GB2145905A - Audible warning device - Google Patents

Abstract

An audible warning device includes a piezo-electric element (36) attached to one surface of a base (31) having a central aperture. A horn (42) with opposite open ends is fixedly secured at one end thereof to the other surface of the base and disposed in communication with the central aperture. A drive circuit (53) is operable to drive the piezo-electric element to produce a sound wave. <IMAGE>

Description

SPECIFICATION Electronic siren This invention relates to an electronic siren for use in alarm devices such as a burglar alarm.

One conventional electronic siren 10 shown in FIG. 1 comprises a molded structure of a synthetic resin composed of a housing 11 of a rectangular crosssection and a horn 12 formed integrally with and extending from the housing 11 The interior of the housing 11 is divided into two chambers 13 and 14 by a partition wall 15. Asound generating unit 17 is accommodated within the chamber 13 while a drive circuit 18 for driving the sound generating unit 17 is accommodated within the chamber 14.

The sound generating unit 17 comprises a permanent magnet 20 of a circular cross-section having an annular groove 20a. A central portion 20b and a peripheral portion 20e separated by the annular groove20a have opposite polarities. Acircular diaphragm21 of a corrugated shape having a central aperture 21 a is fixedly secured at its periphery to the innerwall ofthe housing 11. An oscillatory disk 22 is fixedly secured at its periphery to the edge ofthe aperture 21a ofthe diaphragm 21 Atubular bobbin 24 is fixedly secured at its one end to the peripheral edge ofthe oscillatory disk 22 and is loosely received in the annular groove 20a of the magnet 20. Avoice coil 25 is wound around the bobbin 24.Thevoice coil 25 is electrically connected to the drive circuit 18which is connected toan AC powersource.

In operation, the voice coil 25 is energized to vibrate the oscillatory plate 22 to produce a sound wave which is transmitted tothe exterior of the siren through the horn 12.

With this construction, however, the sound generating unit 17can not be satisfactorily reduced in size in a direction of oscillation ofthe oscillatory plate 22, so thatthe size of the housing 11 is correspondingly increased As a result, the overall sizeoftheconven- tional siren is increased. In addition, the conventional siren is rather heavy in weight because ofthe use of the permanent magnet 20 and is somewhat compli cated in construction and hence is ratherexpensiveto manufacture.

Generally, the conventional electronic siren of the type described produces a sound wave having a constantfrequencywhich sound wave is quite rasping. Therefore, it is desired thatsuch a sound wave should have a predetermined band width by means of modulation or the like to cause itto be less rasping.

It is therefore an object of this invention to provide an electronic siren which is lightweight, compact and inexpensive to manufacture.

Another object is to provide such an electronic siren which is capable of efficiently producing a sound wave of a relatively broad band width.

According to the present invention, there is provided an electronic siren which comprises a base having a central aperture formed therethrough; a piezo-electric element of a disc-shape attached to one surface of the base to coverthe aperture; a horn with opposite open ends fixedly secured at one end thereof to the other surface of the base and disposed in communication with the aperture; and drive means for driving the piezo-electric element to produce a sound wave.

FIG. is a cross-sectional view of an electronic siren provided in accordance with the prior art; FIG. 2 is a cross-sectional view of an electronic siren provided in accordance with the present invention; FIG. 3 is a rear end view of the electronic siren of FIG.

2; FIG. 4 is a cross-sectional view of a base used in the electronic siren of FIG. 2; and FIG. 5 is a block diagram of a drive circuit ofthe electronic siren of FIG. 2.

An electronic siren 30 shown in FIGS. 2 and 3 comprises a circular base 31 having a central tubular boss 32 extending perpendicularlyfrom a front surface 31a ofthe base 31. As best shown in FIG. 4, the base 31 has a recess 33 of a semi-spherical or rounded shape formed at its central portion and extending into the tubular boss 32. The base 31 has first and second peripheral flanges 34 and 35 formed on the front and rear surfaces 31 a and 31 b, respectively.

