Field of the invention

The present invention relates to an electronic warning system, especially to such a system that emits an alarm (warning sound) of variable intensity and frequency.

Background to the invention

Warning devices are required .in many industrial situations in order to give warning to ^' p'ersonnel that machinery is about to start. Such devices have geπer- ally been of the vibrating diaphragm or siren type, having a fixed acoustic output, operated automatically from a contact at the main switch panel of machinery. In order to allow time for personnel to move clear, the start-up of the machinery is delayed for a given period after the alarm ias been sounded.

These prior-art warning devices can be satisfact- ory insofar as they, meet the audible warning require¬ ments of alarms. However, these devices are generally not adjustable for overall noise level, frequency or directivity and therefore suffer from the disadvantage that their performance cannot be optimised for any one specific task or position. Furthermore, the fixed acoustic output, which is often unmodulated, and the omnidirectional radiation characteristics of conven.t- ional warning devices can give rise to the environ¬ mental problem of excessive noise levels beyond the boundary of the works site, thereby causing consider¬ able annoyance to residents in the neighbourhood, the problem being especially acute at night. Clearly, there is a need for a warning device or system that emits a satisfactory on-site warning

- 2 -

signal whilst achieving an acceptable level of noise at the site boundary. It would also be desirable for SL'cr: a warning system to be more flexible in operation than warning devices of the conventional types. Summary of the invention

According to the present invention, an electronic warning system comprises a signal generator, a power amplifier and at least one loudspeaker.

The system according to the present invention overcomes the limitations of the conventional warning devices by enabling. the alarm αperatiπ ^' g level, its acoustic frequency,. its modulation pattern and its direction to be varied, thus providing the facility to optimise the audible impact on the personnel on the site whilst minimising the impact at the site boundary. The present system is suitable for both single and multiple signal source appli-catiαns and is particul¬ arly suitable for applications - such as long conveyor belts, production lines, railways and hosp¬ itals - requiring multiple alarms from one contactor.

Although the overall output is determined by the setting of the volume control of .the power ampli¬ fier, it is desirable to provide means whereby the output level from the or each loudspeaker can be pre-set.

It is also preferred to -pr.ov±d.e -means- whεreb.y . the frequency of the signal can- be adjusted in order to achieve an optimum contrast between the warning signal generated and the background noise. It is also preferred to incorporate means for adjusting the alarm temporal pattern in order to achieve optimum detectability and recognition. Thus, an _atnplitude and frequency modulating device may be provided in

OMPI

- 3 -

order to provide the required pattern (e.g. a character¬ istic wailing, hi-lo or pulsed sound, etc.).

With the present system, central control of the or each loudspeaker can be achieved. Thus, it is 5 preferred to pass the signal .from the amplifier through a multiplex coding unit, each loudspeaker being _. provi¬ ded with a specific multiplex decoding unit. By this means, it is possible to.arrange that a. plurality of loudspeakers operate in a given sequence. This arrange- •JQ meπt is particularly advantageous in t'he case, for example ^' , where warning is to be given of the starting up of a long conveyor belt (which, in a quarry could be as long as 800 metres or more). Sequential operation of loudspeakers sited at predetermined - 15 intervals along the length of the conveyor belt would ensure that adequate warning is given to operatives, _.. whilst avoiding an excessive acoustic total output at any given point in time. Brief description of the drawing 20 The single accompanying figure represents a block diagram of a warning system in accordance with the present invention. Description of preferred embodiments

As illustrated in the accompanying drawing, a 25 preferred embodiment of the present invention comprises a -variable-frequency signal generator "and a power amplifier providing a suitable output signal to one or more remotely sited loudspeakers. The alarm signal is produced in the signal generator and is then codulated 30 by a programme selector in order to provide the means for optimising froth frequency and modulation for any background noise level. The acoustic output cf the system is adjustable in three distinctly separate v/ays

- 4 -

in order to achieve an optimum operating level and noise/warning contrast.

Firstly, the overall acoustic output of the system is adjusted via the main amplifier gain 5 (volume) control.

Secondly, the level of each individual loud¬ speaker is separately adjustable, by means of a secondary volume. control, up to the maximum level set by the main amplifier level control.

^■ JQ In practice, this is checked with a sound-level meter midway betv/eeπ two units to ensure that an adequate πσise-to-background level is obtained. A value of 30 dB has proved satisfactory. Alternatively, the output of each individual loudspeaker could be

15 electronically controlled to any desired level above the ambient noise level, by using the loudspeaker it¬ self or an associated transducer as a feedback device.

Thirdly, a programmable timer is incorporated in order to provide the facility of varying the overall 0 level to produce an optimum setting both for day and for night operation. This last feature is particular¬ ly important as background levels often drop by 10 dB or more at night, during which period any high indust-. rial boundary noise levels are most likely to give rise 5 to complaint (see British Standard (BS) 4142 - Method of Rating Industrial ^' Noise. Aff.ecting Mixed. Residential . and Industrial Areas).

It has proved satisfactory to set the level at -10 dB for the night period. 0 The system of the present invention can be used in applications where only one loudspeaker is required. However, the present system is particularly advantage¬ ous in applications requiring two or more warning

- 5 -

devices associated with a particular item of plant, it being necessary only to extend the number of remotely sited loudspeaker units.

In the case of a system having a plurality or an array of loudspeakers, the individual speakers can be operated either simultaneously or in a desired sequence As shown in the accompanying drawing, a multiplex cod¬ ing unit and multiplex decoding units are provided, whereby sequential operation of the loudspeakers can be achieved. The multiplexing technique allows indiv- idual adjustment of volume for each loudspeaker and also the use of a single pair of signal wires, thus avoiding the need for multiple wiring for each loud¬ speaker . . It is preferred that directional loudspeakers be employed to emit the alarm signal. The choice of directional loudspeakers allows an additional degree of control by permitting adjustment to the polar distr¬ ibution of the soundfield produced; this enhances the alarm signal in the preferred direction and minimises its effect in other directions.

A period timer is provided in order that the duration of the alarm can be pre-set as required to suit the time delay for the specific application. A warning system according to the present inven¬ tion can be designed as _. a .direct, .replacement -of..an existing alarm and can be arranged to be operated by the same contact at the main switch panel of the machinery or plant. The signal (or tone) generator can be of any suit¬ able design, but is preferably adjustable or programm¬ able .

The multiplex switching can be operated by frequency or pulse selection, the latter method being

- 6 -

preferred if a large number of speakers are used.

In the above-described embodiments, the signals are transmitted to the loudspeakers by wires. However, it will be readily understood that the invention applies also tα other systems, e.g. radio-controlled systems.

SUBSTITUf_£-SHEE^ CMFI