GB2323999A

GB2323999A - Variable volume enclosure for a sounder

Info

Publication number: GB2323999A
Application number: GB9706971A
Authority: GB
Inventors: Manhar Amlani
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-04-05
Filing date: 1997-04-05
Publication date: 1998-10-07
Also published as: GB9706971D0

Abstract

A sounder for an alarm system comprises a body 10 having a compartment, a diaphragm 14 mounted to the body 10 over an opening into the compartment, a rocking armature solenoid 20 for vibrating the diaphragm 14 in response to an a.c. signal applied to terminals 21, and a screw 23 on the body for varying the volume of the compartment. The sounder is tuned by applying an a.c. tuning signal to the sounder and varying the volume of the compartment by turning the screw, until the highest sound output level is detected. In this manner, the current drawn by the sounder can be kept at a minimum, whilst maintaining a high sound output. The sounder may also be tuned by creating apertures 30 in the wall of the compartment.

Description

Sounders This invention relates to sounders, such as those for providing an audible warning that an event has occurred.

Sounders are commonly used in alarm systems, for example to warn that a fire has been detected or to warn of a break-in. One such sounder comprises a piezo-electric crystal that vibrates at a frequency within the audible frequency range, thereby producing a sound which can be heard.

A disadvantage of such so-called piezo-sounders is that they consume a heavy current, thereby restricting the number of sounders that can be connected in parallel to any one driver circuit. Furthermore, piezo sounders only operate at a single frequency, whereas I have found that variable or multifrequency sounders are far more effective at alerting people.

Electro-mechanical transducers, such as moving coil or rocking armature loudspeakers, can produce sounds in a wide range of frequencies when driven with a corresponding a.c.

signal.

GB 2 106 748 discloses one such loudspeaker, which can be used as a sounder. The loudspeaker comprises a cup-shaped housing which is closed by a flexible diaphragm. A rocking armature inside the housing is provided with terminals which extend through the rear of the housing for connecting to an a.c. audio frequency signal.

In use, the armature vibrates the diaphragm, thereby creating audible sounds. This type of loudspeaker is compact and works well as a sounder, however a disadvantage of all loudspeakers is that they also consume a high current, thereby restricting the number of sounders that can be connected in parallel.

I have now devised a sounder which alleviates the above-mentioned problems.

In accordance with this invention as seen from a first aspect, there is provided a sounder comprising a body having a compartment, a diaphragm mounted to the body over an opening into the compartment, actuator means for vibrating the diaphragm in response to an a.c. signal, and means on the body for varying the volume of the compartment.

I have found that the volume of the compartment affects the frequency at which the sounder will resonate. At resonance, the ratio of sound output to input current is much higher, and thus the volume of the compartment can be adjusted to make the sounder resonate at its operating frequency (or at frequency which is located within a range of operating frequencies), so as to maximise the above-mentioned ratio. In this manner, the current drawn by the sounder can be kept at a minimum, whilst maintaining a high sound output.

Preferably the volume adjusting means comprises a moveable member which can be moved into or out of the compartment.

Preferably the volume adjusting member comprises a screw which can be screwed into or out of the compartment.

Preferably the body of the sounder comprises a tubular extension in which the screw is mounted.

Preferably the actuator means is mounted within the compartment.

Also in accordance with this invention, as seen from a second aspect, there is provided a method of tuning a sounder as hereinbefore described comprising applying an a.c. signal to the sounder and varying the volume of the compartment until the highest sound output level is detected.

In this manner, a plurality of sounders can be tuned to resonate at the same frequency, thereby overcoming the problem that manufacturing tolerances cause a wide variation in the resonant frequencies of the sounders.

Preferably the frequency of the a.c. signal lies within the desired range of operating frequencies of the sounder.

Alternatively the frequency of the a.c. signal lies intermediate the frequencies of a two-tone operating signal applied to the sounder.

Preferably the volume of the compartment is adjusted by turning a screw into or out of the compartment.

I have found that the less the volume of the compartment, the greater the resonant frequency. The reason for this is that the air inside the compartment acts as a cushion to dampen speaker vibrations. If the volume of the compartment is small, the air cushion appears more rigid because the air inside the compartment does not have room to move: this rigid air cushion prevents low frequency vibrations.

Accordingly, it will be appreciated that the resonant frequency of the sounder is also dependent on the rigidity of the volume of air inside the compartment. Thus, in accordance with this invention as seen from a third aspect, there is provided a method of tuning a sounder of the type having a compartment disposed behind a flexible diaphragm arranged to produce sound, the method comprising applying an a.c. signal to the sounder and varying the size of an opening in a wall of the compartment until the highest sound output level is detected.

Preferably the opening is initially formed by forming an aperture in the wall of the compartment.

Preferably the size of the opening is increased either by enlarging the aperture or by forming additional apertures in the wall.

