A. The most familiar application for them are the tone controls on stereos. As you adjust the bass and treble on a stereo, you are adjusting the response to low and high frequencies, respectively.
In graphic equalizers, the audio spectrum is divided up into bands of frequencies by filters. The graphic equalizer allows you to adjust the volume for each band individually.
Another application for filters is in signal processing circuits in communications systems. One signal may be in one band of frequencies, While another signal may be in another band. Filters are used to select the desired band, and thus separate the signals. Often, an interfering signal is present at a single frequency, or in a narrow band of frequencies.
A filter can be inserted to block out the interfering signal, while allowing the desired signal to pass unaffected.