GB2502982A

GB2502982A - Swimming pool entry alarm and swimmer inactivity alarm

Info

Publication number: GB2502982A
Application number: GB201210313A
Authority: GB
Inventors: Jeremy Ross Nedwell
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-06-12
Filing date: 2012-06-12
Publication date: 2013-12-18
Also published as: GB201210313D0

Abstract

In a first aspect, an apparatus 10 is provided to detect noise in a body of water 22 indicative of a person falling into the body of water and to raise an alarm in response to such detection. The body of water may comprise a swimming pool. A hydrophone 14 may be immersed in the body of water in order to detect noise indicative of a person falling into the body of water. In another aspect, an apparatus is provided including a device carried by a swimmer (see figure 9), the device having a means for sensing movement in a body of water, wherein the apparatus raises an alarm if there is a substantial lack of movement. The movement sensing means may comprise a hydrodynamic pressure sensor. An alarm may be raised if the lack of movement persists for a predetermined period of time. In a further aspect an alarm apparatus is provided including both of the above mentioned apparatus.

Description

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TITLE

Alarm apparatus for a swimmer and/or for a body of water

DESCRIPTION

This invention relates to an alarm apparatus for a body of water and/or to an alarm apparatus ffir a swimmer in a body of water.

The invention was conceived for use with swimming pools, but it does have other applications. Therefore, in this specification, the term body of water' is intended to cover, for example, swimming pools and baths, whether public, commercial or private and whether indoors or outdoors, ponds, lakes, the contents of water storage tanks, and other hodies of water.

Many children, animals and adults die every year from falling into domestic or commercial swimming pools. If not detected, they may drown. While it is clearly possible to provide fencing around ground level pools, there must he access provided to the pool, and accidents are sometimes caused by children getting through a gate that has not been locked.

An aim of a first aspect of the invention, or at least of specific embodiments of it, is to raise an alarm when somehody falls into a swimming pool or other hody of water.

In accordance with the first aspect of the invention, there is provided an alarm apparatus for a hody of water, the apparatus comprising: means for detecting noise in the water indicative of a person falling into the body of water; and means for raising an alarm in response to such detection. Another person noticing the alarm can therefore take appropriate action to check the situation and save the first person if necessary.

The noise detecting means preferably includes: a hydrophone (or underwater microphone) arranged to be immersed in the water and to produce a source signal corresponding to sound to which the hydrophone is exposed; and means for conditioning or processing the source signal. The conditioning or processing means preferably includes means for pre-amplifying the source signal to produce an amplified signal.

It is obviously very important that the apparatus has the highest probability of detecting an immersion, and the alarm could simply he raised if the level of the amplified signal exceeded a predetermined threshold. However, it is also very important that the apparatus gives the lowest possible probability of a false alarm when there is no immersion, since repeated false alarms would typically lead to the alarm being ignored.

It has been found that the noise from an immersion may be most readily detected above pre-existing background and other noise over the hand, or frequency range, from about I kllz to about 100 kl-Iz. It is probable that the noise in this range is created by the noise from the movement within the water or bursting at the surface of bubbles that are carried down into the water by the immersion, and subsequently rise to the surface. The most sensitive range is from 4 kHz to 80 kHz, which may be here termed the preferred band of frequencies. Within the preferred hand, the noise of a splash from an immersion is most readily detected above, for instance, the noise from the circulation pumps usually used to swimming pool is, which is at lower frequencies of a few hundred Hz. It has been determined that the noise from persons in the vicinity of the pool walking, mowing lawns, talking etc. is typically also well below the preferred band.

The conditioning or processing means therefore preferably includes means for filtering out low frequencies in the source signal or amplified signal below a first cut-off frequency (such as 4 kHz) and/or for filtering out high frequencies in the source signal or amplified signal above a second cut-off frequency (such as 80 kllz) to produce a first filtered signal.

Hence, the noise of the splash is more reliably detected by monitoring the preferred band of frequencies, while providing a high degree of immunity to accidental triggering of the alarm by other noise, which would clearly be undesirable.

The conditioning or processing means preferably includes: means for rectil'ing the source signal or the amplified signal or the first filtered signal to produce a rectified signal; and means for filtering the rectified signal by smoothing with a first time constant to produce a first level signal.

In a simple embodiment, the alarm means may be arranged to raise the alarm when the level of the first level signal exceeds a predetermined threshold. Since the signal from an immersion typically dominates over other noise sources in the preferred band, such a circuit would allow a splash to be detected, raising an alarm. This simple circuit is suitable for detecting immersion in many applications.

