The holy grail of loudspeakers is a sound source that provides a sound field whose three-dimensional radiation pattern is constant over a wide frequency range.

This type of source provides an acoustic output whose spectral content does not vary with direction. Traditionally, these speakers are called constant-directivity or constant-beamwidth devices.

The CBT provides nearly perfect polar behavior with extremely low side lobes and constant directivity over a very wide frequency range. The distribution pattern of the array is found to be essentially independent of distance from the CBT, which effectively means the CBT has no nearfield. The CBT is found to be uniformly loaded because the surface velocity and surface pressure are in phase and exhibit the same dependence on angle.

CBT, which stands for Constant Beamwidth Transducer, is a loudspeaker line array technology pioneered by Don Keele in nine Audio Engineering Society technical papers (listed here), and is based on unclassified military underwater sound research.

A traditional line array speaker with multiple drivers have comb filtering and lobing issues, thus leading to a non-uniform response. This not only effects listeners at off-axis but on-axis as well due to the reflected energy being highly colored.

The CBT technology avoids these phase misalignments and gives a very uniform response. The result is a loudspeaker with excellent sound everywhere and a speaker that is much easier to integrate into a living room.

The CBT technology can be applied in several ways with different dispersion characteristics. The most common type is a ground plane CBT with multiple monopole drivers in a circular arc.