GB2316784A - Acoustic alarm employing a resonant cavity arrangement - Google Patents

Abstract

An alarm comprises a casing containing a cavity 16 with at least one opening 18. Within said cavity 16 is disposed an acoustic transducer 7 which is connected to drive means which is activated by switch means 44, 46, 48. The drive means, when activated, induces said acoustic transducer 7 to operate at a resonant frequency of said cavity 16. Resonant chambers or tubes 11 may be linked to the cavity 16. The alarm may be adapted to be worn on the wrist in combination with a time piece 5. The transducer 7 may comprise a Helmholtz resonator, diaphragm 22 and magnet material 32 arrangement or a piezoelectric device (see figure 7). The drive means may comprise a transformer and high-frequency pulse modulation circuitry.

Description

DESCRIPTION PERSONAL ALARM The present invention relates to personal alarms adapted to be carried or worn by a user and to be activated to produce an audible signal in an emergency.

Personal audible alarm units are known in a variety of forms, ranging from simple whistles to compressed gas driven horns and electronic sirens.

Such devices are carried about the user's person, to be activated if the user requires assistance, for example if he/she is threatened or assaulted, the audible signal serving to discourage the assailant and/or to summon help.

To ensure that such alarms are accessible when needed, it is known to produce them in a form similar in size and shape to a wrist watch, complete with a strap, so that they can be worn on the wrist.

Such an alarm is described in UK patent 2267373.

The constraints on the size and shape of a wristworn alarm are severe. Into a small, thin package must be fitted a power source and, connected thereto, means for generating the alarm sound, as well as whatever switching apparatus is to be used to activate the alarm.

Furthermore, the alarm must be loud if it is to be effective, and it is desirable for the sound to last a considerable length of time before the power source is exhausted, so that considerable energy must be stored in the alarm unit and the power source tends to be bulky.

Thus, a major problem to be overcome in relation to wrist-worn alarm units is how to produce a sufficiently small unit of a shape adapted to be comfortably worn on the wrist, which is nevertheless capable of producing a loud and enduring audible signal.

More specifically, there is a need for a wrist worn alarm unit in which stored energy is efficiently used to produce a loud alarm sound despite the small size of the alarm.

A further problem arises in connection with the activation of the audible signal. This must be achieved by a simple action on the part of the user; it would be no good, when confronted by an attacker, to have to perform a fiddly or complex operation to set the alarm off. However, it is clearly undesirable for the alarm to be prone to accidental activation.

In accordance with the present invention, there is provided a personal alarm adapted to be worn on the wrist, comprising a casing within which is defined a resonant cavity having at least one opening, an acoustic transducer disposed within the resonant cavity, electrical circuitry for driving the acoustic transducer at or near a resonant frequency of the resonant cavity, and switching means actuable by the wearer to activate the electrical driving circuitry.

In the alarm according to the invention resonance effects augment the volume of the alarm and its energy efficiency. The resonant cavity together with the opening can act as a Helmholtz resonator.

Preferably, the resonant cavity is acoustically coupled to one or more resonant tube(s).

The tube(s) act(s) as (a) "tuned pipe(s)", adding to the resonance effects and further increasing volume. It is especially preferred that the resonant tube(s) is (are) formed in or by the casing. This allows a neat, attractive and compact construction.

The resonant tubes preferably have a resonant frequency at or near the drive frequency of the acoustic transducer.

The length of the tube(s) is preferably substantially half the wavelength of a sound emitted, in use, by the acoustical transducer.

In this way, it is ensured that sound waves travel down the tube(s), are reflected back toward the resonant cavity and arrive back in the cavity in phase, so that they constructively interfere.

In a preferred embodiment of the present invention, the resonant tube(s) is (or are) disposed around the resonant cavity.

In a particularly preferred embodiment of the present invention, the casing comprises a base portion coupled to a cover portion by a peripheral wall, one or a plurality of partition(s) being provided within the casing, extending between the base portion and the cover portion and thus defining the resonant cavity and the resonant tube(s) within the casing.

