BACKGROUND:

If a diaphragm submerged in water is caused to vibrate by electrical means, it has mechanical energy of motion and this energy is communicated to the water. If another diaphragm is submerged in the water near the vibrating diaphragm, the acoustic energy in the water will excite mechanical vibrations in the second diaphragm. These vibrations may be detected by electrical means to complete a flow of mechanical energy from the first diaphragm to the second. The first diaphragm is called the source, or transducer, and the second is called a receiver or hydrophone. In Aquacom®, the transducer and hydrophone are one and the same.

FACTORS THAT AFFECT SOUND IN WATER:

The sound intensity from a source varies inversely as the square of the distance from the source. This sort of variation is referred to as spherical spreading. Other factors also influence the variation of sound intensity with distance. As the sound passes through the water, some of the energy is absorbed and converted to heat (attenuation) and some of the energy is scattered by fish, pilings, seaweed, bubbles, etc. (diffraction). In addition, both the surface and bottom may affect the sound intensity by reflecting sound back into the water. The sound reflected by the surface and bottom may raise the intensity above normal levels (reinforcement) or it may introduce destructive interference. The bending of the sound waves by temperature variations also has a great effect on the sound intensity at points remote from the source.

In addition to these factors, water density is also important. Because the density of sea water varies with the temperature, the salt content, and the static pressure, the effect on sound of each of these three factors is usually considered separately.

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 frequently the case), the velocity of sound decreases with depth and the waves of sound are bent downward.

Figure 1

Thermoclines will effect the ultrasonic signal. Divers must report thermocline depth/s so surface and/or other divers can be as close to the same depth as possible. 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 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.

If the temperature of the water decreases with depth at the rate of 1° F. 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.

Figure 2

ZONES OF SILENCE:

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. (see Figure 2).

Figure 3

Marine organisms play an important role in underwater acoustics. They are important primarily because of the effect they have on sound transmission, but they sometimes serve as sources of underwater noise as well. High background noise, whether man-made, animal or environmental (waves, rain) can interfere with good communications. Communication through indirect and direct paths. Note kelp scattering sound. Without surface and bottom bounce, diver would lose communications.

ZONES OF SILENCE CAUSED BY DIVING SUITS:

Under some conditions, when your diving suit is directly between the transmitting source and your AQUACOM®, a small zone of silence may be created which prevents reception. This affect becomes greater at longer ranges. Turning approximately 45° in any direction eliminates this zone of silence. Since most divers are in constant motion, it is unlikely that anything more than a momentary signal loss will occur when acoustic energy is blocked by air inside a dry suit, by gas bubbles within neoprene wet suit material, or when both are present such as with a partially inflated dry suit.

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.

The most common of these facts are marine biological noise which can be suppress by the use of the Squelch function and thermoclines. 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 (see Figure 7, one & two).

Now that you have read this operator manual and have become familiar with the system, you are ready for your first communications dive. If you apply what you've learned, especially the proper positioning of the earphone, transducer and slow, deliberate, relaxed speech, you can look forward to a new dimension in underwater experience.