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Where When How N/D 2017 - J/F 2018 : Page 90

Figure 1: The compressed areas represent regions of high pressure, while the expanded areas are regions of low pressure. So a sound wave alter-nately compresses and expands whatever medium it travels through. Figure 2: A sound wave is like a transverse wave in that when particles are compressed under high pressure, the wave goes up; and when the parti-cles expand under low pressure, the wave goes down. Sound moves in a wave but these wave particles move back and forth (pushed and pulled), not up and down, as is the case with a wave on the water’s surface. Water waves are called transverse waves because their particle movement is up and down, not back and forth. However, as shown in Figure 2, we can depict a sound wave like a transverse wave by showing the change in pressure as the wave moves through a medium. When particles are compressed under high pressure, the wave goes up; and when the particles expand under low pres-sure, the wave goes down. We often describe sound as being loud or soft and high-pitched or low-pitched. But these are perceptions, not physical characteristics. To understand and analyze sound, it must be characterized in ways that can be measured using instruments. “Loud or soft” relates to the intensity or amplitude of the sound, whereas “pitch” is the frequency. A sound’s amplitude (loud-ness) is the change in pressure as the wave passes. And, like adjusting your iPod vol-ume, the amplitude of a wave is related to the amount of energy it carries. A high-am-plitude wave carries a large amount of en-ergy, while a low-amplitude wave carries a small amount. The average amount of en-ergy passing through a unit area per unit of time in a specified direction is termed wave intensity. As the amplitude of the sound wave increases, so does the inten-sity of the sound. Sounds with higher in-tensities are perceived to be louder. Relative sound intensities are often given in units termed decibels, abbreviated as dB. As sound travels through seawater its intensity decreases because of spreading, scattering and absorption. Intensity loss in-creases with distance, while scattering oc-curs when sound waves bounce off bubbles, suspended particles, marine life, the surface, the seafloor and other objects. Eventually sound waves are absorbed and converted into a very small amount of heat. The absorption of sound is also a function of its wavelength, with higher fre-quencies being absorbed before lower fre-quency ones. In dry air at sea level with a tempera-ture of 70 degrees Fahrenheit (21 degrees Celsius), the speed of sound is 1,130 feet (342 m) per second (770 mph [1,232 kmph]). In seawater with a temperature of 90 • • • • • NOV/DEC/JAN/FEB 2017/2018 “Where When How -Turks & Caicos Islands”

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