The electronic siren 30 is provided with a sound generating means 36 in the form of a piezo-electric element The piezo-electric element 36 comprises an oscillatory plate or disc 37 of brass, a ceramic disc 38 of a smaller diameter secured to one surface of the oscillatory plate 37, and an electrode film 39 of a circular shape bonded to the ceramic disc 38. A circular counterbore 40 is formed in the rear surface 31 b ofthe base 31 and disposed immediately adjacent to the recess 33. The oscillatory plate 37 is received in the counterbore 40 and fixed to the base 31, the rear surface ofthe oscillatory plate 37 lying flush with the rear surface 31 b of the base 31. The oscillatory plate 37, the ceramic disc38 and the electrode film 39 are disposed in coaxial relation to the recess 33 and the boss 32.

For example, a piezo-electricelement manufactured and sold by Nihon Tokushu Tougyo Limited (Japan) underthetradename of PZ-R45 can be used as the piezo-electric element 36. Also, as the piezo-electric element 36, there can be used a piezo-electric element which comprises an oscillatory plate and a piezoelectric film bonded to the oscillatory plate and made offlexible polymer composite material.

The oscillatory plate 37 and the electrode film 39 serve as electrodes of the piezo-electric element 36.

Two electric leads (not shown) are electrically connected at one ends thereof to the oscillatory plate 37 and the electrodefilm 39, respectively, while the other ends ofthe electric leads are electrically connected to a drive circuit as later described.

As bestshown in FIG. 4,the inner surface of the tubular boss 32 is stepped intermediate opposite ends thereofto provide a shoulder portion 41. An inner horn 42 is snuglyfitted in and fixed to the tubular boss 32 at one end thereof, with the edge ofthe one end held againstthe shoulder41. The inner horn 42 has the opposite open ends and flares toward itsfree or outer end. Three support lugs 43 are formed on and extend perpendicularlyfrom the front surface 31 a of the base 31 in equally circumferentially spaced relation, the support lugs 43 being disposed in surrounding relation to the tubular boss 32.Each of the support lugs 43 has a depression 43a formed in its outer surface adjacent to its distal end and has a screw hole 43bformedtherethrough adjacentto its distal end.

A cup-shaped intermediate horn 45 is attached to thethree support lugs 43 with its open end 45a engaged with the depressions 43a so as to enclose the inner horn 42. The intermediate horn 45 is fixed to the suppor: lugs43 by screws (not shown) passing through the open end 45a into the respective screw holes 43b ofthe support lugs 43. The intermediate horn 45 is disposed coaxiallywith and spaced circumferentiallyfrom the inner horn 42, and also the distal end 42a ofthe inner horn 42 is spaced from the distal end 45b ofthe intermediate horn 45 to enable thetransmission ofthesoundwavethroughthe intermediate horn 45.

An outer horn 47 has opposite open ends and has a radial flange 47a formed at one end thereof. The outer horn 47 is snugly fitted on and fixed to the base 31 with the radial flange 47a engaged with the rear surface 31 b of the base 31, the outer horn 47 flaring towards its free or outer end.

A protective disc 50 is fixedly secured to the distal end of the second flange 35 of the base 31. A cup-shaped casing 51 is snuglyfitted on the second flange 35 ofthe base 31 at an open end thereof and fixed thereto,the open end of the casing 51 being disposed in engagement with the radial flange 47a of the outer horn 47. The casing 51 accommodates the drive circuit 53 for driving the sound generating means 36 comprising the piezo-electric element.

Each ofthe inner, intermediate and outer horns 42, 45 and 47 is made of a synthetic resin and comprises an exponential horn whose cross-sectional area increases exponentially with the axial distance.

As shown in FIGS. 2 and 3, a support member 60 is attached to the casing 51 forfacilitatingthe installation ofthe electronic siren 30. The support member 60 is made of a metal plate having a pairofstamped-out arms 60a and 60a. The casing 51 has an integral projection 51a which is interposed between and fixed to the arms 60a and 60a by a bolt61 passing through them.

