Parabolic loudspeaker

A parabolic loudspeaker is a loudspeaker which seeks to focus its sound in coherent plane waves either by reflecting sound output from a speaker driver to a parabolic reflector aimed at the target audience, or by arraying drivers on a parabolic surface. The resulting beam of sound travels farther, with less dissipation in air, than horn loudspeakers, and can be sent to isolated audience targets, unlike line array loudspeakers. The parabolic loudspeaker has been used for such diverse purposes as directing sound at faraway targets in performing arts centers and stadia, for industrial testing, for intimate listening at museum exhibits, and as a sonic weapon.

A parabolic loudspeaker can send sound farther than traditional loudspeaker designs. The focused waves of a parabolic loudspeaker tend to dissipate in air at about 3 dB SPL per doubling of distance, rather than the usual 6 dB of conventional loudspeakers.

In a parabolic reflecting loudspeaker, one or more speaker drivers are mounted at the focal point of a parabola, pointing away from the audience, toward the parabolic surface. The sound is bounced off the parabolic dish and leaves the dish focused in plane waves. The lowest frequency that can be directed into a narrow beam is dependent on the size of the parabolic dish. A parabolic reflector type of loudspeaker must have a diameter twice that of the lowest desired wavelength, so to obtain directional control of frequencies down to 20 Hz, the dish would have to be over 50 feet (15 m) wide.

Limitations of parabolic reflector loudspeakers include the fact that they are comparatively large and bulky, and that they have a fixed beam width with no ability to broaden or narrow the coverage pattern without changing the curvature of the dish. Their beam width is wider for low frequencies than it is for high frequencies, so at the periphery of the coverage pattern there is a region of sound coverage that doesn't receive the full strength of the high frequencies. In addition, some frequencies are reflected more efficiently than others, so the frequency response is uneven unless audio signal processing correction is applied before the signal reaches the amplifier. The presence and placement of the speaker driver prevents the center of the parabolic dish from reflecting sound outward, as that sound would reflect back into the speaker driver itself. In some loudspeaker designs, a hole is cut at the center of the parabolic dish, or damping material placed, such that no sound is reflected directly at the speaker driver.

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