CA2141619A1 - Ultrasonic paging device for underwater divers - Google Patents
Ultrasonic paging device for underwater diversInfo
- Publication number
- CA2141619A1 CA2141619A1 CA002141619A CA2141619A CA2141619A1 CA 2141619 A1 CA2141619 A1 CA 2141619A1 CA 002141619 A CA002141619 A CA 002141619A CA 2141619 A CA2141619 A CA 2141619A CA 2141619 A1 CA2141619 A1 CA 2141619A1
- Authority
- CA
- Canada
- Prior art keywords
- divers
- diver
- ultrasonic
- water
- devices
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B13/00—Transmission systems characterised by the medium used for transmission, not provided for in groups H04B3/00 - H04B11/00
- H04B13/02—Transmission systems in which the medium consists of the earth or a large mass of water thereon, e.g. earth telegraphy
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B11/00—Transmission systems employing sonic, ultrasonic or infrasonic waves
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
- Y02D30/00—Reducing energy consumption in communication networks
- Y02D30/70—Reducing energy consumption in communication networks in wireless communication networks
Abstract
In devices used by underwater divers to summon other divers from a distance, an audible sound is transmitted through the water and must be heard by a diver's ear. In this invention, each diver carries an electronic device that transmits modulated ultrasonic acoustic waves through the water when he presses a pushbutton switch. This causes a similar device carried by another diver to emit an audible sound that the diver can easily hear. This device is effective over a greater distance because noise from breathing apparatus and the barrier of a diver's protective hood do not interfere with ultrasonic acoustic signals as they do with the detection of sound by a diver's ear. A number of different waveforms are used to modulate the acoustic carrier wave. These are used to convey information between divers, and to achieve separate operating channels so that independent groups of divers can use these devices in one area.
Description
~ 2l~l6l9 Sp~ifi~li 11 This invention }elates to devices used to summon u..~ divers from a distance.
Common devices used to summon underwater divers produce audible sound in the water by rn~h ~ l or ek,~ v..~ ' i means. They rely on the ears of divers to detect the sound. The distance over which they are effective depends on the level of interfering noise from breathing apparatus, ocean surf, boats, and other sources. It is typically under 30 meters. In cold water diving, divers wear a protective hood made of foam rubber which blocks sound, so that the effective distance is less than 3 meters. As a result, such sound generators are seldom used by cold water divers.
We have invented an ~.dc;~ g (paging) device that operates by ~
and receiving modulated ultrasonic acoustic ~vaves in the water. It transmits when a push-button switch is pressed. When it receives an ultrasonic signal, it produces a audible sound that can be heard easily by the diver who carries the device. It is superior to audible sound generators in three ways.
First, the device is effective over a greater distance because it's receiver is more sensitive than the human ear. Second, it's range is not reduced by sources of audible noise or by the use of a foam rubber diving hood. Third, a number of different waveforms can be used to modulate the carrier wave. These different waveforms are used to convey simple messages between divers by causing the receiving device to produce various different sounds. Different ~ i"" waveforms are also used to achieve separate operating channels, so that a number of I, ~ groups of divers can use these devices within range of each other.
Such ~ devices are required because vocal and visual ~.,"." .~ .., between divers is difficult. It is not possible to speak clearly with commonly used breathing apparatus, and 2~41619 .
vocal~y produced sounds can typically be heard less than 10 meters away. Peripheral vision is poor through a scuba diving mask, so that two divers do not always see each other for visual ,~ f~n Visibility can be limited by suspended particles in the water. As a result of these fliffif.-lltif c a diver who stops to make O~a~l v~lLivl-~ or has an emergency situation is oflen lefl behind by his partners.
Following is a list of the drawings which illustrate one ., . i ,ofl;, .. .1 of this invention. These drawings specifically illustrate the uu~ u~,lioll and function of a prototype device which we have built.
Figures I, 2, and 3 are views that show the major ~.,..1,~,....~t~
Figure 4 shows how the device may be carried by a diver.
