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Question 1157

How does a parabolic microphone work? — KL, Regina, Saskatchewan
A parabolic microphone is effectively a mirror telescope for sound. When sound waves strike the dense, rigid surface of the parabolic dish, they partially reflect. This reflection occurs because sound travels much faster in a rigid solid than in the air and changes in the speed of a wave cause part of it to reflect. In this case, the reflection redirects the sound waves inward because the reflecting surface is curved and the sound waves form a real image of the distant source that produced them. While you can't see this real image with your eyes, you can hear it with your ears. If you were to mount a large parabolic dish so that it faced horizontally and then moved your ear around in the focal plane of the dish, you would hear sounds coming from various objects far away from the dish. The same effect occurs for light when it bounces off a curved mirror—a real mirror telescope. A TV satellite dish is the same thing, but this time for microwaves! In all three cases, the real images that form are upside down. To make a parabolic microphone, you normally put a conventional microphone in the central focus of a parabolic surface so that the microphone receives all the sound coming from objects directly in front of the parabola. To listen to different objects, you simply steer the parabola from one to the other. This is exactly what a TV satellite dish does when it wants to "listen" to a different satellite—it steers from one to the other.

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