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Microphone Month

Acoustic Lens

In loudspeakers, a mechanical device used to improve the dispersion of high frequencies, so that dispersion is much more uniform across the audible spectrum. The lens is a product of post-World War II Bell Labs research, first described in 1949. The intent is to focus sound in much the same way that an optical lens focuses light. An axiom called Snell’s law describes the refraction of sound as it passes through an interface between two materials of differing sound speed.

A high-frequency loudspeaker mechanical acoustic lens spreads a single-point sound source into a parallel wave front. Originally introduced commercially by JBL in the 1950s, they appeared in two primary designs. First was the slant-plate lens, which utilizes a series of plates with carefully calculated hyperbolic shapes, which results in a horizontal response pattern. This is the most commonly seen acoustic lens type. Second is the perforated-plate lens assembly, which consists of a collection of perforated barriers at the horn mouth. These perforated screens are actually ring shaped with varying sizes of center cutouts. Although acoustic lenses gradually fell out of favor through the 1970s and 1980s (partially due to their fragility, which made them risky to use in portable sound reinforcement systems), the technology has re-emerged in some high-end home audio systems (notably Bang & Olufsen) in recent years.

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