Figure 5 is a block diagram of the electronic circuitry.
Figure 6 illustrates the ultrasonic acoustic signal.
Figure 7 lists the set of mflf~ stif~n signal rlt;4..~,.1ci~ ~.
Figures 8, 9, and 10 are views that show the acoustic transducem;u~ l u~,liu~.
Figure 11 shows the radiation pattem of the acoustic transducer.
The device illustrated in Figures I, 2, and 3 consists of a ~vaterproof housing I that contains electronic circuitry on a printed circuit board 2 which is powered by a battery 3. A speaker ~, mounted in a recess beneath the printed circuit board, vibrates the housing to create sound that can be heard by the diver who carries the device. Two ~ ,lc pushbutton switches 5, labeled "page"
and "emergency," are used to activate the transmitter. A transducer 6, which transmits and receives the ultrasonic acoustic waves, is mounted on the outside of the housing. A large stainless steel clip 7 2~
is used to mount this device to the strap of a diver's mask as shown in Figure 4. The device can also be mounted on other straps on various pieces of the diver's equipment.
Figure S shows, in the form of a block diagram, the electronic circuits that comprise the device's transmitter and receiver. The transmitter is activated when one of the push-button switches I
is pressed. The mor~ tjnn generator 2 produces a different frequency of square wave "c ' ' on signal in response to each push-button switch. A carrier oscillator 3 generates the carrier wave at an ultrasonic frequency (200 ki'~z). This car ier is modulated by the amplitude modulator 4 and then appiied to a power amplifier 5 which increases it's amplitude to a high voltage. This electrical signal is applied to the p;~.~.U~ l iC transducer 6, which generates ultrasonic acoustic waves in the water.
The receiver operates if no switch is pressed. An electrical signal from the transducer 6, produced in response to acoustic waves in the water, is applied to a bandpass filter 7 which passes oniy signals near the ultrasonic carrier frequency. The signal's amplitude is increased by an amplifier 8 before it is applied to the i~ ' ' 9, which recovers the m~ lqti(m signal. This signal is applied to a detector 10 which detects the presence of one of the expected modulation signals and triggers the sound generator 11 to produce an .~",.~",, ;~L~ signal at audible r.~,u~l.ci~ (800-1100 Hz in this example~. The speaker 12 then produces a sound that the diver can hear. Different sounds are produced depending on which m~ lqtjl~n signal is detected.
Figure 6 illustrates the amplitude-modulated ultrasonic acoustic wave that is transmitted through the water. Since the ~"0 ~ ; -, signal in this case is a square wave, the 200 kHz acoustic carrier wave is simply tumed on and offat the frequency ofthe ",.~.1.,1.~;.." signal. Nine different ""~ ;, .., L~ ,.c;.,, are used for different purposes, as listed in Figure 7. Eight modulation 21~1~19 , ~
~IG.~u.,.~G;-.~ are used by different groups of divers to page only the other divers of the same group.
These are used when the push-button labeled "page" is pressed. Each device transmits and receives only one of these eight ~of~ otif n .',~ ,.,. The ninth ~ ' ' " frequency is used when the push-button labeled "~ ,y" is pressed. This is used by any diver to indicate to all other divers that there is an emergency situation. All of the paging devices can transmit and receive with this modulation frequency.
The acoustic signal illustrated in Figures 6 and 7 is that used by our prototype device. Other carrier Gc~4~ may be used, and many other types of modulation are possible. The carrier wave may be frequency-modulated rather than amplitude--~ ~(' ' ' 1 Modulation waveforms other than a square-wave Gan be used, and messages other than 'page" or C'~ ""_y" can be conveyed.
Figures 8, 9, and 10 show the construction of the ultrasonic transducer assembly. A smgle transducer is used for both frs~omiooi~n and reception of the ultrasorlic acoustic waves. It consists of a cylinder of l.: " ~ ceramic material I which is ~ ,.lly resonant at the ultrasonic carrier frequency. Each circular end of the ceramic cylinder is plated with metal to serve as an electrode. A
cylinder of cork 2 surrounds the ceramic to prevent acoustic waves from escaping from it's sides. A
sleeve of heat-shrink plastic tubing 3 surrounds the cork to hold it tightly to the ceramic cylinder.
