GB2322502A - Piezoelectric sounder - Google Patents

Abstract

A piezoelectric sounder assembly has a sounder 3 mounted on an O-ring 15 which is located in a shoulder 13 on a cover plate 16. The cover plate may be provided with flexing means (24, Fig 4) to enable installation of the device. The cover plate is retained in position by snap-fit protrusions (20, Fig 5) that engage slots 21.

Description

PIEZOELECTRIC SOUNDER This invention relates to piezoelectric sounders. In particular, it relates to the construction of piezoelectric sounders for emergency use, such as sounders used in fire alarms, intruder detection or similar systems.

Piezoelectric type sounders generally use a piezoelectric plate which is connected to an electronic circuit for oscillation and coupled into a resonant chamber for emission of sound. It is important that the piezoelectric disc be rigidly held and also that it be precisely located since the highest efficiency only occurs with one specific set of resonant conditions in association with a specific positioning of the plate.

Previously known piezoelectric sounder assemblies have involved placing a piezoelectric disc on a low ledge on a plate having an aperture therein and placing a cover over the disc to secure it in place. For such a system, too much pressure can be easily exerted by the cover or by the user when putting the cover in place, which can tend to damage the disc. Other systems involve resilient means such as O-rings placed underneath the disc but again a cover element is then generally directly applied on top of the disc with the consequent risk of damage to the disc and undue pressure being applied thereto. Furthermore, a larger air volume is then required under the piezoelectric disc, to accommodate the O-ring, which dramatically affects acoustic performance.

The cover plate is also generally difficult to apply with previously known assemblies.

The present invention arose in an attempt to provide an improved piezoelectric sounder assembly.

According to the present invention there is provided a piezoelectric sounder assembly, comprising a piezoelectric disc, a coupling means for receiving the piezoelectric disc and for coupling sound emissions from the disc to a horn, an O-ring mounted upon the disc at its periphery, and a cover means mounted upon the disc and O-ring and having a shoulder portion spaced radially inwards from its periphery such that the O-ring is held at the radial edge of the shoulder, between the cover means and the disc, when the cover plate is mounted thereon.

The coupling means may be integral with the horn (for example, formed and use as a single moulding).

In a second aspect the present invention provides a piezoelectric sounder assembly comprising a piezoelectric disc mounted directly upon a coupling means for coupling sound from the disc to a horn, and a resiliently compressible member mounted between the disc and a locating member for locating and securing the disc.

The resiliently compressible member may be an o- ring.

In a third aspect the present invention provides a piezoelectric sounder assembly comprising a piezoelectric disc, a coupling means upon which the disc sits, the coupling means having means for coupling sound emitted from the disc to a horn, and an elongate cylindrical wall, and a cover means arranged to be secured within the wall to locate and secure the disc in place, wherein the cover member comprises a plurality of outwardly extending protrusions arranged to locate in a corresponding plurality of slots in the wall of the coupling member and means for enabling flexing of the cover member to enable the protrusions to locate.

The flexing means may comprise a plurality of slots extending from the top edge of the cover member a distance towards its lower edge. Alternatively, or additionally, the flexing means may comprise a flexing groove in one or more surfaces of the cover member.

According to the present invention there is further provided a piezoelectric sounder assembly comprising one or more of the novel features herein described.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a cross sectional view through a piezoelectric sounder assembly; Figure 2 shows the horn and back box; Figure 3 shows the piezo assembly for fitting in the horn and back box; Figure 4 shows a top view of the cover plate of the piezo assembly; and Figure 5 shows a side view of the cover.

Referring to the figures, a piezoelectric sounder comprises a horn 1 which is shaped to provide a specific resonance structure and is mounted upon a back box 2. The back box is intended to be affixed to the ceiling or wall and to receive the electrical cabling enabling the sounder to operate.

Before the horn and back box are connected together (generally by a snap fit) a piezo assembly is mounted on the horn. The assembly itself is shown more clearly in Figure 3 and Figure 1 shows the completed assembly when mounted in situ with respect to the horn.

