One of the first horns successfully designed for directivity control was the multicell horn. Multicell horns are expensive to produce, and therefore other solutions were developed. These included the radial horn, the Electro-Voice Conical-Exponential horn Altec's Manta-Ray horns, and the JBL Biradial horns. All these horns are what can be called single-surface horns, i.e. they are not divided into cells.
However, single cell horns have severe restrictions on what is possible in terms of upper and lower break frequencies and acoustic loading. In fact, given the throat size, upper break frequency and coverage angles, the design is fixed, and only the mouth size can be varied. This paper will explain why.