It is widely recognised that safety is greatly increased if a train has some form of warning siren fitted which operates to warn people where the train is located throughout a period of time and the approach of a train.

Such train horns typically take the form of an audible sound, commonly referred to as a horn, which is operated when a warning of the location or the change of location of the train is desired.

Train horns generally used two sound levels, a high or loud level for use when the train is in normal operation, and a low or soft level for use during shunting operations and within close proximity to ground personnel to avoid excessive noise and/or hearing damage to bystanders.

Previously proposed train horns have a distinctive sound and emit high then low tones separately and serially. A common sound range would include a first tone at 370 Hertz followed immediately by a tone of 311 Hertz.

Whilst such an arrangement is effective in being recognised as a train horn, partly due to the common usage of such sounds for that purpose, train horns using this feature have disadvantages.

The human brain is very poor at being able to locate the source of a sound which consists of a single frequency or just a few frequencies. Therefore, known train horns will alert a person to the fact that a train is nearby, but will provide little guidance as to the location of the train. This is clearly a significant problem, given that a train could be moving and potentially endangering a person but is obscured from view, by, for example, a building, a corner or trees, or allowing for the fact that the person may be visually impaired. Thus, the impetus for the person to take appropriate evasive action is not as great as is clearly needed.

It has been previously proposed to use Broad Band Sound to enable the listener to locate the source of the noise. However the range of Broad Band Sound that is capable of being produced by previously proposed generators may not be suitable for use on trains where a greater range of noise is required to better warn a listener of the train's position and approach.

It is an aim of the present invention to address the aforementioned disadvantage.

Accordingly the present invention is directed to a train horn for a train which in use produces an audible signal comprising broad band sound (BBS) which can be bandpass filtered to create an improved range of high and low tones. Preferably the train horn comprises a signal generating device to generate a broad band sound signal and bandpass filtering means. This enables the frequency content of the white noise broad band sound to be"coloured"by reducing the high or low frequencies. This gives the sound a higher or lower tone. It is known that the human ear and brain can process BBS in such a way that the location of the noise can be determined, whereas this is not possible with a sound containing only one or a few frequencies. The general usefulness of BBS to facilitate sound source location has been disclosed in GB 2303235 and GB 2318662. The ability of a listener to locate the source of a warning sound emitted by a train vastly increases the usefulness of such a horn, as the location of the train is readily apparent even in circumstances where a clear view of the train is unavailable.

Preferably the high and low sound levels are achieved by the use of a digitally controlled attenuator.

Advantageously, the train horn comprises a digital to analog converter (DAC).

Preferably the digital to analog converter is used between a central processing unit (CPU) output and a power amplifier input. This enables the audio signal to be digitally manipulated if so desired before being outputed to the speaker.

Preferably the central processing unit contains the broad band sound signal and timing, filtering and other pre-determined signals.

Preferably the central processing unit can be programmed to recognise signals from inputs and automatically activate the train horn.

Advantageously the input can be a braking system.

Advantageously the input can be a train speedometer. This gives the advantage that the horn can be automatically activated if the emergency break is being actuated or a sharp application of breaks such as would be the case in an emergency.

Advantageously the central processing unit comprises a microprocessor.

Advantageously the central processing unit comprises a pre-programmed memory.

Advantageously the pre-programmed memory is in the form of read only memory (ROM).

Preferably the train horn further comprises an interface unit to act as a buffer between external signals and the central processing unit inputs.

The central processing unit inputs can be damaged by voltage spikes or extremes. A buffer which is resilient to typical voltage spikes or extremes can be used between the external devices and the central processing unit to avoid such problems.

Advantageously the interface unit may be included in the central processing unit.

Preferably the attenuator may be any type of analog switch capable of outputting controllable analog signal levels that can be digitally controlled.

Advantageously the attenuator is an analog to digital converter.

Preferably the audio power amplifier is a high power audio amplifier.

Advantageously the audio power amplifier is capable of amplifying audio signals from 500 Hz to at least 12 kHz.

Preferably, the power control circuit is powered from 24 volts DC and applies reduced voltage levels to the audio power amplifier, central processing unit and the power control circuit.

Advantageously a DC-DC or AC-DC converter is used to reduce and/or convert the train's power supply from Direct Current (DC) or Alternating Current (AC) to the 24 volts DC required.

There is therefore no need for a dedicated power source to be provided for the train horn, as it can be connected to the existing power supply of the train.

Preferably the power applied to the power amplifier during power up or turn on must be soft-started.

Preferably, the standard Broad Band Sound is produced, followed by bandpass filtered Broad Band Sound. The listener will perceive the bandpass filtered Broad Band Sound as a lower tone, producing the overall impression on the listener of a white sound train horn.

Preferably the horn comprises a very high power speaker.

Preferably the speaker is housed in a housing capable of being subjected to the mechanical stresses of being mounted to the outside of a train and subjected to harsh weather conditions.

Advantageously the housing is further capable of withstanding the impact of rocks, stones and other debris. Such debris may be thrown up at the horn at high impact speeds when the train is in motion.

An example of a train horn made in accordance with the present invention will now be described with reference to the accompanying drawing, in which: Figure 1 shows a block diagram of a train siren according to the present invention. The blocks represent various components of an electronic circuit.

Figure 1 shows a central processing unit and interface unit 1 capable of receiving signals from train status inputs based on train speed, type, brakes 10 and producing signal outputs 14,16 and 18. The central processing unit and interface unit can be together or separate. The interface unit is the buffer between the central processing unit and the outside world. The central processing unit and interface unit 1 is also connected to a switch 12 for the control of High or Low sound emission.

Audio output signal 14 and control signal 16 are connected to an attenuator 2. The attenuator 2 is further connected to and audio power amplifier 3 which in turn is connected to a horn 4. The control signal 18 is directed to a power control circuit 5.

The power control circuit 5 receives an input voltage from the trains power system 20, via a voltage converter 6 which can vary according to the power system of the train, and regulates and stabilises it to produce an output voltage of approximately 24 V DC. This output voltage is used to power the train horn according to the present invention. Thus if used on a train the horn can be powered by a train's existing ordinary power system, and no additional power source may be required.

The operation of the warning siren is as follows : When the horn is not in use the circuitry is off except for the interface unit which is powered and ready to accept an input from the horn switch 12. When the horn switch 12 is operated either to the high or low mode, the input signal is sent to the central process unit 1 via the interface unit. The central processing unit is then activiated and sends a signal to the power control unit 5 which then applies power to

the attenuator 2. This soft starts the power amplifier 3. The broad band sound signal is generated by the Central Processing Unit 1 and applied to the high power amplifier 3 via the attenuator 2. The audio power amplifier 3 applies a high power broad band sound to the horn speaker 4 which emits the Broad Band Sound signal as an audible sound.

Ideally, the horn 4 is designed to be waterproof, dust-proof, shock-proof, and generally highly durable, so that it can perform optimally and over a long period when the horn is fitted to an object. The preferred position of the horn is for it be mounted on the exterior of the train, so the speaker needs to be generally resilient.

The combination of features described herein result in a train horn which efficiently and effectively generates, processes and emits a good quality, audible, locatable sound over a widely defined range from a few simple components, allowing a compact and inexpensive safety device to be simply produced.

However the present invention is not limited to this particular circuit arrangement. Other circuits having the same effect as that shown in Figure 1 also fall within the scope of the present invention.

Alternative embodiments of the present invention are possible. For example, the train horn could be further provided with a light which is activated when the train horn is switched on, so that the train horn provides a visual warning signal in conjunction with an audible one, all contained in a single unit.