COMMERCIAL DIVER SUPPORT INTERNATIONAL

OUTFITTING, SUPPORT, CONTRACT DIVER SERVICES - TECHNOLOGIST CENTER | EST 1979

Underwater Wireless Diver Communication

Sound Underwater

Sound travels in water at a variable speed depending on the immediate conditions it encounters along its path. Sound waves in water travel near 1500 meters/second. The speed of sound will vary depending on the level of temperature, salinity and pressure. These variations can greatly change the way sound travels through water, as changes in sound speed between bodies of water, either vertically in the water column or horizontally across the ocean, cause the trajectory of sound waves to bend.

Underwater Wireless Technology

Acoustical energy has been used for decades to wirelessly transmit voice communications underwater. However, The acoustical energy from a personal is not strong enough to travel very far through water. The acoustical energy is amplified and transmitted through a transducer, through water as acoustical energy. Each diver carries a wireless diver-intercom, which has a transducer attached. Surface diver-intercoms would have a remotely attached transducer suspended in the water. The voice transmissions are sent through the attached transducers as acoustical energy, and picked-up by the other dive-intercom's transducers. We have been introducing divers for many years to this exciting affordable technology. This technology is well proven to work, and in use by sport, rescue, safety, research, movie production, law enforcement, and military divers for decades.

Applied Technology

As the acoustical energy passes through the water, some of this energy is absorbed and converted to heat (attenuation) and some of the energy is reflected and scattered by fish, physical objects, plants, and even bubble (diffraction) and other variables including:

Surface & Bottom

In addition, both the water's surface and bottom material affects the sound intensity by reflecting. The sound reflected by the surface and bottom may raise the intensity above normal levels (reinforcement) or it may introduce destructive interference. Bending sound waves by temperature variations also has a great effect on sound intensity.

Water Density

Sea water density varies with temperature, salt content, and static atmospheric pressure; each effecting sound separately.

Water Temperature

Variations in water temperature affect sound transmission most. In some areas of the ocean, the temperature changes at a fixed rate over large ranges of depth. If the temperature increases with depth at a fixed rate, the velocity of sound increases at a rate constant with depth and sound waves are refracted toward the surface. If, however, the temperature decreases with the depth, as is typical, the velocity of sound decreases with depth and the waves of sound are bent downward.

Thermocline

Thermoclines will effect the ultrasonic signal. The thermocline depth must be determined for surface/diver and diver/surface transmissions to work; or, all divers remain in close distance to each other as possible and in the same thermocline region (above or below it). There are also areas in the sea where, at some depth, temperature changes rapidly over a small depth range. Such a layer is sometimes referred to as a thermocline and sometimes as a thermal layer. Such layers, in addition to producing rather sharp bending of the sound waves by refraction effects, can serve as reflecting surfaces.

The best method to deal with thermoclines is to get the divers and/or transducers as close to each other as possible. If a diver enters and thermocline, he should report it to everyone so everyone knows which depth the thermocline is at. All divers should stay within that depth and the surface station should try and lower the surface transducer below or above which ever is the case.

Acoustic Shadows

If the source of the sound is near the surface, there is some point beyond which sound is not received from the source. This point is said to be in a "SHADOW ZONE". The distance from the source to the 'shadow zone' is determined by the rate of change of temperature with depth, the depth of the source, and the depth at which the reception is made.

If the temperature of the water decreases with depth at the rate of 1F for each 30 feet (starting at the surface), most of the sound energy originating at the source near the surface will travel along paths that are bent rather sharply downward. Therefore the sound energy may not reach a shallow detector positioned l000 yards from the source but may reach a deeper detector position further from the source. Greater temperature variations can cause these paths to bend more sharply.

The velocity of sound-transmission changes only about one percent for a temperature change of l0? F. However, the resultant bending of the sound path has great effect over a distance of several hundred yards

Large natural or man-made objects can block acoustical transmission under certain conditions, in much the same way that a rock blocks a fast moving current of water. Close to the backside of the rock, in this example, the current is absent and the water seems still. A short distance away, the current is flowing again.

Similarly, acoustic energy in the water can be blocked if the transmitting source is close to a large object. On the backside of the blocking object, a "ZONE OF SILENCE" is formed in which reception of the transmitted signal is not possible. Divers can reduce "ZONES OF SILENCE" by moving away from, around, or above the blocking object until communication is reestablished (see Figure 3). Most Single Sideband signals are efficient enough to permit communications around blocking objects. Reception is made mainly through surface and bottom reflections. Man-made noise may be present underwater in busy harbors, shipping lanes and many coastal locations, particularly at lower frequencies. The outstanding characteristic of this coastal ambient noise is its great variability from place to place in the same harbor and from time to time at the same place.

Marine Organisms

Aquatic organisms affect underwater acoustics. They effect sound transmission and can be the source for background noise.

Environment Noise

Man-made and natural noises, animal or environmental (waves, rain), can interfere with good wireless underwater communications.

Dive Suit

When a dive-suit comes directly between a transmitting source and the diver's wireless intercom, a small zone-of-silence (ZOS) may be created, preventing acoustical energy reception. The effect becomes greater when the source becomes further distance from the receiver. Acoustic energy can be blocked by air inside a dry suit, by gas bubbles within neoprene wet suit material, and in combination when encountering neoprene dry suit.

Understanding

There are many factors that affect the propagation of sound in water. All of these factors vary depending upon location, depth, and time of day. The net result is that communication in water can be affected by local conditions and the kind and depth of dive being conducted. Fluctuations in range and intelligibility are to be expected.

After reading and understanding the 'operator/user manual' and becoming familiar with the diver intercom hardware, a operator is ready to try diver communications. If applying proper positioning of the earphone, transducer and slow, deliberate, relaxed speech, one can look forward to a new dimension in the underwater experience.