The piezo assembly comprises a piezoelectric disc or plate 3 which operates in known manner to vibrate when an electrical field is applied across it. The piezoelectric plate is mounted within a coupling piece 4 which is preferably a one piece moulded article of a suitable plastics material. The coupling piece includes a cylindrical lower extension 5 which sits in use upon the open edge of a frustoconical portion 6 of the horn. The cylindrical extension 5 is provided with four equispaced ribs 6 on its external periphery, which extend beyond the bottom face of the extension 5 and are each tapered between that point and their ends by a taper 7. Their tapered portion enable the coupling plates to be properly seated and located upon portion 6, as illustrated more clearly in Figure 1.

Cylindrical portion 5 extends from a floor portion 8 of the coupling member and this in turn sits upon a ledge portion of the horn. Additional protrusions in the form of pins or pillars 30 may be formed on the ledge 9 and cooperating slots or holes are formed in floor panel 8 to accommodate these location pins.

The coupling members comprises a cylindrical wall portion 10 and a flange 11, having a generally rectangular or square peripheral configuration, extending from the cylinder laterally. Four vertically extending pins 12 are equiangularly spaced around flange 11 and these may act to support an additional circuit board such as an addressable PCB, in the finished product. Between the flange and floor panel 8 the member is provided with a plurality of slots 21. In one embodiment six slots are provided, equispaced around the cylindrical wall and each slot is of length of about twelve millimetres and depth of about five millimetres. The slots are through-slots. The upper side of floor panel 8 is provided with an annular raised part 13 extending inwardly a short distance from cylindrical wall 10. At its inner edge, this forms a shoulder portion 14 of about 0.5 mm in height. The piezoelectric disc 3 is mounted upon shoulder 13,14 and is of a diameter to fit within the diameter of wall 10.

Since the disc sits directly upon shoulder 13, its central portion is spaced slightly above floor 8 to enable correct operation of the sounder. An O-ring 15 is then applied on top of the piezo assembly, at its periphery, and lies between ledge 13 and wall 10 as shown. A cover plate 16 is then mounted on top. The O-ring is an elastomeric, resiliently-compressible one, of rubber or a plastics material for example.

Cover plate 16 is again a one piece moulded plastics structure and comprises a flat circular floor panel 17. A cylindrical wall 18 extends above and below floor panel 17. At its bottom surface, an annular ridge 19 is provided, which is spaced inwardly from the outer edge of cylinder 18, as shown in Figure 3. The spacing is such that the O-ring can fit between the outer surface of ledge 19 and the cylindrical wall of coupling member 4.

Cover member 16 is also provided with a plurality of outwardly extending protrusions 20 which are equiangularly spaced around wall 18. A corresponding plurality of these is provided to the number of slots 21 formed in the coupling plate. In a preferred embodiment, the number is 6. Each outwardly extending protrusion 20 is of a length designed to cooperate with the length of each slot, typically about ten millimetres long and comprises a flat top surface 20a and an edge surface which slopes back towards the cylinder wall 20b.

The cover plate 16 is also provided with a venting means 21 comprising a upstanding cylinder having a through hole which opens at the top of sounder 21 and also at the bottom surface of floor panel 17. This prevents build up of pressure under floor panel 17. A further, shorter, protrusion 22 is provided on the opposing edge of floor panel 17.

To assemble the structure, the piezoelectric plate is mounted upon shoulder 13 and the O-ring is then mounted on top of the plate. Cover plate 16 is then pushed down, inside cylindrical wall 10 of coupling plate 4 until its protrusions 20 each enter a corresponding slot 21 to latch. This is shown in Figure 1. At this position, the o-ring 15 is held captive between ledge 19 and cylindrical wall 10 and exerts the required amount of downwardly compressive force upon the peripheral edge of the printed circuit board. The O-ring absorbs any excess pressure from above on the piezoelectric plate 3.

It is desirable that the cover plate 16 be allowed to flex slightly to enable its protrusions to enter the corresponding slots 21. A shown in Figure 5, the cover plate 16 is additionally provided with six cuts 22, each extending from the top face of cylindrical wall 18 to a depth approximately level with the protrusions 20.

In one embodiment, the slots may be about one millimetre wide and about six millimetres in length. The slots are conveniently mounted midway between adjacent protrusions 20. The slots enable some flexing of the cylindrical wall 18 to occur, the facilitate location and latching of the protrusions 20 into slots 21.

Additionally, a flexing grove 24 may be provided in the floor panel 17. In a preferred embodiment, a groove 23 is provided in the upper surface of the floor panel and is of width of about 1 millimetre and a depth of about half a millimetre.

