making the vertical coverage pattern too small to be useful. Mantaray Subsequent to Keele's work and using his principles, Clifford A. Henricksen and Mark S. Ureda of Altec designed a strikingly different hybrid horn displaying constant directivity traits, the horizontal diffraction or "Mantaray" horn. The Mantaray horn separates desired vertical coverage pattern from horizontal, making it possible to design horns for a variety of coverage patterns. The Mantaray shape starts with a vertically oriented JBL-style diffraction horn, leading into a conical waveguide (earliest designs), or a square or rectangular horn with four planar sides. For midrange beaming control, the outer mouth is expanded further with a short, flared flange in the Keele style, or with added planar sides of a greater flare angle. Low frequency efficiency is not as pronounced as the constant directivity design. Unlike previous designs, the apparent apex, which is the focal point of pattern dispersion, is not the same for every frequency, making for an ellipsoidal wavefront rather than spherical. Because of this, the Mantaray can only be arrayed satisfactorily in one plane (rather than multiple planes). Its abrupt breaks in flare rate causes diffraction, reflection and distortion components. Bi-Radial By 1980, Keele was at JBL where he took both his and Altec's designs a step further. He mated a JBL-style diffraction horn to a secondary horn consisting of exponentially curved sides derived by using two radial formulas. This resulted in a hybrid constant directivity horn that was free from the distortion components associated with abrupt angle changes. The market responded well to the design in products such as the JBL model 4430 studio monitor with its 100° × 100° model 2344 Bi-Radial high frequency horn often called "butt-cheeks". The Bi-Radial design had problems with apparent apex and arrayability in the same manner as the Mantaray. Twin Bessel Ramsa, the professional audio division of Panasonic Corporation, introduced a twin Bessel constant directivity horn shortly after the Mantaray appeared. The design was very similar to the Mantaray and the Bi-Radial but it used a dual series Bessel expansion formula to determine the flare rate of the secondary horn section. CD horn characteristics Most popular constant directivity horns (also known as CD horns) suffer from non-spherical wavefronts, limitations in arrayability, and distortion at high sound pressure levels as well as reflections and distortions related to the transition from diffraction slot to secondary horn. They tend toward a narrowing