However, rain has been found to also create a reasonably high level of noise in the preferred hand, and hence where such a simple circuit is used to protect a pool in the open there is a risk of it being accidentally triggered by rainfall noise. In order to decrease the risk of this.

the threshold level at which the alarm triggers could simply be increased. However, this would have an adverse effect, in that a relatively low level of splash caused by an accidental immersion, for instance by a small child falling relatively gently into the water, would be less likely to he detected.

In order to deal with this, the conditioning or processing means preferably includes means for filtering the rectified signal or the first level signal by smoothing with a second longer time constant to produce a second level signal. The second time constant is preferably about equal to or greater than the duration of the splash when a person falls in the water.

In one version, the conditioning or processing means then preferably includes means for differencing the first and second level signals, to produce a difference signal. The alarm means is then arranged to raise the alarm when the level of the difference signal exceeds a predetermined threshold.

In another version, the conditioning or processing means may also include means for delaying the first level signal (for example by about one half of the second time constant) to produce a third level signal, and it is then the third and second level signals which are differenced.

In a further version, the preamplifier may have a controllable gain arranged so that the gain increases as the level of the second level signal decreases and vice versa. The alarm means is then arranged to raise the alarm when the level of the first level signal exceeds a predetermined threshold.

As will be explained in greater detail later, with each of these versions, the alarm is not triggered by slow changes in the level of background noise, such as a rain shower beginning, hut is triggered by fast changes in noise such as the splash of an immersion.

It will be appreciated that due to the splashing caused by normal swimming activities, the apparatus described above would normally he turned off while swimming is in progress in the pool. Indeed, it would probably be of no use during such activity. In most cases, protection of the poo1 from accidental immersion during periods it is not in use would he more than adequate. However, in some cases, it may also be required to monitor the swimming of swimmers in swimming pools while they are in use. An aim of a second aspect of the invention, or at least of specific embodiments of it, is to raise an alarm when a swimmer stops swimming.

In accordance with the second aspect of the invention, there is provided an alarm apparatus for a swimmer in a body of water, the apparatus comprising: a device arranged to be carried by the swimmer and including means for sensing movement of the device in the body of water; and means responsive to the movement sensing means for raising an alarm is there is a substantial lack of movement.

The apparatus could therefore raise the alarm if a swimmer stopped swimming, or if somebody wearing the device fell into a swimming pool and did not move afterwards.

The movement sensing means preferably includes means for sensing hydrodynamic pressure fluctuations around the device.

The alarm means is preferably operable to raise the alarm if the lack of movement persists for longer than a first predetermined period of time. This prevents the alarm being triggered if a swimmer stops moving for only a short period of time.

The apparatus preferably further includes: means for providing a warning to the swimmer for a second predetermined period of time after a substantial lack of movement has been detected but before the alarm is raised; and means activatable by the swimmer to prevent the alarm being raised on that occasion.

The preventing means may be responsive to the movement sensing means to prevent the alarm being raised on that occasion if movement is detected during said second predetermined period of time. Therefore, if the swimmer stops moving, receives the warning, and responds by moving, the alarm will not he triggered that time.

The apparatus preferably includes means for sensing whether or not the device is submerged in water.

In this case, the preventing means may be responsive to the submersion sensing means to prevent the alarm being raised on that occasion if the device is sensed as being taken out of the water during said second predetermined period of time. For example, in the case where the device is worn on the swimmer's wrist, if the swimmer stops moving, receives the warning, and responds by lifting their wrist out of the water, the alarm will not be triggered that time.

The movement sensing means and/or the warning means and/or the alarm raising means are preferably arranged to he deactivated when the device is not submerged in water. This helps prevent false alarms and, in the case of a battery-powered device, conserves battery life.

The means for raising the alarm preferably comprises: means provided in the device for transmitting an underwater alarm signal; and a remote device (for example at the side of the swimming pool) for receiving the underwater alarm signal and for raising the alarm in response thereto.

The apparatus may include a transducer (such as a piezo-electrie element) which serves as part of the movement sensing means and/or the warning providing means and/or the underwater alarm signal transmitting means.

The remote device preferably includes: a hydrophone arranged to be immersed in the water and to produce a received signal corresponding to the underwater alarm signal; and means for conditioning or processing the received signal.

