A noisy truck approaches you from behind a building. Initially you hear it but cannot see it. When it emerges and you do see it, its sound is suddenly “brighter”—you hear more of the high-frequency noise. Explain. [Hint: See Section 11–14 on diffraction.]
When the truck is behind the building, sound waves of a longer wavelength reach the person. As the truck starts coming ahead of the building, the wavelength of all frequencies is reaching to the ears.
Thus, as the truck appears, the higher frequency noise starts coming in.
Long-wavelength waves are diffracted more than short-wavelength waves. So, the sound wave of longer wavelength and shorted frequency are audible than the sound wave of short wavelength.
The waves spread out as they travel, and when they encounter an obstacle, the larger wavelengths bend around the building almost as if they were not there. When the truck emerges from behind the building, the sound suddenly increases, and we hear more high-frequency noise because now, even the smaller wavelengths reach us in addition to the larger wavelengths, and hence high-frequency noise is heard.
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