A horn provides more sound pressure level (SPL) at a given listening area by increasing the directivity of the sound towards the listener. There is more sound at the listening area, and less sound outside of that area. By analogy, think of focusing a beam of light (from a flashlight or torch). A widely focused beam spreads the light around, reducing the intensity at any one point. However, a narrowly focused beam provides much more light intensity at the center, and much less in the surrounding area. Properly designed horns can also act as a waveguide that actually serves to spread higher frequency sounds out in a much more consistent manner than would otherwise happen.
Round horns and radial horns tend to change their angles of spread (their directivity, measured by the directivity index, or DI) as the frequency changes. This means that high frequencies, for instance, might be more highly directed, and therefore sound louder to someone in a central location than to someone else outside of the center (but still within the horn’s low-frequency area of enhancement). To cope with this problem, the constant directivity (CD) horn was invented. The design goal of the CD horn is to provide the same SPL at all frequencies within the designed coverage angles.
The term “Constant Directivity” is a trademark of ElectroVoice but has become somewhat of a catchall phrase to describe constant-beamwidth horns. In 1975, Electro-Voice introduced a single-cell horn that consisted of three-stages. The design incorporates a hybridized hyperbolic/exponential throat section coupled to a conical, vertically flared, radial bell section. Flanges that correct for midrange beaming caused by edge diffraction are comprised of a second, wider conical, vertically flared, radial bell-section. As with classic radial horn designs, the sidewalls are straight, but in two flange sections. Having constant beamwidth in both the vertical and horizontal directions, and an unprecedented high directivity index, these horns became the model for virtually all-new horn designs for the next decade. Additionally, they horn loaded the driver well, and as a result sounded very good.