Preferably, the resonant tube(s) is (are) defined between a radially inner face of the peripheral wall and radially outer face(s) of the partition(s), the resonant cavity being defined by radially inner faces(s) of the partition(s).

It is preferred that the resonant cavity communicates with the exterior via at least one aperture for emission of sound.

Such an aperture or apertures contribute to the Helmholtz cavity resonance.

The switching means may comprise a button which acts on or comprises an inertial contact, such that the alarm is actuated only upon sufficiently rapid depression of the button. This can help to avoid accidental triggering of the alarm, without making the switching means fiddly and hence difficult to use in an emergency.

For generating sound, the acoustic transducer preferably comprises a diaphragm. Preferably, this has a resonant (or "natural") frequency at or near the audio frequency at which it is driven by the electrical circuitry.

This frequency is preferably in the region of 3.3kHz, this being the frequency of maximum sensitivity of the human ear.

The acoustic transducer may be electromagnetic.

In particular, it is preferred that the diaphragm is formed of magnetisable material and is magnetically and mechanically coupled to an electromagnet via a chassis, such that the diaphragm can be moved by a magnetic field produced by the electromagnet.

An electromagnetic transducer can have a low electrical impedance and be efficiently driven by the low voltage produced by conventional batteries.

Preferably, the chassis is substantially cup shaped, the diaphragm being coupled to the peripheral edge of the chassis.

It is particularly preferred that the electromagnet comprises a permanently magnetised core.

The magnetisation of the core may be such that during operation of the alarm the net magnetic field produced by the electromagnet does not reverse in direction, despite reversal of the current in the electromagnet.

This ensures that the diaphragm oscillates with a frequency equal to the audio driving frequency.

Alternatively, the transducer may be piezoelectric.

This raises problems of impedance matching. The high impedance of a piezoelectric crystal means that a high driving voltage is required, which conventional electric cells/batteries cannot supply directly.

Further, a conventional oscillator and transformer to step up the voltage could (in view particularly of the bulk of the transformer) be too large to accommodate in a wrist worn alarm.

Preferably, the electric circuitry for driving the acoustic transducer comprises a high frequency oscillator for generating alternating current at a supersonic frequency, the output of the oscillator being connected to a step up transformer (or "flyback" inductor) to produce a driving voltage for the piezoelectric transducer.

By operating the transformer at a supersonic frequency, its core and windings can be greatly reduced in size.

The frequency of the oscillator is preferably greater than, 50 kHz.

The supersonic input drive signal to the step up transformer can be modulated at an audio frequency to provide the drive signal to the piezoelectric transducer. Alternatively, the output driving voltage can be switched to provide the necessary audio modulated signal for driving the transducer.

The electrical power source used in the personal alarm can be a rechargeable battery. A nickel cadmium battery is preferable for the high power it can produce, but suffers from the problem that it discharges over a period of days. To recharge the battery, the personal alarm may comprise an inductive loop connected or connectable thereto, or other charging means.

The personal alarm may further comprise a time piece. Preferably, the time piece is movably mounted in or on the casing, and is movable by the wearer to activate the electrical driving circuitry.

The resonant structure and other features of the personal alarm described herein can also be advantageously utilised in other types of alarm.

In accordance with a second aspect of the present invention, there is provided an alarm comprising a casing within which is defined a resonant cavity having at least one opening, one or more resonant tubes acoustically coupled to the resonant cavity, an acoustic transducer disposed within the resonant cavity, electrical circuitry for driving the acoustic transducer at or near a resonant frequency of the resonant cavity, and switching means for activating the electrical driving circuitry.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: Fig. 1 illustrates a first specific embodiment of the present invention in cross section; Fig. 2 is a partly sectional view of the Fig. 1 embodiment seen from above; Fig. 3 is an exploded view of the embodiment shown in Figs. 1 and 2; Figs. 4, 5 and 6 show circuits forming part of an embodiment of the present invention for driving a piezoelectric transducer; and Fig. 7 shows a piezo-electric diaphragm for use in an alarm according to the invention.

