The D'Appolito arrangement is a special arrangement and control of loudspeakers in a speaker cabinet. This arrangement is named after the American physicist Joseph D'Appolito and has the advantage over other speaker boxes that the vertical dispersion is very narrow.
The D'Appolito arrangement reduces sound distortion due to floor and ceiling reflections and improves room sound. In this concept, a tweeter is located between two midrange drivers, and they are arranged in a vertical line at the same distance from the tweeter.
Optimal sound dispersion is achieved when the distance between the tweeter and the midrange drivers is less than two-thirds of the wavelength of the cutoff frequency, where the distance between the drivers refers to the center of the diaphragms.
For example, if the crossover frequency is 1.5 kHz, then at a speed of sound of 330 m/s, the wavelength is 22 cm. This means that the distance between the tweeter and midrange centers should not exceed 15 cm. So, in order for the D'Appolito principle to be implemented, both the arrangement and the crossover frequencies for the tweeters and midrange/midrange drivers are crucial.