Crossovers are often described in terms of symmetrical pairs of high- and low-pass filters with a common denominator, usually Butterworth, double Butterworth (Linkwitz-Riley), or notched. The native response of the usual closed-back tweeter is second-order high-pass, but its cone excursion goes to a compliance-limited maximum at frequencies below its cutoff. It therefore needs further high-pass filtering to prevent excessive power dissipation and cone excursion produced by components of the program signal at frequencies lower than its passband. Thus the overall high-pass transfer function must be of at least third order, and preferably higher. In the low-pass channel, on the other hand, such high-order filtering is often unnecessary, so the cost and complexity of the crossover can be reduced significantly by using an asymmetrical crossover. Various possibilities are explored, with comments on their advantages and disadvantages compared with symmetrical systems.
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