The alarm signal preferably has a predetermined characteristic (such as a sinusoidal waveform of a predetermined frequency); in which case the conditioning or processing means preferably includes means for filtering the received signal to remove components thereof not having the predetermined characteristic to produce a filtered signal. Preferably, the conditioning or processing means is arranged to detect the level of the filtered signal; and the remote device is arranged to raise the alarm when the level of the filtered signal exceeds a predetermined threshold.

The apparatuses of the first and second aspects of the invention may he combined into a single apparatus sharing some common components.

Specific embodiments of the present invention will now he described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a portion of a swimming pool and a pool alarm apparatus and/or one part of a swimmer alarm apparatus: Figure 2 is a block diagram of components of a first embodiment of the apparatus of Figure 1 when functioning as a pool alarm; Figures 3A-D a example graphs of a signal level in the apparatus of Figure 2 in different circumstances; Figure 4 is a block diagram of components of a second embodiment of the apparatus of Figure 1 when functioning as a pooi alarm; Figures 5A-D a example graphs of one signal level in the apparatus of Figure 4 in different circumstances; Figures GA-D a example graphs of another signal level in the apparatus of Figure 4 in those circumstances; Figure 7 is a block diagram of components of a third embodiment of the apparatus of Figure 1 when functioning as a pool alarm; Figures 8A-D a example graphs of a signal level in the apparatus of Figure 7 in different circumstances; Figure 9 is a schematic diagram of device which forms another part of the swimmer alarm apparatus of Figure 1; Figure 10 is a state diagram for the device of Figure 9; Figures 1 1A-D are block diagrams of operational parts of the device of Figure 9 in its various states; Figure 12 is a block diagram of components of a fourth embodiment of the apparatus of Figure 1 when functioning as a swimmer alarm; Figure 13 is a block diagram of components of a fifth embodiment of the apparatus of Figure 1 when functioning as a swimmer alarm; and Figure 14 is a block diagram of components of a sixth embodiment of the apparatus of Figure 1 when functioning as a pool or swimmer alarm.

Referring to Figure 1 of the drawings, a pool alarm apparatus 10 embodying the first aspect of the invention comprises a housing 12 containing various electrical components, and a hydrophone 14 connected tn the electrical components by a cable l& In normal use, the housing 12 is placed on the ground 18 near the side of a swimming pool 20 and the hydrophone is suspended by the cable 16 in the water 22 of the pool 20. Alternatively, the hydrophone 14 may be mounted on a rigid bent arm fixed to the housing 12. The hydrophone senses the pressure of underwater sound and/or its particle velocity and/or some other parameter of the sound.

Preferably, the hydrophone has a reasonably tiat response in the range from I kFIz to 100 kFIz.

The apparatus may he powered by its own battery (not shown) or by a low-voltage mains adapter (not shown).

Referring to Figure 2 of the drawings, the housing 12 contains a preamplifier 24 which amplifies the signal from the hydrophone 14 with a preset gain. The amplified signal is fed through a hand-pass filter 26 having a lower cut-off frequency of about 4 kT-Jz and an upper cut-off frequency of about 80 kHz so that components of the amplified signal outside the preferred band of 4 kT-Tz to 80 kI-Iz are severely attenuated. The preferred band is chosen to exclude some sources of noise, such a pool circulation pumps (which typically produce noise at a frequency of a few hundred hertz), walking and lawn-mowing. The filtered signal is rectified by a rectifier 28 and smoothed by a smoothing filter 30 having a time constant of about 0. 1 seconds to produce a signal Si which is fed to a level detector 32. The smoothing filter attenuates for example any brief spikes in the signal cause by electronic interference. When the level of the signal 51 exceeds a preset threshold level TI, the level detector 32 activates an audible and/or visible alarm 34. The aiarm 34 can be reset by a manually operable element 36.

When the apparatus 10 is used with an indoor swimming pool 20, the background noise that is picked up by the hydrophone 14 might produce a reasonably steady and low-level signal SI as shown in Figure 3A. When a person falls into the pool 20, a peak 38 is formed in the level of the signal 51. as shown in Figure 3B. The threshold level Tl of the detector 32 is set, as shown by the dash-dot lines in Figures 3A and 3B, so that the background noise does not trigger the alarm, hut so that the combination of the background noise and the splash of somebody falling into the pooi 20 does trigger the alarm.

