A Wide-Range Coincident Louspeaker Drive Unit

Enrico Cecconi, B.Sc.(Hons), M.Sc., AES Member

Loudspeaker system design, by the spatial summation of cascaded acoustical and electrical filters, has reinforced the requirement for a sound source whose position does not vary with frequency. This had, hitherto, been particularly difficult to achieve using moving coil transducers. The practical implementation of such a device, which relied heavily on the availability of a high energy permanent magnet material for its realisation, is described. The implications regarding subsequent crossover design and system off-axis acoustic response are also discussed.

Conventional loudspeaker systems; those using separate drive-units covering different frequency ranges mounted adjacent to each other on the front of a cabinet, all share a common characteristic. Their summed frequency response cannot vary in a symmetric manner as the summation point is moved away from the chosen design axis. This will be the case even in the acoustically neutral environment of a half space baffle (2p steradian).

The reason for this phenomenon is the physical separation between drive-units and the attendant time delays to the arrival of sound on the particular measurement axis.

It is possible to design conventional loudspeakers to have a smooth amplitude response and produce a coherent stereo image within a restricted listening window.

Unfortunately, even the best of such designs invariably suffer serious response changes and variable stereo image when the listener moves outside that window. Design of drive-units, crossover networks and cabinets can minimise off-axis differences over a limited range of listening angles, but it is impossible to eradicate them completely. This effect restricts the positions in which a listener can sit in order to hear the sound which the designer intended.

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