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Sound Sound is the pattern of waves produced in a medium by a vibrating object. The pattern is transmitted through the molecules of the air, water, or other medium. Robert Boyle was the first to demonstrate that sound does not propagate through a vacuum. We perceive sound when vibrations reach our ears. The vibrations induced in our eardrum are transmitted through a series of small connecting bones to the inner ear. In the inner ear, small hairs respond to vibration and stimulate nerve endings that in turn respond to different frequencies and pass the information on to our brain. Our brain interprets the stimuli as sounds. Sound travels best through a dense medium, the closer the molecules of the medium the more efficiently the sound is transmitted. Water is much denser than air and is a much better transmitter of sound. In air at standard temperature and pressure, the speed of sound is about 330 meters per second (1,080 feet per second). In much denser water, the speed of sound is about 1,440 meters per second (4,725 feet per second). Also, because sound is transmitted very efficiently underwater, sounds underwater can be heard over great distances. We perceive the direction of a sound because our brain is able to measure and interpret the difference in time between the sound reaching our two ears. This time delay works very well for us in air, the environment to which we are adapted. In water however, sound travels over four times faster than in air, and the time delay is about four times less. As a result it is almost impossible to locate the direction of a sound source underwater. Underwater you can hear sounds generated in the water very well, but it is as though someone had turned off the stereo on your radio. Heat Heat and temperature are very important to the diver. Being comfortably warm is the difference between having an enjoyable dive and a miserable experience. Exposure to cold water is not only uncomfortable, it can be much more dangerous and life-threatening than exposure to air of the same temperature. Chapter 3- Diving Physics Heat, or thermal energy, is associated with the kinetic energies or motions of all of the molecules in a substance. Heat is closely related to temperature, but it is not the same. Different substances may have the same temperature, but have very different quantities of heat available for transfer. A substance may even absorb heat without any change in temperature by changing from one physical state to another, as from a solid to a liquid (melting ice) or from a liquid to a gas (boiling water). Heat is a form of energy, and temperature is a measure of the amount of heat energy present in a body. If two objects of different temperatures are brought together, heat energy always flows from the hotter body to the colder body. Since all energy is the ability to do work, units of energy are expressed in units of work. Heat is no exception. The two most common units of heat are the calorie (SI/metric) and the British Thermal Unit or Btu (U.S./Imperial). The calorie is the amount of heat required to raise the temperature of one gram of pure water 1°C. The British Thermal Unit is the amount of heat required to raise one pound of water 1°F. A gramcalorie is not the same as a food Calorie (capital “C”). A Calorie is a kilogram-calorie, equal to 1,000 calories. One Btu equals 252 calories. The specific heat of a substance is the ratio of the amount of heat necessary to raise the temperature of a body by one degree compared to the amount of heat required to raise an equal mass of water by one degree. It requires one calorie to raise one gram of water by one degree Celsius, so the specific heat of any substance is numerically equal to the amount of heat required to raise one gram of that substance by one degree Celsius. Heat capacity is the heat absorbed by a material in calories per degree Celsius in terms of the actual amount of material under consideration. Heat capacity equals the specific heat of the body times its mass. Water has a high specific heat (1.0) compared with air (0.24). About four times as much heat is required to heat a quantity of water equal in mass/weight to a quantity of air. Volume for volume, water also has a heat capacity that is nearly 3,000 times that of air. It can store enormous quantities of heat. Because water Diving Physics 99


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