When the apparatus 10 is used with an outdoors swimming pool 20, the background noise that is picked up by the hydrophone 14 might produce a far more variahle signal SI.

Figure 3C shows the signal level 51 at the onset of a shower of rain, and it is possible that, as shown, the level of the signal SI may rise above the threshold level TI and cause false triggering of the alarm. in order to deal with this, the apparatus of Figure 2 may be modified as shown in Figure 4.

in Figure 4, the signal SI from the smoothing filter 30 is fed to a delay circuit 40 and also to a smoothing filter 42 having a time constant of about 10 seconds to produce a signal S2.

The delay circuit 40 produces a delay in the signal SI of about one half of the time constant of the smoothing filter 42, so a delay of about 5 seconds. The delayed signal and the smoothed signal 52 are fed to a differencing circuit 44 which produces a level signal S3 equal to the difference between the level of the delayed version of the signal Si and the level of the smoothed signal S2. The difference signal S3 is fed to the level detector 32, which has a preset threshold level T2.

The forms of the smoothed signals 52 corresponding to the four forms of the signal SI of Figures 3A to 3D are shown in Figures 5A to 5D respectively. Also, the forms of the difference signals S3 for those four scenarios are shown in Figures 6A to 6D respectively. As shown in Figure 6A, for steady background noise, the level of the signal S3 is substantially zero. For a splash with steady background noise, the level of the difference signal S3 has a substantial oscillation 46, as shown in Figure 6B. As shown in Figure 6C, for a combination of steady background noise and the onset of a shower of rain, there is a slight ripple about zero in the level of the difference signal 53. For a combination of steady background noise, a splash and the onset of a shower, the level of the difference signal S3 again has a substantial oscillation 46, and there is also a minor ripple, as shown in Figure GD. The threshold level T2 can therefore readily he chosen so that it is exceeded by a splash, as shown by the oscillations 46 and the dot-dash lines in Figures GB and 6D. but not by the onset of a shower or any other slow change in the level of noise.

In order to deal with the problem of false triggering by slow changes in the level of background noise, the apparatus of Figure 2 may alternatively be modified as shown in Figure 7.

In Figure 7, the signal from the rectifier 28 is fed (i) to the smoothing filter 30 having a time constant of about 0.1 seconds to produce a level signal S4, and also (ii) to a smoothing filter 42 having a time constant of about 10 seconds to produce a control signal S5. The preamplifier 24 has a variable gain set by the level of the control signal S5 so that the gain of the preamplifier 24 is approximately inversely proportional to the level of the control signal S5.

The level signal S4 from the smoothing filter 30 is fed to the level detector 32. which has a preset threshold level T3. The circuit therefore provides automatic gain control so that the level of the signal S4 for any constant level of the signal from the hydrophone 14 is a fixed value, which in Figures 8A to SD is shown to he unity.

The forms of the level signals S4 corresponding to the four scenarios represented by Figures 3A to 3D are shown in Figures 8A to SD respectively. As shown in Figure 8A, for steady background noise, the level of the signal S4 is substantially unity. For a splash with steady background noise, the level of the difference signal S4 has a positive oscillation 48 followed by a single overswing, as shown in Figure 8B. As shown in Figure SC, for a combination of steady background noise and the onset of a shower of rain, there is a single small ripple 50 in the level of the signal S4. For a combination of steady background noise, a splash and the onset of a shower, the level of the signal S4 is a combination of the small ripple and the oscillation with overswing, as shown in Figure 6D. The threshold level T3 can therefore readily be chosen so that it is exceeded by a splash, as shown by the oscillations and the dot-dash lines in Figures 8B and 8D, hut not by the onset of a shower or any other slow change in the level of noise.

A swimmer alarm apparatus embodying the second aspect of the invention will now he described. The swimmer alarm apparatus includes a device 52 as shown in Figure 9 of the drawings having a waterproof housing 54 and a strap 56.58 for attaching the housing 54 to a part of a swimmer's body such as their wrist. The housing 54 contains or mounts a battery 60, a controller 62, an LED 64, a water sensor 66 and a piezo-electric element 68. The water sensor 66 may include a couple of electrodes which are exposed to the outside of the housing 54 and an electrical resistance sensor so that it can he detected whether the housing 54 is under water or not. The piezo-electric element 68 is also coupled to the outside of the housing 54 and (i) can be driven by an electrical signal when the housing 54 is under water to transmit sound into the water (i.e. act as an underwater loudspeaker) and also to vibrate the housing and (ii) can produce an electric signal when the housing is underwater and exposed to hydrodynamic pressure fluctuations (i.e. act as an underwater microphone or hydrophone).

