon, which electrode extends substantially to the edge of the disc, and wherein the combination is also stressed against a second contact stud provided on the baseboard adjacent the first contact stud, which second contact stud is in contact with the third electrode.

5. A sound transducer as claimed in any previous Claim wherein the baseboard is a printed circuit board having respective wiring tracks connected to at least one of the three pins and to the contact stud(s).

6. A sound transducer as claimed in Claim 4 including at least one transistor mounted on said baseboard and connected to the disc electrodes so as to cause said disc, in use, to vibrate at a resonant frequency thereof.

7. A sound transducer as claimed in any previous Claim, including a further board which extends parallel to the baseboard and which carries a tube thereon extending coaxially towards the disc, but not touching the diaphragm/disc combination, so as to form a Helmholtz resonator for the disc.

8. A sound transducer as claimed in Claim 7 wherein the distance between the baseboard and the further board is adjustable whereby the distance between the disc and the end of the tube remote from the board may be adjusted.

9. A piezoelectric sound transducer substantially as herein described with reference to Figures 1 to 4, or to Figures 1 to 4 as modified by Figure 6, of the accompanying drawing.