5.2 Application of Linear-Phase Digital Crossover Filters to Pair-Wise Symmetric Multi-Way
Part 1: Control of Off-Axis Frequency Response
ULRICH HORBACH(1) & D. B. (DON) KEELE, JR.(2)
(1) Harman Consumer Group, Northridge, California, USA. (2) Harman/Becker Automotive Systems, Martinsville, Indiana, USA.
Various methods exist for crossing over multi-way loudspeaker systems. These methods include those loosely
classified as Linkwitz-Riley filters, constant-voltage filters, and D’Appolito configurations. All these methods do not
provide broad-band constant-beamwidth or constant-directivity operation because their vertical radiation patterns
change shape as a function of frequency. This paper describes a simple, non-iterative linear-phase crossover filter
design technique that provides uniform frequency responses vertically off-axis for a given multi-way loudspeaker.
Distances between the individual drivers, and desired off-axis attenuation are prescribed as input parameters for
the design process, the outcome of which is a set of crossover frequencies and unique filter frequency responses
in each band. In order to obtain wide-band constant-beamwidth, a loudspeaker array configuration composed of a
single central tweeter surrounded symmetrically by pairs of lower-operating-frequency transducers arranged in a
vertical line is required. Practical implementation issues are outlined in the paper by means of various design
examples. Two design methods are presented in two-parts: Part 1: a general method which emphasizes flatness of
arbitrary off-axis frequency responses and Part 2: a simplified method that emphasizes frequency uniformity of
beam shape and coverage angle (vertical beamwidth) of the polar patterns.