As shown in Figure 10, the device 52 has four states, standby', detect', warning' and alarm', and changes between states in the manner shown in Figure 10. The device 52 may also have an off' state controlled by a manually-operable switch (not shown) for the battery 60. The controller 62 may he provided by hard-wired logic circuiny or by a programmed microcontroller.

As shown in Figure hA, when the device 52 is in the standby state, the controller 62 receives an input from the water sensor 66, and the piezo-electric element 68 and the LED 64 are not active. If and when the controller 62 determines that the device 52 is under water, the device 52 changes to the detect state, as shown by the state-change 70 in Figure 10.

As shown in Figure 1 lB. when the device 52 is in the detect state, the controller 62 receives an input from the water sensor 66. If and when the controller determines that the device 52 is no longer under water, the device 52 changes back to the standby state, as shown by the state-change 72 in Figure 10. Also, in the detect state, the controller 62 receives an input from the piezo-electric element 68. if the controller 62 determines that the piezo-electric element 68 is not being subjected to changes in hydrodynamic pressure above a threshold level, the device 52 changes to the warning state, as shown by the state-change 74 in Figure 10. The controller 62 may cause the state-change 74 immediately, hut more preferably detects whether -10 -the lack of variation in hydrodynamic pressure persists for a predetermined time such as 45 seconds before making the stace-change 74.

When the device 52 enters the warning state, the controller 62 starts an internal timer.

While the device 52 is in the warning state, if and when the timer reaches a count of, ffir example. 15 seconds, the controller 62 causes the device to change to the alarm state, as shown by the state-change 76 in Figure 10. Also, while the device is in the warning state and as shown in Figure 1 IC, the controller 62 sends signals to the piezo-electric element 68 and to the LED 64. The signal to the piezo-electric element 68 is an AC signal such that the housing 54 vibrates at a frequency that can he felt by the swimmer. The signal to the LED 64 may he an intermittent DC current so that the LED flashes. In the warning state the controller 62 also receives an input from the water sensor 66. If and when the controller 62 determines that the device 52 is no longer under water, the device 52 changes back to the standby state, as shown by the state-change 78 in Figure 10.

As shown in Figure liD, when the device 52 is in the alarm state, the controller 62 sends signals to the piezo-electric element 68 and to the LED 64. The signal to the piezo-electric element 68 is a sinusoidal AC signal of a predetermined frequency (for example a single frequency in the range 20 kHz to 30 U-k) such that a sound signal is transmitted into the water as an alarm signal. The signal to the LED 64 may he an intermittent DC current so that the LED flashes. In the alarm state, the controller 62 also receives an input from the water sensor 66. If and when the controller 62 determines that the device 52 is no longer under water, the device 52 changes hack to the standhy state, as shown by the state-change 80 in Figure 10.

It will therefore he understood that, when the device 52 is switched on and worn on a swimmer's wrist, while the swimmer is out of the water the device 52 will he in the standhy state. When the swimmer enters a swimming pool and puts their wrist under water, the device 52 will change to the detect state. While the swimmer's wrist is under water, if no movement of their wrist is detected for a period of 45 seconds, the device 52 will change to the warning state, in which the swimmer's wrist will be vibrated and the LED 64 will tlash. Once in the warning state, if the swimmer responds by lifting their wrist out of the water, the device 52 will revert to the standby state. However, if such a response is absent within 15 seconds, the device 52 will change to the alarm state in which the alarm signal is transmitted into the water and the LED 64 continues to flash.

The swimmer alarm apparatus embodying the second aspect of the invention also includes a remote unit 82 as shown in Figure 1. Similarly to the pool alarm apparatus 10 -11 -previously described, the remote unit 82 comprises a housing 12 containing various electrical components, and a hydrophone 14 connected to the electrical components by a cable 16. In normal use, the housing 12 is placed on the ground 18 near the side of a swimming pool 20 and the hydrophone is suspended by the cable 16 in the water 22 of the pool 20. The unit 82 may be S powered by its own battery (not shown) or by a low-voltage mains adapter (not shown).

