Electric Sound-producing Device

Abstract

An electric sound producing device includes a ferromagnetic diaphragm, an electromagnet for deforming the diaphragm and an oscillator for repeatedly supplying short duration current pulses to the electromagnets. The oscillator may be driven by a multivibrator to produce intermittent sound pulses or by a saw-tooth pulse generator to produce sound with an apparently cyclically varying frequency.

Description

SUMMARY OF THE INVENTION

This invention relates to an electric sound producing device and has as an object to provide such a device in a convenient form.

An electric sound-producing device in accordance with the invention comprises a resilient diaphragm of ferromagnetic material, an electromagnet whereby said diaphragm is deformed when current is supplied to said electromagnet, and an oscillator for repeatedly supplying current to said electromagnet in short duration pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a section through an example of a device in accordance with the invention;

FIG. 2 is a fragmentary section on line 2--2 in FIG. 1,

FIG. 3 is a view like FIG. 1 showing another example of the invention, and

FIGS. 4, 5 and 6 are circuit diagrams of three electrical circuits for use with either of the embodiments shown in FIGS. 1 to 3, and

FIG. 7 is a view like FIG. 1 showing yet another example of the invention, and

FIG. 8 is a circuit diagram which is specifically intended for use with the example of FIG. 7 but which could also be used with the examples of FIG. 1 and 2 or FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The device shown in FIGS. 1 and 2 includes a body formed of a synthetic resin material in two parts, namely a base 10 and a cap 11. The base 10 and the cap 11 are fitted together with mating cylindrical surfaces so that the cap can be turned relative to the base 10. A plurality of abutments 12 on the base 10 coact with a series of inclined ramp faces 13 in the cap to determine the relative axial positions of the cap and base. An adhesive 14 is used to secure the two parts together after adjustment as will be explained hereinafter.

A diaphragm member 15 is carried by the cap 11. This diaphragm member is formed from thin steel sheet and has a flat portion 16, a rim 17, a cylindrical marginal portion 18 having an external rib 19 thereon and a lip 20 around its free edge. The rib 19 is a snap fit in an internal groove in the cap. The flat portion 16 of the diaphragm is free to vibrate in a direction perpendicular to its plane and by making its diameter 1 inch and its thickness 0.01 inch, a natural frequency of approximately 3,000 Hz can be obtained. This is a frequency to which the normal human ear is particularly sensitive.

The base 10 has an integral spigot 21 therein extending perpendicularly to the plane of the flat portion 16 of the diaphragm member 15. Mounted on this spigot 21 is a mild steel electromagnet core 22 of longitudinally split tubular form. A coil 23 is mounted on this core 22. A shallow cup-shaped steel member 24 is fitted on the base part 10 with the core 22 in contact with it around a hole in the member 24 surrounding the spigot 21. The edge of the member 24 receives the lip 20 of the diaphragm member 15, so that the diaphragm member 15, the core 22 and the member 24 form a magnetic circuit with an air-gap between the core 22 and the centre of the flat portion 16 of the diaphragm member 15.

The drive circuit for the coil 23 has its components supported on an annular insulation member 25 itself carried by wires 26 extending through holes in the member 24 and the base 10 and connected to power input terminals 27. One of the wires 26 is insulated to prevent the terminals 27 being short-circuited by the member 24.

As shown in FIG. 4, which illustrates a very simple circuit for the device, the coil 23 has two windings 28 and 29. The winding 28 has one end connected to a positive input terminal 27 and its other end is connected to the collector of an n-p-n transistor 30. This transistor 30 has its emitter connected to the other terminal 27. A resistor R.sub.1 is connected between the collector of the transistor 30 and the anode of a diode 31 which has its cathode connected to the positive terminal 27.

The other winding 29 is connected at one end to the base of the transistor 30 and at the other end to one electrode of a capacitor C and one end of a resistor R.sub.2. The other electrode of the capacitor C is connected to the negative terminal 27 and the other end of the resistor R.sub.1 is connected to the positive terminal 27.

The circuit described forms an oscillator which operates to produce a train of pulses in the coil 23. When the circuit is first connected to the battery the capacitor C charges up through the resistor R.sub.2 until current starts to flow to the base of the transistor 30. The resulting collector current through the winding 28 induces a voltage in the winding 29 which drives the transistor 30 into a fully conducting condition. The current in winding 28 increases until the electro-magnet is saturated when the induced voltage falls and the transistor becomes nonconductive. The current in winding 28 then flows through the resistor R.sub.1 and the diode 31 and decays. Meanwhile, the capacitor C has been partially discharged and this then recharges to initiate another pulse. The pulse repetition frequency can be varied by varying the values of the resistor R.sub.2 and the capacitor C, and this frequency is chosen so as to be substantially less than the natural frequency of the diaphragm. For efficient operation the resistor R.sub.1 is chosen so that the pulse length is approximately equal to half the natural period of the diaphragm.