Glue secures these three ,v ~ ~I .v~, ,1. to a metal bracket 4 which is fastened to one of the two mounting screws 5. A thin wire G is connected, using solder, between each of the mourlting scre~vs and one of the metal-plated end surfaces of the ceramic cylinder. All of the ~, , are then encased in molded epoxy plastic 7. The plastic protects the ~UIII,UU ' yet conducts the acoustic waves well between the water and the two circular faces of the ceramic cylinder. The two screws 5 ~ 214~19 erve to mount the assembly to the housing of the paging device. They also serve as the electrical to the l.~ . ,r crystal.
E1igure 11 shows the acoustic radiation pattern of the transducer for a 0.4 inch diameter ceramic cylinder which is resonant at 200 kHz. It radiates wide beams from both the front and rear surfaces. It therefore transmits and receives well in many directions. This is essential for successful operation of the paging device.
We have d~lllUII~ I that our prototypes operate reliably to distances over 30 meters when randomly oriented, and to over 130 meters when the acoustic transducer points at another diver.
Transmitter power is ~ 200 milliwatts. The receiver circuitry detects signals as small as 3 microvolts RMS from the transducer. The circuitry is designed for low power . , so that a common 9 volt battery lasts for 12 hours of normal operation.
A carrier frequency of 200 kHz was selected for our prototype. Ultrasorlic &~4..~,.1GiC~ from 10 kHz to ~ lo 'y 2000 kHz can be used for these devices. Higher OEequency waves suffer more attenuation as they pass through the water, and a wide beam is mote difficult to achieve.
Greatest operating range is therefore achieved at lower carrier fi~l~u.,.~;.,;,. However, ~ du~,~,,s for r,~ .,c.~,s below 100 kHz require large ceramic cylinders and additional c....~ , and so are more expensive to construct.
A frequency range of 800-1100 Hz was selected for the sounds ptoduced by the speaker. This is near the resonant frequency of the ~ o~ lic speaker that we used. We have found that a foam rubber diving hood attenuates r,~4u.,..c.~, . higher than 1500 Hz, and it is desirable to use low 21~
cics for best ~ . through the rubber. However, r.c4 ~lc;~ below 800 Hz are difficult to generate at sufficient amplitude with a small speaker.
Ultrasonic diver pagers may be used in ~U~ liUIl with a similar device located on a boat tbat is serving as a diving tender. From the boat, it is possible to page a group of divers, monitor for emergency situations, recall divers to the boat, etc A boat-based device is functionally similar to the UIA~ . ;t~,l diver devices, but can be powered from a boat's electrical system and need not be . Powem;~ iv - and size are not critical, so higher transmitter power and receiver sensitivity can be achieved to give greater range The ultrasonic transducer can be dropped into the water or mounted on the boat's hull, and connected to the device by a cable.
Common devices used to summon underwater divers produce audible sound in the water by rn~h ~ l or ek,~ v..~ ' i means. They rely on the ears of divers to detect the sound. The distance over which they are effective depends on the level of interfering noise from breathing apparatus, ocean surf, boats, and other sources. It is typically under 30 meters. In cold water diving, divers wear a protective hood made of foam rubber which blocks sound, so that the effective distance is less than 3 meters. As a result, such sound generators are seldom used by cold water divers.
We have invented an ~.dc;~ g (paging) device that operates by ~
and receiving modulated ultrasonic acoustic ~vaves in the water. It transmits when a push-button switch is pressed. When it receives an ultrasonic signal, it produces a audible sound that can be heard easily by the diver who carries the device. It is superior to audible sound generators in three ways.