As Figs. 1, 2 and 3 show, a first embodiment of the present invention comprises a two part casing shaped like a conventional wrist watch and comprising a lower body part 2 to which is attached an upper lid part 4. Contained by the casing are a movably mounted chronometer 5 and an acoustical transducer 7.

The form of the body part 2 is most clearly seen in Fig. 3; it comprises wings 13 for attachment of a strap, and a circular base 6 integrally formed, at its periphery, with an upstanding annular outer wall 8, thus defining a cylindrical internal cavity to contain working components of the alarm.

This internal cavity is divided by three sectoral walls 10, which lie radially inward of the annular outer wall 8 and which themselves project upward from and are integrally formed with the base 6. Three sectoral cavities 11 are thus defined between the radially outer surfaces of respective sectoral walls 10 and the radially inner surface of the annular outer wall 8.

A respective first end of each sectoral cavity 11 (the end lying furthest in a clockwise direction in Fig. 3) is closed by a respective battery receiving portion 12 of the body part 2, which projects upward from the base 6 and joins the sectoral and outer walls.

A respective second end of each sectoral cavity 11 (the end lying furthest in an anti-clockwise direction in Fig. 3) is open by virtue of a rectangular aperture 14 between the end of the respective sectoral wall 10 and the adjacent battery receiving portion 12.

The rectangular apertures 14 connect the sectoral cavities 11 to an inner cylindrical cavity 16 defined by the radially inner faces of the sectoral walls 10.

The important acoustical properties of this structure are discussed below.

The lid part 4 upwardly closes the three sectoral cavities 11 and, along with the chronometer 5, substantially upwardly closes the inner cylindrical cavity 16, although it should be noted that the inner cylindrical cavity 16 communicates with the outside via three sectoral, through going slots 18 in the lid part 4. When the alarm is activated, these slots facilitate emission of sound, and also form, in combination with cavity 16, a Helmholtz resonator.

Each battery receiving portion 12 defines a cylindrical battery receiving cavity 17, which is open upwardly when the lid part 4 is removed to permit insertion of a respective battery 19. Electrical contacts 21,23 are provided above and below the battery, i.e. the said contacts are mounted at the base of the cylindrical cavity 17 and on a lower portion of the lid part 4.

The acoustical transducer 7 lies within the inner cylindrical cavity 16, being driven by an electronic oscillator circuit formed on a printed circuit board (pcb) 20 mounted on the circular base 6 and comprising a diaphragm 22, a chassis member 24, a magnetic core 32 and associated windings 34. The diaphragm 22 lies immediately above the pcb 20 and is formed as a thin, circular sheet of resilient material in which magnetisation can be induced. In the present embodiment, the diaphragm is steel.

Immediately above the diaphragm is the chassis member 24, formed in the shape of an inverted cup that is, the chassis member comprises a frustroconical wall 26 integrally formed, at its upper edge, with a planar, circular wall 28. The lower periphery of the frustro-conical wall is joined to or at least in contact with the periphery of the diaphragm 22.

The frustro-conical wall 26 is penetrated by a number of through going holes 30, making it effectively transparent to sound. The chassis member 24 is formed from a material in which magnetism can be induced steel according to the present embodiment.

A magnetic core 32, formed as a rod, projects downward from the centre of the circular wall 28, so that its lower end is separated by only a short gap from the diaphragm 22. The magnetic core 32 is permanently magnetised in a direction parallel to its longitudinal axis.

Surrounding the magnetic core 32 are the magnetising windings 34 connected to the oscillator circuit.

With regard to the operation of the transducer 7, it should firstly be noted that the magnetic core 32, chassis member 24 and diaphragm 22 together form a "magnetic circuit", the "poles" of which are at the lower end of the rod 32 and the diaphragm 22 opposite thereto. Interaction between the magnetic core 32 and the diaphragm 22 occurs because of induced magnetisation of the diaphragm 22, generating a force which attracts the diaphragm to the core. Completion of the "circuit" referred to above serves to maximise this force.