The device thus produces a train of pulses of high frequency sound. The quality of the sound can be varied by a suitable choice of the natural frequency of the diaghragm, and the pulse repetition rate.

The device can be inexpensively produced and can produce a relatively loud noise for a small power consumption. It has the advantage over conventional electromechanical devices incorporating contact breakers that there are no wearing parts and dirt and corrosion will not significantly affect the operation of the device.

In the circuit shown in FIG. 5 there are three additions to the circuit of FIG. 4. Firstly a resistor R.sub.3 is connected in series with the capacitor C and the winding 29. In the circuit of FIG. 4 the pulse length may tend to vary somewhat with variations of temperature owing to possible variations in the incidental resistance of the base-emitter circuit of the transistor 30. The inclusion of the resistor R.sub.3 stabilizes this resistance and hence the pulse length. Secondly the positions of the resistor R.sub.1, and the diode 31 are interchanged and a capacitor C.sub.2 is connected between the cathode of the diode 31 and the negative terminal 27. This capacitor C.sub.2 reduces the level of electrical interference injected back into the supply by operation of the device and also gives a measure of protection to the transistor 30 from high peak transient voltages in the supply which may result from the operation of other equipment, from the same supply.

Finally a diode 32 is introduced between the positive terminal 27 and the resistor R.sub.1 to protect the transistor from damage caused by reverse voltages resulting from inadvertently connecting the terminals 27 to the incorrect terminals of the battery or other supply.

As a further possibility the device may be arranged to operate intermittently either by connecting the terminals 27 to the supply via a thermal-type flasher unit or as shown in FIG. 6. In this case the circuit 33 which corresponds to the circuit of FIG. 5 with two exceptions operates as before to supply pulses to the winding 28. The capacitor C.sub.2 is, however, shown in an alternative position between the cathode of the diode 32 and the negative terminal 27. In this position it still provides suppression of electrical interference, but safeguards the transistor 30 against high peak transient voltages to a lesser extent. The other modification is the disconnection of the winding 29 from the cathode of the diode 32 and its connection instead to the output terminal 35 of a multivibrator 34. This multivibrator is of conventional form including two n-p-n transistors 36, 37 with their collectors connected via resistors 38, 39 respectively to the cathode of diode 32 and their emitters connected to the negative terminal 27. The bases of the transistors are connected via resistors 40, 41 respectively to the cathode of the diode 32. A capacitor 42 connects the base of the transistor 37 to the collector of the transistor 36. A capacitor 43 is connected between the base of the transistor 36 and the anode of a diode 44 having its cathode connected to the collector of the transistor 37. A resistor 45 is connected between the anode of the diode 44 and the cathode of the diode 32. The output terminal 35 is connected to the collector of the transistor 37.

It will be appreciated that the multivibrator produces an intermittent output. The circuit 33 will operate only when an output is received from circuit 34, i.e. during these periods when the transistor 37 is non-conductive.

In the alternative embodiment of the invention shown in FIG. 3, the cap 11 is replaced by a resonator 46 which fits on to the base 10 like the cap 11 and has an internal groove to receive the rib 19 on the diaphragm member. The resonator 46 has tubular portion 47 connected at one end to an annular wall portion 48 adjacent the diaphragm member. The other end of the portion 47 has an end wall 49 which is perforated so that the tubular portion 47 is effectively and acoustically open at both ends. The length of the portion 47 is chosen so that its fundamental frequency is approximately equal to the natural frequency of the diaphragm. Thus when the diaphragm is caused to vibrate a resonance condition will be established and the quality and volume of the sound produced will be affected.

In the example shown the internal diameter of the tubular portion 47 is less than the diameter of the flat portion 16 of the diaphragm member. This too has an effect on the quality of sound produced by the device. In the case where the tubular portion 47 has a diameter equal to or greater than the diameter of the diaphragm the acoustic loading on the diaphragm is low and the diaphragm/resonator combination would tend to "ring" at a relatively low volume after initiation of vibration of the diaphragm. With a reduced diameter resonator tubular portion 47 the acoustic loading is increased so that a shorter and more intense wave train is produced.

Any of the three circuits of FIGS. 4 to 6 may be used in conjunction with the device of FIG. 3.

