█ LARRY GILMAN
A parabolic microphone is an ordinary microphone mounted inside a sound-reflecting dish having a parabolic cross section. Sound waves passing straight into the parabolic reflector are focused by it on the microphone; sounds entering the reflector dish from other angles impinge directly on the microphone, but are not focused on it by the reflector. Thus, the parabolic microphone is highly directional, that is, more sensitive to sound sources at which it is directly pointed than to other sources. This makes the parabolic microphone useful for recording localized sources of relatively faint sounds, such as conversations or bird calls, at a distance.
A paraboloid reflector is used because of the unique geometrical properties of the parabola. A parabola is an open curve resembling a V with a rounded point. (Mathematically, the two arms of the curve go on forever; in building a parabolic reflector, the arms are cut short.) The "axis" of the parabola is a straight line that passes through it like a vertical line drawn through the center of a V. All rays that enter a parabola parallel to its axis and are reflected from the curve (like light rays from a mirror) pass through a single point inside the parabola, the focus. In a parabolic microphone system, the microphone is placed at this point; sound waves entering the dish parallel to the axis are focused on the microphone and, thus, amplified.
Another type of directional microphone, the shotgun mike, attains directionality by embedding the microphone in a long, narrow, open-ended tube; only sound approaching the mike along the axis of the tube can reach the mike. On one hand, the shotgun design does not focus sound on the mike, and so is not as sensitive as the parabolic design; on the other, the shotgun mike is less cumbersome and less open to off-axis sounds.
Parabolic reflectors are also used to create light beams from point sources. All light emanating from a point source placed at the focus of a parabolic reflector will exit the reflector in the direction of the parabola's axis. (The bulbs of car headlights are placed at the foci of parabolic reflectors.)
█ FURTHER READING:
Weisstein, Eric W. "Parabola." MathWorld (Wolfram Research). <http://mathworld.wolfram.com/Parabola.html> (April 17, 2003).