Editing Audio crossover (section)

====Second order====
Second-order filters have a 40 dB/decade (or 12 dB/octave) slope. Second-order filters can have a [[Bessel filter |Bessel]], [[Linkwitz-Riley filter |Linkwitz-Riley]] or Butterworth characteristic depending on design choices and the components that are used. This order is commonly used in passive crossovers as it offers a reasonable balance between complexity, response, and higher-frequency driver protection. When designed with time-aligned physical placement, these crossovers have a symmetrical [[wiktionary:polarity|polar]] response, as do all even-order crossovers. 

It is commonly thought that there will always be a [[Phase (waves)|phase]] difference of 180° between the outputs of a (second-order) low-pass filter and a high-pass filter having the same crossover frequency. And so, in a 2-way system, the high-pass section's output is usually connected to the high-frequency driver 'inverted', to correct for this phase problem. For passive systems, the tweeter is wired with opposite polarity to the woofer; for active crossovers the high-pass filter's output is inverted. In 3-way systems the mid-range driver or filter is inverted. However, this is generally only true when the speakers have a wide response overlap and the acoustic centers are physically aligned.