Mention is made above of axial adjustment of the position of the cap 11 (or resonator 46) relative to the base 10 with a view to varying the air gap between the end of the core 22 and the flat portion 16 of the diaphragm member 15. Such adjustment is effected during assembly of the device by fitting the cap 11 and base 10 together with the adhesive 14 between them and adjusting the cap 11 before the adhesive sets. The terminals 27 are connected to a supply and the cap 11 is turned to reduce the airgap until the diaphragm is heard to strike the core, the cap is then turned in the opposite direction until such striking ceases. The adhesive is then left to set permanently fixing the cap in its adjusted position and also sealing the body against the ingress of dirt and liquids. The holes in the base 10 through which the wires 26 are passed would likewise be sealed.

Alternatively sealing of the body could be effected by ultrasonic welding after adjustment of the gap.

Adjustment of the airgap to the minimum is of some importance in obtaining maximum efficiency from the device. As mentioned above the pulse length is arranged to be approximately equal to one half of the natural vibration of the diaphragm. Thus if the diaphragm commences at rest in its normal position it will be moved towards a new equilibrium position appropriate to the magnetic field set up by the current in the coil. On reaching this position the diaphragm will continue to move under its own momentum with the magnetic attraction progressively increasing.

Finally the diaphragm reaches a rest position and, ideally it is at this point that the pulse to the electro-magnet terminates. Clearly, therefore, the total displacement of the diaphragm will increase with reduction of the air gap for the same power consumption.

In the example shown in FIG. 7 the cap 11 or resonator 46 is replaced by an exponential horn 50. Once again the narrow end of this horn is of smaller diameter than that of the diaphragm to place a high acoustic load on the diaphragm. A high intensity sound can thus be obtained.

The circuit of FIG. 8 utilises an oscillator 33 as before, with a saw-tooth pulse generator circuit 59 including a capacitor 51 connecting the resistor R.sub.2 to the negative terminal 28 and a further resistor 52 connecting it to the positive terminal 27. Also connected to the resistor R.sub.2 is the emitter of a p-n-p transistor 53 the collector of which is connected to the negative terminal via a resistor 54. The base of the transistor 53 is connected via a resistor 55 to the negative terminal and via a resistor 56 to the base of another -p-n-p transistor 57, the collector of which is connected to the base of the transistor 53. A capacitor 58 connects the base of the transistor 57 to the collector of the transistor 53. The emitter of the transistor 57 is connected to the positive terminal.

The cycle of operation of the circuit may be regarded as commencing with the capacitor 51 uncharged and the transistor 53 non-conductive. When the supply is connected the capacitor 51 charges up so that initially the voltage on the resistor R.sub.2 is at the negative terminal voltage but this rises as the capacitor 51 charges, progressively increasing the pulse repetition frequency of the oscillator. When the voltage across the capacitor 51 exceeds the bias applied to the base of the transistor 53, the latter starts to conduct and the changing voltage at its collector, caused by current flow through the resistor 54, is applied to the base of the transistor 57 via the capacitor 58. This in turn causes the bias on the base of the transistor 53 to be reduced and the regenerative action causes transistor 53 to switch fully on and discharge the capacitor 51 extremely rapidly. The cycle then restarts.

Thus the device emits pulses of sound at the resonant frequency of the diaphragm, which is preferably as mentioned above, in the region of 3,000 Hz. The pulse repetition frequency varies cyclically from a low frequency rising to a high frequency and then commencing at the low frequency again. The total effect is a sound of which the apparent frequency follows this pattern, although in fact all the sound emitted is at or near the aforementioned resonant frequency.

If desired, an additional transistor may be introduced between the resistor R.sub.2 and the capacitor 51 to allow the magnitude of the current handled by the "time-base" circuit to be reduced. Alternatively the additional transistor may be connected to the base of the transistor 30. In each case the additional transistor would be connected as an emitter follower.

The horn of FIG. 7 may also be used in conjunction with any of the circuits shown in FIGS. 4, 5 and 6.

Claims

I claim:

1. An electrical sound-producing device comprising a resilient diaphragm of ferromagnetic material having a substantially flat portion, an electromagnet including a tubular core having one end adjacent said flat portion of the diaphragm and spaced therefrom by an air gap, the other end thereof being mounted on a spigot, whereby said diaphragm is deformed when current is supplied to said electromagnet, and an oscillator for repeatedly supplying current in short duration pulses to said electromagnet.

2. A device as claimed in claim 1 in which the oscillator produces pulses of duration approximately equal to half the period of the natural vibration of the diaphragm.

3. An electrical sound producing device comprising a resilient diaphragm of ferromagnetic material, an electromagnet whereby said diaphragm is deformed when current is supplied to said electromagnet, said electromagnet including a core having one end adjacent said diaphragm, a coil comprising a pair of windings on said core and an oscillator of which said core and windings form an essential part for repeatedly supplying current to the windings in short duration pulses.

