Sonic Booms to Bow Wakes What happens to the sound produced by a moving source, such as a jet airplane, that approaches or even exceeds the speed of sound? The answer to this question applies not only to sound but to all other waves as well. Suppose a jet airplane is coming nearly straight at you, emitting a sound of frequency s . The greater the plane's speed s , the greater the Doppler shift and the greater the value observed for obs . Now, as s approaches the speed of sound, obs approaches infinity, because the denominator in obs=s(wws) approaches zero. At the speed of sound, this result means that in front of the source, each successive wave is superimposed on the previous one because the source moves forward at the speed of sound. The observer gets them all at the same instant, and so the frequency is infinite. (Before airplanes exceeded the speed of sound, some people argued it would be impossible because such constructive superposition would produce pressures great enough to destroy the airplane.) If the source exceeds the speed of sound, no sound is received by the observer until the source has passed, so that the sounds from the approaching source are mixed with those from it when receding. This mixing appears messy, but something interesting happensa sonic boom is created. (See Figure 17.16.)