Referring to Figure 12 of the drawings, the housing 12 contains a preamplifier 84 which amplifies the signal from the hydrophone 14 with a preset gain. The amplified signal is fed through a band-pass filter 86 having very narrow pass band centred on the frequency of the alarm signal that is transmitted by the piezo-electric element of the device 52 when in the alarm state. The filtered signal is rectified by a rectifier 88 and smoothed by a smoothing filter 90 having a time constant of about 0.1 seconds to produce a signal which is fed to a level detector 92. When the level of the signal Si exceeds a preset threshold level, the level detector 92 activates an audible and/or visible alarm 94. The alarm 94 can be reset by a manually operable element 96.

The threshold level of the level detector 92 is set so that when an alarm signal is transmitted from the device 52 at any position in the swimming pool 20, the level of the signal input into the level detector 92 is sufficient to trigger the alarm 34. The intention is that the alarm will attract the attention of an attendant or other nearby person who will take whatever action is necessary. In the case where there are many swimmers in the pool 20, the flashing LED 64 of the device 52 that caused the alarm to be triggered may assist the attendant in determining which swimmer is in need of assistance.

There is a risk with the unit 82 of Figure 12 of a false detection of an alarm being made if the receiver circuit is triggered by a loud sound such as a splash. rather than by a genuine alarm signal from a swimmer device 52. In order to deal with this, the unit 82 of Figure 12 may he modified as shown in Figure 12. In Figure 12, the output of the preamplifier 84 is also fed in mm through a narrow band-pass filter 98. rectifier 100 and smoothing filter 102 which are similar to the narrow band-pass filter 86, rectifier 88 and smoothing filter 90 except that the pass band of the filter 98 is close to, hut does not overlap, the pass hand of the filter 86. The outputs of the two smoothing filters 90,102 are fed to a differencer 104 which outputs to the level detector 92. The differencer 104 and level detector 92 are arranged so that the alarm 94 is triggered only if the level of the signal from the smoothing filter 90 exceeds the level of the signal from the smoothing filter 102 by more than a threshold amount. Noise, such as a splash, occurs across a spectrum of frequencies and therefore is likely to affect the levels of the signals -12 -output from the smoothing filters 90,102 approximately equally so that the alarm 94 is not triggered. By contrast, the alarm signal transmitted by the swimmer device 52 and received by the hydrophone 14 has a single frequency which is passed by the filter 86 hut attenuated by the filter 98 so that the alarm 94 is triggered.

The pooi alarm apparatuses 10 described with reference to Figures 1 to 9 and the remote units 82 of the swimmer alarm apparatuses described with reference to Figures 10 to 13 may he combined into a single apparatus 106, for example as shown in Figure 14 which combines the pool alarm apparatus of Figure 4 with the remote unit of Figure 13. As can be seen, some of the parts can he used in common in the apparatus 106 for both functions, and a switch 108 may be included to set the mode of operation of the apparatus 106.

Many modifications and developments may be made to the embodiments of the invention described above.

For example, the hydrophone 14 may be mounted in the housing 12, and the latter may he designed to he used underwater.

Particularly in the case of a large body of water, more than one hydrophone 14 may he employed.

Instead of using a piezo-electric element 68 in the device 52 to sense movement of the swimmer, an accelerometer or geophone may he used and may employ MEMS devices.

Instead of using a resistive submersion sensor 66, submersion could he detected by a hydrostatic pressure sensor, in which case a common sensor would be employed for sensing submersion and movement.

When in the warning state, instead of warning the swimmer by vibration of the device 52, the device could be modified to emit a warning sound audible through water, or air and water.

Instead of requiring submersion of the device 52 in order to change it from its warning state to its standby slate, the device 52 could be provided with. fir example, a manually-operable push-button switch.

The device 52 could be arranged to be attached to some part of the swimmer's body other than their wrist.

-13 -It should he noted that the embodiments of the invention have been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.

Claims

-14 -CLAIMS1. An alarm apparatus for a body of water, the apparatus comprising: means for detecting noise in the water indicative of a person falling into the body of water; and means for raising an alarm in response to such detection.
2. An apparatus as claimed in claim 1, wherein: the noise detecting means includes: a hydrophone arranged to he immersed in the water and to produce a source signal corresponding to sound to which the hydrophone is exposed; and means for conditioning or processing the source signal.
3. An apparatus as claimed in claim 2, wherein: the conditioning or processing means includes means for pre-ampliting the source signal to produce an amplified signal.
4. An apparatus as claimed in claim 2 or 3. wherein: the conditioning or processing means includes: means for filtering out low frequencies in the source signal or amplified signal below a first cut-off frequency and/or for filtering out high frequencies in the source signal or amplified signal above a second cut-off frequency to produce a first filtered signal.
5. An apparatus as claimed in any of claims 2 to 4. wherein: the conditioning or processing means includes: means for recti'ing the source signal or the amplified signal or the first filtered signal to produce a rectified signal: and means for filtering the rectified signal by smoothing with a first time constant to produce a first level signal.