4. A device as claimed in claim 3 in which said oscillator comprises first and second terminals, an n-p-n transistor with its collector connected via one winding of said coil to the first terminal and its emitter connected to the second terminal, a first resistor and a diode in series circuit bridging said first winding, a capacitor connected in a circuit with the second winding of the coil connecting the second terminal to the base of the transistor and a charging circuit for said capacitor including a second resistor.

5. A device as claimed in claim 4 in which said charging circuit consists of said second resistor connected between the end of the second winding which is connected to the capacitor and the first terminal.

6. A device as claimed in claim 4 in which said charging circuit comprises a further resistor connecting the second resistor to the first terminal, a further transistor with its collector/emitter path connecting the interconnection of the second and further resistors to said second terminal and means for applying a signal to the base of said further transistor.

7. A device as claimed in claim 6 in which said further resistor and said further transistor form part of a multivibrator whereby the charging circuit is energized intermittently so that the device produces sound intermittently.

8. A device as claimed in claim 6 in which said further resistor and said further transistor form part of a sawtooth pulse generator circuit whereby the pulse repetition frequency of the oscillator is varied cyclically.

9. A device as claimed in claim 4 further comprising a further diode with its anode connected to the first terminal to prevent reverse voltages being applied to the first mentioned transistor.

10. A device as claimed in claim 4 in which the anode of the diode is connected to the collector of the transistor and its cathode is connected to the first resistor, and further comprising a further capacitor connecting the cathode of said diode to the emitter of the transistor.

11. A device as claimed in claim 4 further comprising a further capacitor interconnecting the first and second terminals.

12. A device as claimed in claim 4 further comprising a third resistor connecting said capacitor to the second winding.

13. An electrical sound producing device comprising a body formed in two parts, a flexible resilient diaphragm of ferromagnetic material supported by one body part, an electromagnet supported by the other body part whereby the diaphragm is deformed when current is supplied to the electromagnet, means for adjusting the position of said two parts relative to one another during assembly of the device to vary the air gap between the electromagnet and the diaphragm, sealing means joining the body parts together in an adjusted relationship and an oscillator contained within said body for repeatedly supplying current to the electromagnet in short duration pulses.

14. A device as claimed in claim 13 in which said parts have mating cylindrical surfaces and are rotatable relative to one another about the common axis of said surfaces during assembly, said parts having interengaging means for displacing the two parts axially relative to one another on turning of one part relative to the other.

15. A device as claimed in claim 14, in which said interengaging means comprises a plurality of abutments on one part and a plurality of inclined ramps on the other part.

16. A device as claimed in claim 13, in which the part of the body which supports the diaphragm has an internal groove into which an external rib on a member of which the diaphragm is an integral part is a snap fit.

17. A device as claimed in claim 13, in which the part of the body which carries the electromagnet has an integral spigot on which there is fitted a tubular core forming part of said electromagnet.

18. A device as claimed in claim 17, in which a coil of the electromagnet is mounted on the core.

19. A device as claimed in claim 18, in which there is provided an annular insulation member surrounding the coils and supporting the component parts of the oscillator.

20. A device as claimed in claim 17, in which the electromagnet further comprises a cup-shaped member, having a hole fitted on said spigot and engaged by the core, forming a magnetic connection between the core and the periphery of the diaphragm, an air gap being defined between the core and the center of the diaphragm.

21. An electrical sound-producing device comprising a body, a flexible resilient diaphragm mounted in said body, a tubular acoustic resonator on the body tuned to the natural frequency of the diaphragm and whose axis is perpendicular to the plane of the diaphragm, an electromagnet in the body for deforming the diaphragm when current is supplied thereto and an oscillator contained in the body for repeatedly supplying current to the electromagnet in short duration pulses of lengths approximately equal to half the period of the natural vibration of the diaphragm, and wherein the internal diameter of the resonator is less than the diameter of the diaphragm.

22. An electrical sound-producing device comprising a body, a diaphragm within the body dividing the interior of the body into two chambers, a horn on the body opening into one chamber and of increasing cross-sectional area in a direction away from said one chamber, an electromagnet having a pair of windings in the other chamber for deforming the diaphragm when the current is supplied to the electromagnet, and an oscillator circuit including said pair of windings within said other chamber for repeatedly supplying current to the electromagnet in short duration pulses.

23. A device as defined in claim 22, further comprising a pulse rate control circuit in said other chamber connected to said oscillator and cyclically varying the pulse repetition rate of the oscillator.

24. A device as defined in claim 23, in which said pulse rate control circuit is a saw-tooth pulse generator circuit whereby the pulse repetition frequency of the oscillator is caused to commence each cycle at a low frequency and rise continuously to a high frequency, thereafter recommencing at said low frequency.