First, the device is effective over a greater distance because it's receiver is more sensitive than the human ear. Second, it's range is not reduced by sources of audible noise or by the use of a foam rubber diving hood. Third, a number of different waveforms can be used to modulate the carrier wave. These different waveforms are used to convey simple messages between divers by causing the receiving device to produce various different sounds. Different ~ i"" waveforms are also used to achieve separate operating channels, so that a number of I, ~ groups of divers can use these devices within range of each other.
Such ~ devices are required because vocal and visual ~.,"." .~ .., between divers is difficult. It is not possible to speak clearly with commonly used breathing apparatus, and 2~41619 .
vocal~y produced sounds can typically be heard less than 10 meters away. Peripheral vision is poor through a scuba diving mask, so that two divers do not always see each other for visual ,~ f~n Visibility can be limited by suspended particles in the water. As a result of these fliffif.-lltif c a diver who stops to make O~a~l v~lLivl-~ or has an emergency situation is oflen lefl behind by his partners.
Following is a list of the drawings which illustrate one ., . i ,ofl;, .. .1 of this invention. These drawings specifically illustrate the uu~ u~,lioll and function of a prototype device which we have built.
Figures I, 2, and 3 are views that show the major ~.,..1,~,....~t~
Figure 4 shows how the device may be carried by a diver.
Figure 5 is a block diagram of the electronic circuitry.
Figure 6 illustrates the ultrasonic acoustic signal.
Figure 7 lists the set of mflf~ stif~n signal rlt;4..~,.1ci~ ~.
Figures 8, 9, and 10 are views that show the acoustic transducem;u~ l u~,liu~.
Figure 11 shows the radiation pattem of the acoustic transducer.
The device illustrated in Figures I, 2, and 3 consists of a ~vaterproof housing I that contains electronic circuitry on a printed circuit board 2 which is powered by a battery 3. A speaker ~, mounted in a recess beneath the printed circuit board, vibrates the housing to create sound that can be heard by the diver who carries the device. Two ~ ,lc pushbutton switches 5, labeled "page"
and "emergency," are used to activate the transmitter. A transducer 6, which transmits and receives the ultrasonic acoustic waves, is mounted on the outside of the housing. A large stainless steel clip 7 2~
is used to mount this device to the strap of a diver's mask as shown in Figure 4. The device can also be mounted on other straps on various pieces of the diver's equipment.
Figure S shows, in the form of a block diagram, the electronic circuits that comprise the device's transmitter and receiver. The transmitter is activated when one of the push-button switches I
is pressed. The mor~ tjnn generator 2 produces a different frequency of square wave "c ' ' on signal in response to each push-button switch. A carrier oscillator 3 generates the carrier wave at an ultrasonic frequency (200 ki'~z). This car ier is modulated by the amplitude modulator 4 and then appiied to a power amplifier 5 which increases it's amplitude to a high voltage. This electrical signal is applied to the p;~.~.U~ l iC transducer 6, which generates ultrasonic acoustic waves in the water.
The receiver operates if no switch is pressed. An electrical signal from the transducer 6, produced in response to acoustic waves in the water, is applied to a bandpass filter 7 which passes oniy signals near the ultrasonic carrier frequency. The signal's amplitude is increased by an amplifier 8 before it is applied to the i~ ' ' 9, which recovers the m~ lqti(m signal. This signal is applied to a detector 10 which detects the presence of one of the expected modulation signals and triggers the sound generator 11 to produce an .~",.~",, ;~L~ signal at audible r.~,u~l.ci~ (800-1100 Hz in this example~. The speaker 12 then produces a sound that the diver can hear. Different sounds are produced depending on which m~ lqtjl~n signal is detected.
Figure 6 illustrates the amplitude-modulated ultrasonic acoustic wave that is transmitted through the water. Since the ~"0 ~ ; -, signal in this case is a square wave, the 200 kHz acoustic carrier wave is simply tumed on and offat the frequency ofthe ",.~.1.,1.~;.." signal. Nine different ""~ ;, .., L~ ,.c;.,, are used for different purposes, as listed in Figure 7. Eight modulation 21~1~19 , ~
~IG.~u.,.~G;-.~ are used by different groups of divers to page only the other divers of the same group.
