The RC circuit we have been studying is a special case of a low pass filter (LPF). Notice in Figure 48 that for low frequencies, the magnitude is approximately constant, near 0 dB, while at high frequencies the magnitude drops off at a rate of 20 dB per decade. This means that low frequencies are passed through, but high frequencies are attenuated--essentially blocked. The 3 dB point indicates (roughly) the division between the pass band and the block band. This is called the cut-off frequency. See Figure 50 for some ideal filter types.
HiFi speaker systems usually have a number of speakers in each cabinet. Large speakers or woofers are best for low frequency sounds, whereas small tweeters are best for high frequencies. A mid-size speaker is used for the midrange. A crossover network is a filter circuit that takes the incoming audio signal from the amplifier and splits it into low, mid and high frequency components, see Figure 51 and the PSPICE file cross.sch.
ANU Engineering - ENGN2211