As shown in FIG. 5,thedrivecircuit53 comprisesan oscillator54forgenerating a signal having, for example, a frequency of 2.9KHz, an amplifier 55 for amplifying the signal fed from the oscillator 54, and a transformer 56for matching the impedance of the amplifier 55 with the impedance ofthe piezo-electric element 36. A secondary coil 56a of the transformer 56 is electrically connected to the oscillatory plate 37 and electrode film 39 of the piezo-electric element 36 through terminals 59a and 59b.Thetransformer56 is enclosed by a metal shield casing 57 so as to prevent the transformer 56 from causing electromagnetic interference, the shield casing 57 being grounded.

As shown in broken lines in FIG. 5, a modulation signal may be applied to the oscillator 54to vary the frequency of the output signal ofthe oscillator 54 within a predetermined range.

When an alternating current or a pulsating current is applied from the drive circuit 53 to the piezo-electric element 36, the piezo-electric element 36 oscillates or vibrates ata frequency equal to the frequency of the alternating current or the pulsating current two produce a sound wave. This sound wave is transmitted through the inner, intermediate and outer horns 42,45 and 47 in an efficient manner.

A corner portion 60 into which the surface ofthe recess 33 and the inner surface ofthetubular boss 32 merge has a radius of curvature of 3.5 mm. With this radius of curvature, the electronic siren 30 can efficiently output the sound wave, generated by the piezo-electric element 36, with less loss over a wide range of frequencies.

An experiment was conducted to determine the efficiency ofthe output sound of the electronic siren 30. The parameters ofthe electronic siren 30 were as follows: Radius of curvature of corner portion 60: 3.5 mm Diameter of recess 33 at its edge remotefromtubularboss32: 28mm Radius of curvature of surface of recess 33: 22 mm The signal with a frequency of 2.9 KHz generated by the oscillator 54 was so frequency-modulated to have a sufficient band width, and the drive circuit drove the piezo-electric element 36. As a result, the electronic siren 30 produced an output sound with less loss at a frequency range of 800 Hzto 4 KHz.

A comparative experiment was conducted according to the above procedure exceptthatthe radius of curvature ofthe corner portion 60 was 2 mm. In this case, the electronic siren produced an output sound with less loss at a limited frequency range of 1 to 3 KHz.

While the electronic siren according to this invention has been specifically shown and described herein, the invention itself is notto be restricted bythe exact showing ofthe drawings orthe description thereof. For example, the inner horn 42 may be formed integrally with the base 31 to provide a one-piece construction.

Claims (6)

1. An electronic siren comprising: (a) a base having a central aperture formed therethrough; (b) a generally planar piezo-electric element secured relative to one side of said base to cover said aperture; (c) a first horn with opposite open ends fixedly secured at one end thereof to the opposing side of said base and disposed in communication with said aperture; and (d) drive meansfordriving said piezo-electric elementto produce a sound wave.

2. An electronicsiren according to claim 1,further comprising a second horn of a generally cup-shape fixedly secured to said base at its open end to enclose said first horn and disposed in spaced relation to said first horn, and a third horn having opposite open ends and fixedly secured to said base at one end thereofto surround said second horn.

3. An electronic siren according to claim 1 or2, in which said piezo-electric element comprises an oscillatory disc secured to said base, a ceramic disco a reduced diametersecuredtoone surface of said oscillatory disofacing away from said aperture, and an electrode film secured to said ceramic disc.

4. An electronic siren according to claim 1,2 or3 in which said aperture is enlarged in diameter atthe one surface of said base to provide a semi-spherical recess.

5. An electronic siren according to claim 4, in which a corner portion of said aperture at which an inner end of said semi-spherical recess is disposed has a radius of curvature of 3.5 mm.

6. An electronic siren substantially as hereinbefore described with reference to Figs. 2 to 5 ofthe accompanying drawings.