These are used when the push-button labeled "page" is pressed. Each device transmits and receives only one of these eight ~of~ otif n .',~ ,.,. The ninth ~ ' ' " frequency is used when the push-button labeled "~ ,y" is pressed. This is used by any diver to indicate to all other divers that there is an emergency situation. All of the paging devices can transmit and receive with this modulation frequency.
The acoustic signal illustrated in Figures 6 and 7 is that used by our prototype device. Other carrier Gc~4~ may be used, and many other types of modulation are possible. The carrier wave may be frequency-modulated rather than amplitude--~ ~(' ' ' 1 Modulation waveforms other than a square-wave Gan be used, and messages other than 'page" or C'~ ""_y" can be conveyed.
Figures 8, 9, and 10 show the construction of the ultrasonic transducer assembly. A smgle transducer is used for both frs~omiooi~n and reception of the ultrasorlic acoustic waves. It consists of a cylinder of l.: " ~ ceramic material I which is ~ ,.lly resonant at the ultrasonic carrier frequency. Each circular end of the ceramic cylinder is plated with metal to serve as an electrode. A
cylinder of cork 2 surrounds the ceramic to prevent acoustic waves from escaping from it's sides. A
sleeve of heat-shrink plastic tubing 3 surrounds the cork to hold it tightly to the ceramic cylinder.
Glue secures these three ,v ~ ~I .v~, ,1. to a metal bracket 4 which is fastened to one of the two mounting screws 5. A thin wire G is connected, using solder, between each of the mourlting scre~vs and one of the metal-plated end surfaces of the ceramic cylinder. All of the ~, , are then encased in molded epoxy plastic 7. The plastic protects the ~UIII,UU ' yet conducts the acoustic waves well between the water and the two circular faces of the ceramic cylinder. The two screws 5 ~ 214~19 erve to mount the assembly to the housing of the paging device. They also serve as the electrical to the l.~ . ,r crystal.
E1igure 11 shows the acoustic radiation pattern of the transducer for a 0.4 inch diameter ceramic cylinder which is resonant at 200 kHz. It radiates wide beams from both the front and rear surfaces. It therefore transmits and receives well in many directions. This is essential for successful operation of the paging device.
We have d~lllUII~ I that our prototypes operate reliably to distances over 30 meters when randomly oriented, and to over 130 meters when the acoustic transducer points at another diver.
Transmitter power is ~ 200 milliwatts. The receiver circuitry detects signals as small as 3 microvolts RMS from the transducer. The circuitry is designed for low power . , so that a common 9 volt battery lasts for 12 hours of normal operation.
A carrier frequency of 200 kHz was selected for our prototype. Ultrasorlic &~4..~,.1GiC~ from 10 kHz to ~ lo 'y 2000 kHz can be used for these devices. Higher OEequency waves suffer more attenuation as they pass through the water, and a wide beam is mote difficult to achieve.
Greatest operating range is therefore achieved at lower carrier fi~l~u.,.~;.,;,. However, ~ du~,~,,s for r,~ .,c.~,s below 100 kHz require large ceramic cylinders and additional c....~ , and so are more expensive to construct.
A frequency range of 800-1100 Hz was selected for the sounds ptoduced by the speaker. This is near the resonant frequency of the ~ o~ lic speaker that we used. We have found that a foam rubber diving hood attenuates r,~4u.,..c.~, . higher than 1500 Hz, and it is desirable to use low 21~
cics for best ~ . through the rubber. However, r.c4 ~lc;~ below 800 Hz are difficult to generate at sufficient amplitude with a small speaker.