When alternating current is supplied to the windings 34 by the oscillator circuit, the windings 34 generate a magnetic field component which, in the region of the magnetic core 32, is alternately substantially parallel and anti-parallel to the field component due to the magnetisation of the core itself.

The time varying resultant magnetic field results in a time varying force on the diaphragm 22, causing motion of the diaphragm and so generating sound.

It would be possible, and within the scope of the present invention, to use a magnetic core 32 which was not permanently magnetised. In this case, since the diaphragm would be attracted to the core upon passage of an electrical current through the windings 34, regardless of the direction of the current, and since the current and magnetic field fall to zero twice per current cycle, the diaphragm would be driven at a frequency twice that of the alternating current applied to the windings.

However, according to the present embodiment the strength of the magnetisation of the magnetic core 32 and the maximum strength of the magnetic field generated by the windings are chosen such that the resultant magnetic field does not reverse in direction as the current varies, but only changes in magnitude ie. the resultant field falls to zero, if at all, only once per current cycle. The diaphragm is thus driven at a frequency equal to that of the alternating current, which is chosen to be equal to a resonant frequency of the diaphragm.

The amplitude of the resultant sound is further increased by virtue of the shape of the casing, and in particular the internal space it defines.

The inner cylindrical cavity 16, being open via apertures 14, can be thought of as a Helmholtz resonator, and the sound generated by the diaphragm is chosen to match a resonant frequency of the Helmholtz resonator.

Further, the lengths of the sectoral cavities 11 are chosen such that a sound wave (having the driven frequency of the diaphragm) passing down their length and being reflected to return to the inner cylindrical cavity 16 interferes constructively with oscillations in the inner cylindrical cavity 16, adding to the resonance effect. Specifically, according to the present embodiment, the length of each sectoral cavity is half the wavelength of the generated sound.

The stiffness and proportions of the diaphragm are such that its natural frequency coincides with the sound frequency.

The loud sound thus generated is emitted via the sectoral slots 18.

The sound is, according to the present exemplary embodiment, pitched at the peak of sensitivity of the human ear - in the region of 3.3 kHz - for maximum effect.

By virtue of resonance effects in the alarm unit, the energy provided by the batteries is efficiently used to produce a high volume sound.

The chronometer 5 and the switching mechanism by which the alarm is activated will now be described.

The chronometer 5 of the present embodiment is circular in plan, and in cross section (Fig. 1) is seen to comprise a housing 36 of small depth in which a timekeeping device is enclosed, and a domed glass 38 projecting upward therefrom. The housing 36 is slidably received within an annular portion 40 of the lid part 4, and lies immediately above the chassis member 24, being upwardly biased by small resilient spacers or springs 42 which contact both housing 36 and chassis member 34.

Downward pressure on the domed glass 38 moves the chronometer 5 downward, compressing the spacers/springs 42, and causing two contacts 44 and 48 (see Fig. 1) to meet, actuating the alarm. Electronic means, latching, may be used to ensure that the alarm persists after the contacts separate.

The contacts themselves may take a variety of forms, but a particularly advantageous embodiment thereof is schematically illustrated in Fig. 1. Here, the upper contact 44 is formed as a mass mounted on a resilient, flexible arm 46 and lies below the lower surface of the chronometer 5. A corresponding lower contact 48 lies a short distance below the upper contact 44.

When the chronometer 5 is depressed by the user, its lower face forces the upper contact 44 downward, bending the flexible arm 46. However, the vertical spacing of the contacts is chosen such that if the chronometre 5 is slowly depressed to its fullest extent, the upper contact 44 does not travel far enough to reach the lower contact 48.

To actuate the alarm requires rapid motion of the chronometer 5 (e.g. caused by a sharp "rap" on the glass 38), causing correspondingly rapid motion of the upper contact 44, whereupon the momentum gained by the upper contact causes it to travel on after motion of the chronometer 5 ceases and so to briefly meet the lower contact 48.

