Conventional loudspeaker crossover networks of slope greater than 6 dB per octave, when properly implemented, result in a loudspeaker system whose acoustic transfer function, although of flat magnitude, has all-pass phase characteristics. The system is thus nonminimum phase, and complicated phase equalization using delay equalizers is required in order to render it linear phase and so transient perfect. A number of attempts are currently being made to "acoustically align" such systems by deliberately either introducing or eliminating time delays between the drivers and using conventional minimum-phase equalization to flatten their overall frequency response. It is shown that no choice of interunit time delay can render the system minimum phase, and hence that minimum-phase equalization cannot make such a system both flat and phase linear.
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