Ultrasonic diver pagers may be used in ~U~ liUIl with a similar device located on a boat tbat is serving as a diving tender. From the boat, it is possible to page a group of divers, monitor for emergency situations, recall divers to the boat, etc A boat-based device is functionally similar to the UIA~ . ;t~,l diver devices, but can be powered from a boat's electrical system and need not be . Powem;~ iv - and size are not critical, so higher transmitter power and receiver sensitivity can be achieved to give greater range The ultrasonic transducer can be dropped into the water or mounted on the boat's hull, and connected to the device by a cable.
Claims (4)
1) An electronic paging device for use by underwater divers to summon each other's attention, which transmits modulated ultrasonic acoustic waves when a push-button switch is pressed, and which produces an audible sound when such acoustic waves are received.
2) A device as defined in claim 1 that transmits using a variety of different modulation signals, and produces different audible sounds when it receives those signals, in order to convey simple messages between divers.
3) A device as defined in claim 1 that uses a variety of different modulation signals to achieve isolated operating channels so that independent groups of divers can use these devices in one area.
4) A device as defined in claims 1, 2, and 3 that uses a 200 kHz carrier wave which is amplitude-modulated by a square wave having a frequency in the range of 100 Hz to 10,000 Hz.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002141619A CA2141619A1 (en) | 1995-02-01 | 1995-02-01 | Ultrasonic paging device for underwater divers |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002141619A CA2141619A1 (en) | 1995-02-01 | 1995-02-01 | Ultrasonic paging device for underwater divers |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2141619A1 true CA2141619A1 (en) | 1996-08-02 |
Family
ID=4155163
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002141619A Abandoned CA2141619A1 (en) | 1995-02-01 | 1995-02-01 | Ultrasonic paging device for underwater divers |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2141619A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6125080A (en) * | 1997-08-18 | 2000-09-26 | Divecom Ltd. | Underwater communication apparatus and communication method |
| US6130859A (en) * | 1997-12-01 | 2000-10-10 | Divecom Ltd. | Method and apparatus for carrying out high data rate and voice underwater communication |
-
1995
- 1995-02-01 CA CA002141619A patent/CA2141619A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6125080A (en) * | 1997-08-18 | 2000-09-26 | Divecom Ltd. | Underwater communication apparatus and communication method |
| US6130859A (en) * | 1997-12-01 | 2000-10-10 | Divecom Ltd. | Method and apparatus for carrying out high data rate and voice underwater communication |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US6125080A (en) | Underwater communication apparatus and communication method | |
| US5010529A (en) | Underwater transmitter apparatus | |
| US7388512B1 (en) | Diver locating method and apparatus | |
| US4912464A (en) | Anchor alarm for boats and the like | |
| US3789353A (en) | Diver communication system | |
| US3887896A (en) | Active sonar image perception | |
| US5450060A (en) | Motion detection | |
| US3267414A (en) | Portable underwater communication unit | |
| US8063771B2 (en) | Marine personal locator apparatus | |
| US3908168A (en) | Radio transmission system | |
| CA2141619A1 (en) | Ultrasonic paging device for underwater divers | |
| GB2287564A (en) | Overboard alarm and detector | |
| US3489993A (en) | Ultrasonic homing beacon and communication equipment for underwater swimmers | |
| US4276623A (en) | Underwater audio intercommunication system | |
| JP2851874B2 (en) | Ultrasonic underwater wireless communication device | |
| RU97104707A (en) | DEVICE FOR WIRELESS ALARM AND TECHNOLOGICAL SIGNALING AND COMMUNICATION | |
| GB2205433A (en) | Doorbell operated by radio or ultrasonic waves | |
| GB2133599A (en) | Monitoring system for divers | |
| GB2220483A (en) | A monitor for measuring noise radiated from a ship | |
| JP2946527B2 (en) | Protective radio equipment | |
| Lee et al. | Miniature flextensional acoustic transducers for long range location tracking | |
| KR0127321Y1 (en) | Noise elimination device for signal transmission module in towed acoustic counter module | |
| AU645643B2 (en) | Telephone recording device | |
| Webster et al. | Evaluation of a Modulated Air‐Flow Loudspeaker | |
| Steere | Underwater communications system for use by free swimming divers. |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |