A number of symmetrical, narrow dispersion horns, usually exponential horns, can be used in an array driven by a single driver to create multicellular horns. Patented in 1936 by Edward C. Wente of Western Electric, multi-cage horns have been used in loudspeakers since 1933 to solve the directivity problem at high frequencies and provide excellent low-frequency loading. Their directional control starts the beam both vertically and horizontally in the middle of their target frequency range, tapering further at high frequencies with level changes of up to 10dB between lobes. Multicellular horns are complex, difficult to manufacture and therefore require higher costs. They persisted in public address applications for years because, even with their mistakes, they sounded relatively good. The revolutionary Altec Lansing Duplex 601 and 604 coaxial driver used a multi-cage horn for the high-frequency component from 1943 to 1998.