-15 -
6. An apparatus as claimed in claim 5, wherein: the conditioning or processing means includes: means for filtering the rectified signal or the first level signal by smoothing with a second longer time constant to produce a second level signal.
7. An apparatus as claimed in claim S or 6, wherein: the conditioning or processing means includes means for delaying the first level signal to produce a third level signal.
8. An apparatus as claimed in claim 6 or 7. wherein: the conditioning or processing means includes means for differencing the first and second level signals. or the second and third level signals, to produce a fourth level signal.
9. An apparatus as claimed in any of claim 6 when indirectly dependent on claim 3, wherein: the preamplifier has a controllable gain arranged so that the gain increases as the level of the second level signal decreases and vice versa.
10. An apparatus as claimed in any of claims 5 to 9, wherein: the alarm means is arranged to raise the alarm when the level of the first level signal or fourth level signal exceeds a predetermined threshold.
11. An alarm apparatus for a body of water, substantially as described with reference to the drawings
12. An alarm apparatus for a swimmer in a body of water, the apparatus comprising: a device arranged to be carried by a swimmer and including means for sensing movement of the device in the body of water; and means responsive to the movement sensing means for raising an alarm is there is a substantial lack of movement.-16 -
13. An apparatus as claimed in claim 12, wherein: the movement sensing means includes means for sensing hydrodynamic pressure fluctuations around the device.
14. An apparatus as claimed in claim 12 or 13, wherein: the alarm means is operable to raise the alarm if the lack of movement persists for longer than a predetermined period of time.
15. An apparatus as claimed in any of claims 12 to 14, further including: means for providing a warning to the swimmer fOr a predetermined period of time after a substantial lack of movement has been detected but before the alarm is raised; and means activatable by the swimmer to prevent the alarm being raised on that occasion.
16. An apparatus as claimed in claim 15. wherein: the preventing means is responsive to the movement sensing means to prevent the alarm being raised on that occasion if movement is detected during said predetermined period of time.
17. An apparatus as claimed in any of claims 12 to 16, further including: means for sensing whether or not the device is submerged in water.
18. An apparatus as claimed in claim 17 when dependent on claim 15 or 16. wherein: the preventing means is responsive to the submersion sensing means to prevent the alarm being raised on that occasion if the device is sensed as being taken out of the water during said predetermined period of time.
19. An apparatus as claimed in claim 17 or 18, wherein: the movement sensing means and/or the warning means and/or the alarm raising means are arranged to be deactivated when the device is not submerged in water.-17 -
20. An apparatus as claimed in any of claims 12 to 19, wherein: the means for raising the alarm comprises: means provided in the device for transmitting an underwater alarm signal; and a remote device for receiving the underwater alarm signal and for raising the alarm in response thereto.
21. An apparatus as claimed in any of claims 12 to 20, thrther including: a transducer which serves as part of the movement sensing means and/or the warning providing means and/or the underwater alarm signal transmitting means.
22. An apparatus as claimed in claim 20, or in claim 21 when dependent on claim 20, wherein: the remote device includes: a hydrophone arranged to be immersed in the water and to produce a received signal corresponding to the underwater alarm signal: and means for conditioning or processing the received signal.
23. An apparatus as claimed in claim 22, wherein: the alarm signal has a predetermined characteristic; and the conditioning or processing means includes means for filtering the received signal to remove components thereof not having the predetermined characteristic and produce a second filtered signal.
24. An apparatus as claimed in claim 23, wherein: the conditioning or processing means is arranged to detect the level of the second filtered signal: and the remote device is arranged to raise the alarm when the level of the second filtered signal exceeds a predetermined threshold.
25. An alarm apparatus for a swimmer in a body of water, substantially as described with reference to the drawings.-18 -
26. An alarm apparatus for a body of water and for a swimmer, wherein: the apparatus is operable in a first mode in which the apparatus is as claimed in any of claims I to 11: and the apparatus is alternately operable in a second in which the apparatus is as claimed in any of claims 12 to 25.