Studio monitoring systems 511 about 9 dB down one octave away from the crossover frequency. If a drive unit had a 5 dB response peak one octave away from the crossover point, then its response would only be 4 dB (9 dB ­ 5 dB) below the wanted signal level, and so the irregularities in the response could clearly be audible. Alterna- tively, a 72 dB/octave slope would solve the problem completely, but would introduce other problems, such as greater group delays and possible ringing problems from the high Q filters needed to achieve such a slope. Crossover slopes are therefore chosen as compromises, and there are indeed many options. 20.6.1 Passive crossovers The function of a crossover is to split the frequency bands and send only the appropriate frequencies to each drive unit ­ lows to the woofers, mids to the squawkers, highs to the tweeters, etc. Traditionally this was done with passive networks of inductors and capacitors placed between the amplifier output termi- nals and the loudspeaker drive units; usually inside the loudspeaker cabinet. Typical, historical networks are shown in Figure 20.4. The values of compo- nents were often chosen presuming that the loudspeakers were electrically like resistors, but in fact that is not the case, because they exhibit resistance, induct- ance and capacitance. Their input impedance (resistance plus reactance) there- fore tends to vary with frequency, and two typical impedance curves are shown in Figure 20.5. The traditional crossovers were thus usually mis- matched to the load impedance, and response flattening often took place with