The upper contact 44 thus acts as an inertial contact.

In this way, accidental actuation of the alarm due to static pressures on the glass 38 is avoided, while permitting the alarm to be quickly and reliably actuated when necessary.

The transducer used to generate the alarm sound does not have to be electromagnetic. One alternative is to use a piezoelectric element to move the diaphragm. A suitable form of piezo-electric diaphragm is shown in Fig. 7, and comprises a circular metal diaphragm plate 70, each face of which bears a respective actuating layer 72 of piezo-electric material (in this case, a piezo-ceramic).

The actuating layers are each sandwiched between the diaphragm plate 70, acting as a first electrode, and a respective outer metal layer 74, serving as a second electrode. In this way, the electric field required to change the dimensions of the actuating layer 72 is applied.

The diaphragm may have an actuating layer 72 on only one of its faces.

Magnetic components including the chassis member 24 are omitted where a piezo electric diaphragm is used.

A problem arises, however in that piezoelectric transducers are high impedance devices requiring driving voltages very much higher than can be provided by conventional cells/batteries.

A transformer can of course be used to increase the voltage of an alternating current, but a transformer operated at the audio frequencies at which the transducer is driven would necessarily be bulky and hence difficult to accommodate in a wrist worn alarm.

The problem is overcome in a specific exemplary embodiment of the present invention by providing an oscillator circuit operating at a supersonic frequency whose output is fed to the step up transformer. By using very high frequencies (tens or hundreds of kHz), a very small transformer core can be used to handle adequate power and produce the required voltages.

The driver stage can be turned on and off to generate the audio modulation or a half or a full bridge can be used on the output after rectification, or two such miniature devices can be set to operate alternately. In this way very small components can generate the voltage needed for a piezo transducer to produce loud sounds.

A suitable circuit for driving the piezoelectric transducer is shown in Fig. 4. Switching elements A and B in the form of field effect transistors are shown, but these maybe replaced with bipolar transistors. The output is rectified but no reservoir capacitor is added - the transducer itself has a high capacitance and smooths out the very high generator switching frequency. The generator itself is modulated by turning it on and off. it could be modulated with a shaped waveform by pulse-width modulation in series with an inductor. Most simply, the circuit of Fig. 4 can be turned on for, say, 80s every 300us.

Fig. 4 shows two such circuits which are turned on and off alternately. The turning on and off could be smoothly modulated.

Fig. 5 shows a single circuit plus a bridge. The circuit shown in Fig. 6 is smaller but comprises a full bridge. In both cases the bridge operates at high voltage but could be easily miniaturised and constructed as a single chip circuit along with the rest of the electronic components.

To provide adequate power for producing a high volume alarm sound, the cells/batteries used in the alarm are preferably nickel cadmium. In order to conveniently keep these charged, the alarm may be placed on a charging stand when not in use, charging current being transferred to the alarm either by direct electrical contact or by means of an inductive loop in the alarm in which a current is induced by a corresponding loop in the charging stand.

Claims (28)

1. A personal alarm adapted to be worn on the wrist, comprising a casing within which is defined a resonant cavity having at least one opening, an acoustic transducer disposed within the resonant cavity, electrical circuitry for driving the acoustic transducer at or near a resonant frequency of the resonant cavity, and switching means actuable by the wearer to activate the electrical driving circuitry.

2. A personal alarm as claimed in claim 1, wherein the resonant cavity is acoustically coupled to one or more resonant tube(s).

3. A personal alarm as claimed in claim 2, wherein the resonant tube(s) is (are) formed within or by the casing.

4. A personal alarm as claimed in claim 2 or claim 3, wherein the resonant tube(s) is (are) formed such that it (they) has (have) a natural frequency at or near the frequency at which the acoustic transducer is driven by the electrical circuitry.

