Miscellaneous Phenomena at Underwater
Underwater Voice | Sound: In the air, sound travels at about 335 meters per second, but the speed of sound varies greatly in different substances capable of transmitting it. In water, sound travels more than four times as fast as in air. Both air and water transmit sound well, but unfortunately, all but one ten-thousandth part of the sound energy is lost when being transmitted from air to water; therefore, for divers to hear each other or to hear sounds from the air above them, the sounds must originally be very loud.
Breathing an oxy-helium gas mixture or air at high pressures changes the voice to a higher pitch. The higher the pressure the greater this effect becomes until at great depths the voice becomes so distorted that telephone communication is almost impossible. This effect is due to the different speeds of sound in gases of different densities. Devices have been developed to correct the change of frequency under these conditions and allow intelligible speech to be transmitted when using oxy-helium mixtures.
The fact that sound in water travels much faster than in air makes it almost impossible to pinpoint a source of sound underwater, and it is possible to obtain only a general idea of the direction.
Transfer of heat Underwater
A diver may lose their body heat by conduction, convection, or radiation, but mostly by conduction and minimum by radiation.
conduction: This is the direct transmission of heat through a substance or materials in contact with each other. An unprotected diver will lose heat to the surrounding water by direct conduction through the skin, the rate at which they lose the heat
depends upon the difference in temperature between the body and the water. If the water temperature is less than 32°C to 33°C, the temperature difference between the diver’s body and the water is such that if unprotected they will lose heat faster than their body can produce it and they will chill at rest.
Conductivity: This is a measure of the affluence with which a material transmits heat. A material of very low conductivity placed between the body and the surrounding water acts as an insulator and decreases the conduction of heat from the body to the water. The insulation provided by wool clothing and foam-type materials results mainly from the fact that they comprise a series of air pockets that place a layer of air between the body and the colder surrounding media. The detention of air in these air pockets also greatly reduces the loss of heat by convection.
Convection: This is the transmission of heat by the movement of air or liquids. The unprotected diver standing more or less still underwater will lose heat, not only by conduction but also by the fact that the water in contact with their skin, warmed by conduction, will develop and, becoming slightly lighter than the surrounding water, will rise to the surface to be replaced by cold water. Thus convection currents are set up that take away the water warmed by conduction through the skin and the diver is continuously bathed in cold water taking its place. Provided the water temperature is not less than 15°C, a good-fitting wool under-suit may suffice to prevent the diver from being chilled. In colder water, insulation with a ‘dead’ air space must be provided by wearing an under-suit under a watertight suit or by wearing a ‘wet’ suit of cellular foam material that does not lose its insulating value when wet and which reduces convection effects on the diver’s skin by trapping a film of water.
Radiation: This is the transfer of heat by invisible waves as experienced when standing in sunlight or front of an electric radiator. The amount of body heat the diver loses by radiation is unimportant compared with the loss by conduction.
Note! The high thermal conductivity of helium (approximately 6 times that of air) draws heat away from the diver at a great rate. Therefore when using oxy-helium mixtures the diver must dress appropriately for the circumstances.
Conclusion:
Understand the behavior of Sound and Heat Underwater