The flexing groove may be provided as an alternative, or additionally to the flexing slots 23.

More than one flexing groove may be provided and its or their position may be varied.

Referring back to Figure 1, once the mechanical parts of the piezoelectric sounder assembly have been latched or secured in place, a printed circuit board 25 is mounted in place, within cylindrical portion 18 of cover plate 16 and this is mounted onto mounting parts associated with protrusions 21 and 22 such as to be spaced from the floor of the cover plate as shown in Figure 1.

The circuit board is shown schematically in Figure 4 by dotted lines 25 illustrating how it may be supported.

Figure 1 shows some additional components 26,27 located upon the circuit board 25 and the electrical assembly may then be encapsulated by a suitable encapsulating material 28, eg a resin. This aspect of the present invention thereby enables a piezoelectric assembly, comprising a piezoelectric plate, a coupling member, a cover plate and associated electronic circuitry to be assembled and encapsulated so that it can be easily transported and handled as a complete, rugged, unit, requiring only fitting into a horn assembly and connection to external cables to form an operative unit. The encapsulation also improves the rigidity of the structure and, by more rigidly fixing the piezo disc in place, may improve acoustic performance As shown in Figure 1, the base box 2 is of such depth that a considerable space is left between the top of the piezo assembly and the back of the box 2. This space is sufficient for additional elements, such as an additional circuit board, for example an addressable circuit board, to be mounted and such a board may conveniently be mounted upon pillars 12. Thus, it is a simple matter to add an addressable circuit board to the sounder so that the sounder can be used in an addressable alarm system. Other components may also be mounted in this space as required. This is enabled as a result of the compact arrangement and the encapsulation of the circuit board and components within the piezo assembly.

Previously proposed piezoelectric sounder assemblies have used separate circuit boards and connections to the piezo disc which have taken up more room, and thereby left less room for other components such as addressable PCBs.

The coupling piece 4 may, in alternative embodiments, be formed and used integrally with the horn, eg formed as a single moulding.

Claims (11)

1. A piezoelectric sounder assembly, comprising a piezoelectric element, a coupling means for receiving the piezoelectric element and for coupling sound emissions from the element to a horn, an O-ring mounted upon the element at its periphery, and a cover means mounted upon the element and O-ring and having a shoulder portion spaced radially inwards from its periphery such that the O-ring is held at the radial edge of the shoulder, between the cover means and the element, when the cover plate is mounted thereon.

2. A sounder assembly as claimed in Claim 1, wherein the coupling means is integral with the horn.

3. A piezoelectric sounder assembly comprising a piezoelectric element mounted directly upon a coupling means for coupling sound from the element to a horn, and a resiliently compressible member mounted between the element and a locating member for locating and securing the element.

4. A sounder assembly as claimed in Claim 3, wherein the resiliently compressible member is an O-ring.

5. A piezoelectric sounder as claimed in any of the preceding claims, wherein the cover means comprises a plurality of outwardly extending protrusions arranged to locate in a corresponding plurality of slots in the wall of the coupling member and means for enabling flexing of the cover means to enable the protrusions to locate.

6. A piezoelectric sounder assembly comprising a piezoelectric element, a coupling means upon which the element is mounted, the coupling means having means for coupling sound emitted from the element to a horn, and an elongate cylindrical wall, and a cover means arranged to be secured within the wall to locate and secure the element in place, wherein the cover means comprises a plurality of outwardly extending protrusions arranged to locate in a corresponding plurality of slots in the wall of the coupling member and means for enabling flexing of the cover means to enable the protrusions to locate.

7. A sounder assembly as claimed in Claim 6, wherein the flexing means comprises a plurality of slots extending from the top edge of the cover means a distance towards its lower edge.

8. A sounder assembly as claimed in Claim 6 or Claim 7, wherein the flexing means includes at least one flexing groove in one or more surfaces of the cover means.

9. A sounder assembly as claimed in Claim 8, wherein the flexing groove is provided in the upper surface of a floor panel of the cover means.

10. A sounder assembly as claimed in Claim 8 or Claim 9, wherein the flexing groove is of width about lmm and depth of about mm.

11. A sounder assembly substantially as hereinbefore described, with reference to, and as illustrated by, any of the accompanying drawings.