5. A personal alarm as claimed in claim 4, wherein the length of the resonant tube(s) is (are) substantially half the wavelength of the sound produced by the acoustic transducer.

6. A personal alarm as claimed in any of claims 2 to 5, wherein the resonant tube(s) is (or are) disposed around the resonant cavity.

7. A personal alarm as claimed in any of claims 2 to 6, wherein the casing comprises a base portion coupled to a cover portion by a peripheral wall, one or a plurality of partition(s) being provided within the casing, extending between the base portion and the cover portion and thus defining the resonant cavity and the resonant tube(s) within the casing.

8. A personal alarm as claimed in claim 7, wherein the resonant tube(s) is (are) defined between a radially inner face of the peripheral wall and radially outer face(s) of the partition(s), the resonant cavity being defined by radially inner face(s) of the partition(s).

9. A personal alarm as claimed in any preceding claim, wherein the resonant cavity communicates with the exterior via at least one aperture for emission of sound.

10. A personal alarm as claimed in any of claims 7 to 9, wherein the cover is adapted to be removable for access to the interior of the casing.

11. A personal alarm as claimed in any preceding claim, wherein the switching means comprises an actuating button which acts on or comprises an inertial contact, such that the alarm is actuated only upon sufficiently rapid depression of the button.

12. A personal alarm as claimed in any preceding claim, wherein the acoustic transducer comprises a diaphragm.

13. A personal alarm as claimed in claim 12, wherein the diaphragm is such that it has a resonant frequency at or near the audio frequency at which it is driven by the electrical circuitry.

14. A personal alarm as claimed in claim 12 or claim 13, wherein the diaphragm is formed of magnetisable material and is magnetically and mechanically coupled to an electromagnet via a chassis, such that the diaphragm can be moved by a magnetic field produced by the electromagnet.

15. A personal alarm as claimed in claim 14, wherein the chassis is substantially cup shaped, the diaphragm being coupled to the peripheral edge of the chassis.

16. A personal alarm as claimed in claim 14 or claim 15, wherein the electromagnet comprises a permanently magnetised core.

17. A personal alarm as claimed in claim 16, wherein the magnetisation of the core is such that during operation of the alarm, the net magnetic field produced by the electromagnet does not reverse in direction despite reversal of the current in the electromagnet.

18. A personal alarm as claimed in any of claims 1 to 13, wherein the acoustic transducer is piezoelectric.

19. A personal alarm as claimed in claim 18, wherein the acoustic transducer comprises an actuating layer of piezo-electric material sandwiched between a supporting, conductive diaphragm plate and an outer electrode layer, the electrode layer and the diaphragm plate being connected to the electrical drive circuitry for applying an electric field to the actuating layer.

20. A personal alarm as claimed in claim 18 or claim 19, wherein the electrical circuitry for driving the acoustic transducer comprises an oscillator for generating alternating current at a supersonic frequency, the output of the oscillator being connected to a step up transformer to produce a driving voltage for the piezoelectric transducer.

21. A personal alarm as claimed in claim 20, further comprising an audio oscillator for modulating the drive voltage to the piezoelectric transducer at an audio frequency.

22. A personal alarm as claimed in any preceding claim, comprising a rechargeable battery.

23. A personal alarm as claimed in claim 22, further comprising an inductive loop connected or connectable to the rechargeable battery for recharging thereof.

24. A personal alarm as claimed in any preceding claim, further comprising a time piece.

25. A personal alarm as claimed in claim 24, wherein the time piece is movably mounted in or on the casing and is movable by the wearer to activate the electrical driving circuitry.

26. An alarm comprising a casing within which is defined a resonant cavity having at least one opening, an acoustic transducer disposed within the resonant cavity, electrical circuitry for driving the acoustic transducer at or near a resonant frequency of the resonant cavity, and switching means for activating the electrical driving circuitry.

27. A personal alarm substantially as described herein with reference to, and as illustrated in, the accompanying figures.

28. An alarm substantially as described herein with reference to, and as illustrated in, the accompanying figures.