Sound, in reference to airplanes and their movement through the air, is nothing more than pressure disturbances in the air. As discussed earlier in this section, it is like dropping a rock in the water and watching the waves flow out from the center. As an airplane flies through the air, every point on the airplane that causes a disturbance creates sound energy in the form of pressure waves. These pressure waves flow away from the airplane at the speed of sound, which at standard day temperature of 59°F, is 761 mph. The speed of sound in air changes with temperature, increasing as temperature increases. Figure 3-75 shows how the speed of sound changes with altitude.
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