An embodiment of this invention will now be described by way of example only, and with reference of the accompanying drawings, the single figure of which is a sectional view through an embodiment of sounder in accordance with this invention.

Referring to the drawing, there is shown a sounder comprising a cup-shaped body 10 having a circular base 11 and a peripheral wall 12. The peripheral wall 12 is turned outwardly at its outer end to form a rim 13. A thin metal diaphragm 14 sits on the out-turned rim 13 of the body 10. A front cover 15 is crimped onto the rim 13, in order to secure the diaphragm 14 to the body 10. A spacer disc 16 and a plastic membrane 17 are disposed between the diaphragm 14 and the cover 15.

The diaphragm 14 comprises a concave portion 18 which is connected via a mechanical link 19 of wire to a rocking armature coil assembly 20. Terminals 21 are provided on the rear of the cover 10 for applying an a.c. signal to the coil assembly 20.

The sounder as hereinbefore described is substantially the same as a conventional sounder, of the type disclosed in GB 2 106 748. In accordance with this invention the body 10 of the sounder comprises a tubular extension 22 which projects rearwardly from its base 11. A screw 23 is screw-threaded into the free end of the tubular extension 22.

In use, the sounder may be arranged to produce a twotone sound, say comprising frequencies of 500 and 1,000 Hz.

Thus, in order to tune the sounder a frequency of 750 Hz is applied to the terminals 21. The armature of the assembly 20 vibrates, and this vibration is transmitted to the diaphragm 14 via the mechanical wire link 19. The diaphragm 14 thus vibrates, thereby creating an audible sound at 750 Hz.

The frequency at which the sounder resonates is dependent upon the volume of the air inside its body 10. The volume of the air can be adjusted by varying the position of the screw 23. Thus, in order to tune the sounder to resonate at 750 Hz, the screw 23 is turned until the highest volume is achieved. Turning the screw inwardly decreases the volume and increases the resonant frequency, and vice-versa.

At resonance, the sound output-to-current ratio of the sounder is maximised, and thus a lower current is used to generate the same sound output level.

Fire alarm systems comprise a plurality of sounders connected to a line, which extends from a control panel. In use, when a fire is detected, the control panel applies a voltage to the line which energises the sounders. The current that can be drawn by the line is limited, and thus the number of sounders is correspondingly limited. However, it will be appreciated that tuning the sounders to resonance enables more sounders to be connected to the line.

In an alternative embodiment, one or more apertures e.g. 30 of varying sizes may be formed in the base 11 of the sounder body 10: this may be in addition to or instead of the screw adjustment.

In this embodiment the sounder is manufactured to resonate at a frequency above the desired operating frequency (or frequencies). The sounder is then tuned to the desired resonant frequency by forming one or more apertures of varying sizes in the base 11 of the sounder body 10; this has the effect of reducing the rigidity of the air cushion trapped inside the body, thereby decreasing the resonant frequency.

Claims

1) A sounder comprising a body having a compartment, a diaphragm mounted to the body over an opening into the compartment, actuator means for vibrating the diaphragm in response to an a.c. signal, and means on the body for varying the volume of the compartment.

2) A sounder as claimed in claim 1, in which the volume of the adjusting means comprises a moveable member which can be moved into or out of the compartment.

3) A sounder as claimed in claim 2, in which the volume adjusting member comprises a screw which can be screwed into or out of the compartment.

4) A sounder as claimed in claim 3, in which the body of the sounder comprises a tubular extension in which the screw is mounted.

5) A sounder as claimed in any preceding claim, in which the actuator means is mounted within the compartment.

6) A sounder substantially as herein described with reference to the accompanying drawing.

7) A method of tuning a sounder as claimed in claim 1, the method comprising applying an a.c. signal to the sounder and varying the volume of the compartment until the highest sound output level is detected.

8) A method as claimed in claim 7, in which the frequency of the applied a.c. signal lies within the desired range of operating frequencies of the sounder.

9) A method as claimed in claim 7, in which the frequency of the applied a.c. signal lies intermediate the frequencies of a two-tone operating signal applied to the sounder.

10) A method as claimed in any of claims 7 to 9, in which the volume of the compartment is adjusted by turning a screw into or out of the compartment.

11) A method of tuning a sounder of the type having a compartment disposed behind a flexible diaphragm arranged to produce sound, the method comprising applying an a.c. signal to the sounder and varying the size of an opening in a wall of the compartment until the highest sound output level is detected.

12) A method as claimed in claim 10, in which the opening is initially formed by forming an aperture in the wall of the compartment.

13) A method as claimed in claims 10 or 11, in which the size of the opening is increased either by enlarging the aperture or by forming additional apertures in the wall.

14) A method of tuning a sounder, the method being substantially as herein described with reference to the accompanying drawing.