output level over its frequency range. The benefits of the design were first published by C.R. Hanna and J. Slepian in 1924 for the American Institute of Electrical Engineers (AIEE). A major drawback is that the exponential horn allows for a narrowing of the radiation pattern as frequency increases, making for high frequency 'beaming' on axis and dull sound off axis. Another concern is that a throat of small diameter is needed for high efficiency at high frequencies but a larger throat is best for low frequencies. A common solution is to use two or more horns, each with the appropriate throat size, mouth size and flare rate for best performance in a selected frequency range, with sufficient overlap between the frequency ranges to provide a smooth transition between horns. Another solution tried in the late 1930s by Harry F. Olson of RCA was to use multiple exponential flare rates, either by connecting increasingly larger horns in series or by subdividing the interior of a single horn. Exponential horns continue to be used by some designers, and in some applications. Multicell A number of symmetrical, narrow dispersion, usually exponential horns can be combined in an array driven by a single driver to produce multicell horns. Patented in 1936 by Edward C. Wente of Western Electric, multicell horns have been used in loudspeakers since 1933 to address the problem of directivity at higher frequencies, and they provide excellent low frequency loading. Their directional control begins to beam both vertically and horizontally in the middle of their target frequency range, narrowing further at high frequencies with level changes as great as between lobes. Multicell horns are complex and difficult to fabricate and thus have a higher associated expense. They persisted in public address applications for many years because, even with their faults, they sounded very good, and still do with competent design. The revolutionary coaxial driver, the Altec Lansing Duplex 601 and 604, used a multicell horn for its high frequency component from 1943 to 1998. Radial, sectoral, and diffraction Radial horns have two surfaces based on an exponential flare rate, and two straight walls that determine the output pattern. The radial horn exhibits some of the beaming of the exponential horn. Altec sectoral horns were radial horns with vanes placed in the mouth of the horn for the stated purpose of pattern control. For ease in mounting to loudspeaker cabinets, flat