Phase plug

The phase plug extends high frequency response because it guides waves outward toward the listener rather than allowing them to interact destructively near the driver.

In each case they serve to equalize sound wave path lengths from the driver to the listener, to prevent cancellations and frequency response problems.

[3] Various loudspeaker designs were produced in the 1920s, including General Electric engineers Chester W. Rice and Edward W. Kellogg mating an acoustic horn to the speaker driver in 1925.

At high frequencies, the diaphragm does not act as a perfect piston; instead, it displays rippling, modal properties related to its stiffness and density.

Radial slots in the phase plug do not correct for this small time difference, which affects the highest frequencies.

This design requires a radically different shape of phase plug, but radial slots and concentric rings may still play a part.

In this case, "high frequency" is relative to the intended bandpass; for example, a 12-inch (300 mm) cone woofer might be expected to reproduce 550 Hz energy near the top of its intended range, however, the wavelength of 550 Hz is approximately twice the diameter of the woofer, so wave energy at that frequency traveling laterally from one side to the other will be out of phase and will cancel.

With a phase plug in the center, such lateral wave energy bounces off of the obstruction and is reflected outward toward the listener.